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Sound Reinforcement - Forums for Live Sound Professionals - Your Displayed Name Must Be Your Real Full Name To Post In The Live Sound Forums => AC Power and Grounding => Topic started by: John Hyun on May 03, 2012, 12:13:51 am

Title: Dangerous AC situation in reception hall - PLEASE READ
Post by: John Hyun on May 03, 2012, 12:13:51 am
Providing sound for a banquet at a reception hall this past weekend and I ran into some problems.

Rig
2 Yorkville U15 run off rmx2450 each channel
2 Yorkville UCS1 run off 2 rmx2450 in bridged mono
Mixer (Mackie SR24)
Effects rack (dbx 2231s, Yorkville unity processors, dbx compressors)
Snake (horizon 16x4)

Setup
-Please see attached file

Power
-There was an outlet in the front of the stage and I plugged the two amps for the subs there.
-I plugged the mixer, effects rack, and poweramp for the mains into an outlet at the back of the stage

Signal
-So my tech was working on the tops while I worked on the subs.  He routed it incorrectly, so Iíll explain the chain. 

Tops : Main outs from mixer > Unity Processor (crossover) > Unity Bi-amp/high out > Eq in > Snake A Channel tail end > From snake box, ran a cable back to the poweramp > U15s

Subs: Sub Out from unity processor > Snake C channel > from head of snake, channel C into poweramp > UCS1s via parallel inputs

-I had the tech correct the chain for the tops because he didnít have to go through the snake because the poweramp was right there by the mixer/effects rack.  Also had him bypass the EQ.

New Signal Chain
Tops: Main outs from mixer  > Unity Processor > Unity Biamp out > Poweramp > U15s


Now, hereís where it gets funky.  Had him power up everything.  Mixer turned on, effects turned on, and as we are about to turn on the poweramps, we smell smoke and I have him shut everything off.  At this point I had no idea what was going on because none of the poweramps were on.  My first thought was that I was drawing too much power somewhere.  I ran to unplug everything, and the first piece I went to get was the cable in channel C of the snake (which is for the subs).  It was HOT, so I left it.  I took off the plugs from the outlet.

It was total chaos.  Channel A of the snake on the tail end lost its coating (which probably was what the smoke was) and the wire was glowing red (about a foot of cable).  Now, if you were following along, Channel A was plugged into the EQ output.  A cable on the box end was originally patched to the poweramp (the one in the back for the mains) but it was pulled out.  So, Channel A on the box end had nothing connected to it. 

I still cant explain what happened.
What caused Channel A to short and heat up like that?  It was only connected to the output of the EQ.  And it only melted from the jack to about a foot in.

Why was channel C hot?  Matter of fact, I touched the entire snake cable and it was really warm.  My guess is that the heat from channel A was heating all the cables?  Or was it channel C that was causing the problem?

None of the poweramps were on.  Mixer and effects were on for about 20-30 seconds.

So after everything cooled down, I used another outlet for the mixer, and used separate outlets on opposite walls for the poweramps.  Not knowing how the circuit is split up, I took my best guess.  Powered everything back on and nothing smoked.  Channel C on the snake passed signal for the subs.  Other channels on the snakes worked as well.  Had a flawless night with speech and presentations, and two hours of pumping music at levels barely clipping.

What happened?  Iím glad nothing serious, like fire, started.  But something DID happen, and itís serious and I donít want it to happen again.  Please input, and Iíll try to answer questions if I left things out.

Thanks.
Title: Re: freak accident? help!
Post by: Jordan Wolf on May 03, 2012, 12:36:40 am
John,

It sounds like there was a hot/ground swap in one of the outlets you used.  Anything tied to the chassis ground of the equipment that was plugged into that faulty outlet would have current flowing through it and, likely, through any snake channels connected to it.  My guess is that the smaller gauge wire in the snake heated up faster than the larger gauge metals that made up the shells of the rackmounted gear, so it smoked and became a heating coil.

Unfortunately, a tester cannot detect this fault - a visual inspection is the easiest way to find out and the best way to verify that everything is "ok".  If you were able to use a multimeter to measure the voltage between the outlet's "ground" and the rack rails or some piece of gear that had a metal chassis, you might have seen the voltage.  Anytime you measure an outlet's ground against anything other than a hot leg, and it reads 1V or more, I'd be concerned and investigate further or refuse to use that outlet.

Any other takers?
Title: Re: freak accident? help!
Post by: brian maddox on May 03, 2012, 12:42:39 am
John,

It sounds like there was a hot/ground swap in one of the outlets you used.  Anything tied to the chassis ground of the equipment that was plugged into that faulty outlet would have current flowing through it and, likely, through any snake channels connected to it.  My guess is that the smaller gauge wire in the snake heated up faster than the larger gauge metals that made up the shells of the rackmounted gear, so it smoked and became a heating coil.

Unfortunately, a tester cannot detect this fault - a visual inspection is the easiest way to find out and the best way to verify that everything is "ok".  If you were able to use a multimeter to measure the voltage between the outlet's "ground" and the rack rails or some piece of gear that had a metal chassis, you might have seen the voltage.  Anytime you measure an outlet's ground against anything other than a hot leg, and it reads 1V or more, I'd be concerned.

Any other takers?

i concur that this is a likely cause.  it's always a good idea to test for voltage between neutral and ground, even straight from a wall outlet.  that would have shown this issue immediately.

i suspect it was the outlet that the mixer was plugged into...
Title: Re: freak accident? help!
Post by: Jordan Wolf on May 03, 2012, 12:55:51 am
it's always a good idea to test for voltage between neutral and ground, even straight from a wall outlet.  that would have shown this issue immediately.

One time is one time too many for me. 

I make absolutely certain that I meter my power when :
 - I go to a new venue
 - I think something may have changed since I was there last
 - any type of tie-in is made (cams, twistlock)
 - a generator is used (along with  >:(or without in-wall power)

I don't care as much about smoked gear as I do smoked people; smoked salmon on the other hand...
Title: Re: freak accident? help!
Post by: Tim Perry on May 03, 2012, 01:25:07 am
One time is one time too many for me. 

I make absolutely certain that I meter my power when :
 - I go to a new venue
 - I think something may have changed since I was there last
 - any type of tie-in is made (cams, twistlock)
 - a generator is used (along with  >:(or without in-wall power)

I don't care as much about smoked gear as I do smoked people; smoked salmon on the other hand...

A $3 tester from most any hardware store may save your life... if you use it every time.  The one outlet you fail to check may get you.

Title: Re: freak accident? help!
Post by: Samuel Rees on May 03, 2012, 01:29:55 am
Glad no one was hurt. What would be the perfect storm for that to happen, someone being hurt by this I mean?Touching a rack rail while grounded?
Title: Re: freak accident? help!
Post by: Jordan Wolf on May 03, 2012, 01:43:18 am
A $3 tester from most any hardware store may save your life... if you use it every time.  The one outlet you fail to check may get you.
Tim,

One thing that those testers DON'T show you is whether or not the ground and neutral are swapped.  A multimeter or, better yet. visual inspection will yield an answer, though.
Title: Re: freak accident? help!
Post by: Ivan Beaver on May 03, 2012, 06:40:57 am
John,

It sounds like there was a hot/ground swap in one of the outlets you used. 
I have seen that happen in a couple of cases.  Mostly in Churches-where htey have used "free" help.  Everything may worki fine for years-AS LONG AS nothing common is tieing the outlets together-like a sound system.

My old house actally had 1 whole floor with the hot/neutral swapped.  I lived there for 10yrs and never realized it.  And my shop was on that floor.  It wasn't until I went to sell the house and the buyer home inspecter pointed out the problem.

I took a cheap tester with me to all the new houses I was looking at.
Title: Re: freak accident? help!
Post by: Bob Leonard on May 03, 2012, 07:45:24 am
Tim,

One thing that those testers DON'T show you is whether or not the ground and neutral are swapped.  A multimeter or, better yet. visual inspection will yield an answer, though.

In a 120v single phase circuit 20 amp circuit the ground and the neutral should tie back to the same point. Ground.
Title: Re: freak accident? help!
Post by: Tim Perry on May 03, 2012, 09:59:41 am
Tim,

One thing that those testers DON'T show you is whether or not the ground and neutral are swapped.  A multimeter or, better yet. visual inspection will yield an answer, though.

A lot of places I go would get upset if i disassembled their outlets for visual inspection and even just checking that the colors were right wouldn't guarantee a misswire elsewhere in the circuit.

The cheep checker is easy to use by anyone. Just remember: 'If you see red, you could be dead'   
Title: Re: freak accident? help!
Post by: John Roberts {JR} on May 03, 2012, 10:05:37 am
Yes a miswired outlet. that's the kind of fault that has killed musicians since the product can still work, but it's "safe" outer chassis metal is now a dangerous shock hazard.

JR
Title: freak accident? help!
Post by: Samuel Rees on May 03, 2012, 10:23:33 am
I realize this isn't my thread but this is a safety issue I could encounter and I'm not following, so that's got to be bad..,

What points of contact would your hands need to get in this situation to get hurt? In this case do we suspect hot/ground or neutral/ground to be switched? Which one can the tester identify and is that the dangerous one?
Title: Re: freak accident? help!
Post by: Jay Barracato on May 03, 2012, 11:30:09 am

even just checking that the colors were right wouldn't guarantee a misswire elsewhere in the circuit.

+1

And given that motors are far less critical of these swaps, and since most outlets in bars/ballrooms probably see more vacuum cleaner type equipment than anything else, a miswire could be affecting a whole string of outlets that nobody knows if they are on the same circuit or not.
Title: Re: freak accident? help!
Post by: John Roberts {JR} on May 03, 2012, 12:03:48 pm
I realize this isn't my thread but this is a safety issue I could encounter and I'm not following, so that's got to be bad..,

What points of contact would your hands need to get in this situation to get hurt? In this case do we suspect hot/ground or neutral/ground to be switched? Which one can the tester identify and is that the dangerous one?

The old advice to keep one hand in your pocket has some merit.  For modest voltages it matters what path the current takes inside your body.  I recall one muso who was killed by 120V flowing from one hand to the other, through his body core.

Of course being sweaty lowers your skin resistance so makes you a better conductor.

be careful...

JR
Title: Re: freak accident? help!
Post by: Tim McCulloch on May 03, 2012, 12:39:35 pm
+1

And given that motors are far less critical of these swaps, and since most outlets in bars/ballrooms probably see more vacuum cleaner type equipment than anything else, a miswire could be affecting a whole string of outlets that nobody knows if they are on the same circuit or not.

Our state's Fair Association holds their annual meeting and showcase in a Holiday Inn ballroom.  At the back of the room there is a 20 amp Edison outlet.  I was there as a hired gun for FOH mixing so I didn't pay much attention to the Lampies, but they blew 3 follow spot lamps before I put a meter on that outlet.  Found it wired for 208v.  Apparently it was for either a floor cleaning machine or food service equipment.

It's best to NEVER assume that an outlet is wire correctly or that it has the correct voltage for your use.
Title: Re: freak accident? help!
Post by: Nick Enright on May 03, 2012, 01:22:27 pm
Our state's Fair Association holds their annual meeting and showcase in a Holiday Inn ballroom.  At the back of the room there is a 20 amp Edison outlet.  I was there as a hired gun for FOH mixing so I didn't pay much attention to the Lampies, but they blew 3 follow spot lamps before I put a meter on that outlet.  Found it wired for 208v.  Apparently it was for either a floor cleaning machine or food service equipment.

It's best to NEVER assume that an outlet is wire correctly or that it has the correct voltage for your use.

Is this the correct metering decision tree:

hot - neutral : 120v (little blade -> big blade)
hot - ground : 120v (little blade -> U blade)
neutral - ground: 0v (big blade -> U blade)

if above is true, outlet is safe

is there another way to be sure? (besides bringing your own distro)
Title: Re: freak accident? help!
Post by: Geoff Doane on May 03, 2012, 01:49:44 pm

What points of contact would your hands need to get in this situation to get hurt? In this case do we suspect hot/ground or neutral/ground to be switched? Which one can the tester identify and is that the dangerous one?

We suspect that there was a hot ground in this situation.  You could be electrocuted if you touched the chassis of the equipment plugged into this outlet or anything else it touches, and a real ground like a water pipe or other metal structure.  You might even draw some current standing on a concrete floor if it's a bit damp and your shoes aren't good insulators. 

Hot and ground may have been swapped (in which case the equipment would not be able to power up), but it's possible that the "ground" was hot for some other reason.  The simple 3-light tester should show this condition.  If all three wires were hot, it would not show a fault (except that it would indicate the outlet was "dead").

A swapped ground and neutral is not unsafe by itself (as long as all the connections are good), but it will inject an incredible amount of hum into a system (ask me how I know  :-[), or at least it will with non-pin 1 compliant equipment.  The tester cannot detect this condition, and it would be difficult without internal inspection of the wiring.

A hot ground, especially on installed wiring, should be very rare, but one way it has happened is with isolated ground outlets, often used for computer or other technical equipment.  Until recently, BX cable (flexible armour) was not available with a green wire for the isolated ground, so electricians used 3-wire cable, and used the red wire (appropriately taped) as the isolated ground.  The bare copper wire stayed as the regular ground for the box.

That works OK, as long as all the rules are followed.  On one installation, apparently one guy worked on one end, and another did the breaker panel.  The second guy though hooked it up like any normal split outlet, to a double breaker.  Once the breaker was turned on, there was 120V on the ground pins of that outlet.  The computer that was plugged into it worked fine, and no one even got a shock because it had a plastic case.  It even worked with an ethernet connection because the LAN CMMR was good enough for 120 VAC.  But when they switched to one of those old 50Ω coax LANs.  Once that was hooked up, the sparks flew!  :o

One other wiring fault that gets mentioned from time to time is a swapped hot and neutral.  This is probably far more common than anyone realizes, because under most conditions and with most equipment, it's not a problem.  It's not much different than "balanced" power, which has 60 VAC on the hot, and 60 VAC (opposite polarity) on the "neutral".  The simple tester will show that as a fault.

GTD
Title: Re: freak accident? help!
Post by: John Roberts {JR} on May 03, 2012, 02:38:38 pm
Is this the correct metering decision tree:

hot - neutral : 120v (little blade -> big blade)
hot - ground : 120v (little blade -> U blade)
neutral - ground: 0v (big blade -> U blade)

if above is true, outlet is safe

is there another way to be sure? (besides bringing your own distro)

Unless ground and neutral are both swinging at the same 120VAC, and the hot terminal is at 0V.

Without some external true ground reference like a cold water pipe, relative AC measures can be incomplete. If both ground and neutral are swinging at the same 120VAC a simple VOM will measure 0V between them. I don't know how a floating hand held tester can catch that.

The both neutral and ground hot, is a common mistake when old 2 wire buildings are converted to 3 wire outlets without third independent safety ground wire "and" the polarity of the 2 wires are reversed. The equipment plugged in doesn't know the difference until we interface this gear with other gear that is properly grounded. Then the full mains potential exist between the two chassis. 

Be alert for sparks or arcing between cables. Many experienced Musos will not approach a stage mic without first testing it for sparks to the grounded (?) guitar strings.

Before plugging the snake in at the far end, a quick spark test to the distant ground might have revealed this. Generally this is caught before releasing smoke or killing meat puppets, because there will be subtle (or not) clues, but not always.  A snake burning up is one of the less subtle clues. 

JR
Title: Re: freak accident? help!
Post by: Mike Sokol on May 03, 2012, 05:00:44 pm
Guys... this is not a freak accident. It was caused by something I call a Reverse Polarity Bootleg Ground (or RPBG). This occurs when an old building has new grounded outlets added by bonding the ground screw to the neutral screw because there was no separate ground wire to begin with. That by itself is electrically safe (however, it's illegal per the NEC), but many older buildings had black power wires for both the hot and neutral, and some were simply wired backwards with the white/neutral wire being hot and the black/power line being actual neutral. See the attached diagram. In that case any piece of gear plugged into a Reverse Polarity Bootleg Grounded outlet will have its chassis energized to 120 volts. If you then connect that piece of audio gear to something else that's plugged into a correctly wired outlet, you can have 20 amps or more of current flow down the shield, which melts wires and destroys gear. The really scary thing is that a 3-light tester will tell you that this reversed outlet is wired correctly, when in fact both the neutral and ground contacts are at 120 volts and the hot side is at earth potential. Please see http://www.youtube.com/watch?v=_pwCY4_LwJo&feature=youtu.be&noredirect=1  for a video I did a few weeks ago that describes how you can use a $20 non-contact AC tester in conjunction with a cheap 3-light tester to qualify grounds in power plugs. That's the only easy way to determine if an outlet will blow up your gear.

After discussing this testing issue with a few meter manufacturers, it seems that the entire industry has missed this problem. In fact, electrical inspectors routinely use a 3-light tester to qualify outlets in renovated buildings, but that's where the hot and neutral wires in the wall are most likely to be reversed.

I'm covering a lot of this on www.noshockzone.org and trying to get Lowes and Home Depot to offer training to consumers and electricians on how to check for this condition. Please contact me with any questions or comments.

Mike Sokol - mike@fitsandstarts.com
Title: Re: freak accident? help!
Post by: Al Keltz on May 03, 2012, 05:38:39 pm
Mike Sokol turned me on to this because we recently received a call about a DI's ground wire blown up when connected between a mixer and a powered speaker. The speaker was connected to an outlet that had been recently converted from 2 to 3 prong and obviously wired with a reverse polarity bootleg ground. They told me they had checked it with a 3-light outlet tester which of course said it was wired OK.

And when you have a reverse polarity bootleg ground, measuring from neutral to ground with a meter as suggested earlier will NOT show any voltage, and measuring from hot (which is really now neutral) to ground will measure 120 volts . . . except the ground is the one at 120 volts! So the meter test doesn't help and the three-light tester can't be trusted by itself if it tests an outlet as OK..

Check out Mike's video.

- Al
Title: Re: freak accident? help!
Post by: Mike Sokol on May 03, 2012, 06:43:29 pm
Mike Sokol turned me on to this because we recently received a call about a DI's ground wire blown up when connected between a mixer and a powered speaker. The speaker was connected to an outlet that had been recently converted from 2 to 3 prong and obviously wired with a reverse polarity bootleg ground. They told me they had checked it with a 3-light outlet tester which of course said it was wired OK.

And when you have a reverse polarity bootleg ground, measuring from neutral to ground with a meter as suggested earlier will NOT show any voltage, and measuring from hot (which is really now neutral) to ground will measure 120 volts . . . except the ground is the one at 120 volts! So the meter test doesn't help and the three-light tester can't be trusted by itself if it tests an outlet as OK..

Check out Mike's video.

- Al

The gold standard method for testing is to run a wire to the ground stake ouside the building, but NOBODY will do that. The easiest way is to use a $20 non-contact tester such as a Fluke VoltAlert to confirm if the ground pin is indeed at earth potential. As for a safety issue, it will indeed kill you if you plug a guitar amp into a RPBG (Reverse Polarity Bootleg Ground) and your PA system is connected to a properly grounded outlet. Your guitar amp will operate perfectly fine until you touch the microphone with your lips while your hand is on the strings. And "only" 120 volts can certainly kill you. So don't take chances.... get a non-contact AC tester and use it to confirm that your guitar amp and/or PA system aren't electrified. Here's a video of me electrifying a 40 foot RV trailer with 120 volts. http://www.youtube.com/watch?v=Y8h64X33aKg  You'll note there's no visual warning that the entire RV and tow vehicle is now at 120 volts and will kill you if you stand on the ground and touch the stairs at the same time. This could just as easily be a tour bus, so a non-contact tester will confirm if your tour bus power hookup is safe as well as your stage power hookups.

Mike Sokol
Title: Re: freak accident? help!
Post by: Jordan Wolf on May 03, 2012, 07:19:32 pm
Al, et, uh...al  ::)

I had no idea that such a thing existedÖ"reversed-polarity bootleg ground" - who woulda' thunk it?

Whether it was a hot/ground swap or the ground is bonded to the neutral at the receptacle, the ground would still be energized and could cause the issue, right?  Would there be an acceptable method to test using a multimeter as compared to visual inspection?

It would be cool if the OP could get the venue to check out what actually is the problem, especially because damage was incurred on his equipment.
Title: Re: freak accident? help!
Post by: John Hyun on May 03, 2012, 07:21:50 pm
okay thanks for the explanations.  i'm still trying to understand all this so I'll take a couple of reads before I ask any technical questions.

On the other hand, I have couple of other questions.

1) I definitely feel obligated to inform the reception hall.  Our liaison came in when she saw all the smoke and asked if everything was okay.  She didn't seem to think it was serious since nobody spoke to me afterwards.  If I'm the first to get this problem, I won't be the last.  How should I go about this?  I have a feeling it might just get brushed by.

2) I didn't get to test my gear yet (the EQ which the heated cable was connected to).  Is there any damage I should inspect?  My snake seemed to work fine, but would the heat cause any damage (other channels, the box, etc.)

3) So seeing that the channel with the EQ was the only one frying, it was probably the outlet the rack was connected to?  If that's the case, how come none of the other stuff connected heated? the mixer and rack was connected to same outlet

4) the rack has a power conditioner.  guess that doesnt help in this situation does it?

5) what exactly do I need to test?  seem's like people are posting different things.

thanks again.

i'm really glad nothing serious happened but it was sure one the scariest moments of my life.
Title: Re: freak accident? help!
Post by: Mike Sokol on May 03, 2012, 07:39:04 pm
Al, et, uh...al  ::)

I had no idea that such a thing existedÖ"reversed-polarity bootleg ground" - who woulda' thunk it?

Whether it was a hot/ground swap or the ground is bonded to the neutral at the receptacle, the ground would still be energized and could cause the issue, right?  Would there be an acceptable method to test using a multimeter as compared to visual inspection?

It would be cool if the OP could get the venue to check out what actually is the problem, especially because damage was incurred on his equipment.

A hot-ground swap will cause the 3-light outlet tester to show red, warning you that the ground is hot. But a RPBG (Reverse Polarity Bootleg Ground) will appear to be wired properly using a 3-light outlet tester. And it will meter as correctly wired using a voltmeter as well. Even a $300 GLIT (Ground Loop Impedance Tester) like I use in my proof-of-concept video will not recognize a RPBG outlet. The only way to know for sure is to reference the earth itself by using a non-contact tester which capacitively couples to the earth via your body, or run a wire to known good ground (vice grips on a cold water pipe in the basement).

I'm pitching the idea of adding the VoltAlert test to the NFPA 70E National Electrical Code. This should be performed on EVERY outlet in any facility that had strange power problems. It only takes one RPBG outlet to trash your sound system.

BTW: I have a new test I've developing that will predict ground loop induced hum without having to plug in a single piece of audio gear. More fun!!!!

Mike Sokol
Title: Re: freak accident? help!
Post by: Mike Sokol on May 03, 2012, 07:47:53 pm
okay thanks for the explanations.  i'm still trying to understand all this so I'll take a couple of reads before I ask any technical questions.

On the other hand, I have couple of other questions.

5) what exactly do I need to test?  seem's like people are posting different things.

thanks again.


Ask Al Keltz from Whirlwind to post pictures of an IMP-2 DI box and an active audio distro that were hooked up to RPBGs. We did autopsies of the gear and found that the internal grounding wires were fried and circuit board traces vaporized. Funny thing is, those grounds operated as a very expensive fuse, and the audio transformer of the IMP-2 could still pass audio even though its ground wire was melted. Also note that something like an IMP-2 can be plugged between two pieces of gear with different chassis potentials (one grounded and the other at 120 volts due to a RPBG) and operate perfectly as long as the ground-lift switch is in the lift position. If you ground the switch, the box will basically blow up right in your hands.

Interesting, isn't it?

Mike Sokol
Title: Re: freak accident? help!
Post by: John Hyun on May 03, 2012, 07:52:50 pm
Ask Al Keltz from Whirlwind to post pictures of an IMP-2 DI box and an active audio distro that were hooked up to RPBGs. We did autopsies of the gear and found that the internal grounding wires were fried and circuit board traces vaporized. Funny thing is, those grounds operated as a very expensive fuse, and the audio transformer of the IMP-2 could still pass audio even though its ground wire was melted. Also note that something like an IMP-2 can be plugged between two pieces of gear with different chassis potentials (one grounded and the other at 120 volts due to a RPBG) and operate perfectly as long as the ground-lift switch is in the lift position. If you ground the switch, the box will basically blow up right in your hands.

Interesting, isn't it?

Mike Sokol

just reading that freaks me out.  I'm gonna have trouble plugging stuff into outlets now...
Title: Re: freak accident? help!
Post by: Ivan Beaver on May 03, 2012, 08:06:10 pm
If you ground the switch, the box will basically blow up right in your hands.


I had a bass player friend of mine that this actually happened.  He was at a church and they "grabbed" power from various outlets and when he plugged in the DI to his amp-the whole thing went up in his hands-along with other wires melting on the snake and so forth.

It DOES happen in the real world-not just in the labs or in theory.
Title: Re: freak accident? help!
Post by: Guy Luckert on May 03, 2012, 08:11:18 pm
Glad no one was hurt. What would be the perfect storm for that to happen, someone being hurt by this I mean?Touching a rack rail while grounded?

whether it's a stove and a refrigerator or a guitar and a mike a "perfect storm" would be both plugged into hot grounds of opposite phase for a whopping 208 or 240 volts passing through your heart at 60 time a second U.S.A. (55 times a second at higher voltage abroad)

hot sticks are a good idea
now I'm gonna watch the video
Title: Re: freak accident? help!
Post by: Mike Sokol on May 03, 2012, 08:24:01 pm
whether it's a stove and a refrigerator or a guitar and a mike a "perfect storm" would be both plugged into hot grounds of opposite phase for a whopping 208 or 240 volts passing through your heart at 60 time a second U.S.A. (55 times a second at higher voltage abroad)

hot sticks are a good idea
now I'm gonna watch the video
Yes, possible... but pretty unlikely. However, that would make for a VERY serious meltdown if it did occur on a live gig. I could easily mock that up on my test bench for a "really beeg shew"....
Title: Re: freak accident? help!
Post by: Mike Sokol on May 03, 2012, 08:37:25 pm
Here's a picture of the IMP-2 that Al Keltz from Whirlwind sent me. Note the blown "fuse" which was actually the grounding wire connected between the chassis and pin-1 on the XLR via the ground lift switch. We're both 99% sure it was caused by a powered monitor wedge plugged into a RPBG stage outlet. The sound guy actually tried this TWICE after burning up the first box and monitor using a second active monitor wedge new out of the box. So they blew up two active monitors and a small digital board totaling around $6K I guess. Fluke Voltalert would have been a lot cheaper.... For those who missed it earlier in the thread, here's the video of me testing for RPBGs (Reverse Polarity Bootleg Grounds). http://www.youtube.com/watch?v=_pwCY4_LwJo&feature=youtu.be&noredirect=1

FYI: I have an entire test rig I built that lets me introduce up to 40 amps ground loop current and 120 volts AC electrification into any mixer, amp, speaker, mic or refrigerator. Al Keltz has seen me do this demonstration at the Whirlwind office in Rochester, and it's really cool to watch all the engineers step back from the table when I crank a mixer and microphone up to 120 volts bias, and there's no sparks, hums, or buzzes. But the VoltAlert will beep and light up from 4 to 6 inches away from the mic.

Mike Sokol
Title: Re: freak accident? help!
Post by: John Roberts {JR} on May 03, 2012, 09:46:45 pm
If we're done scaring each other, to offer some practical advice, you need a reliable external ground reference to determine which outlets are trying to kill you, BUT you can safely make it through 99.9% of your gigs, plugging into random outlets that are available to you, just by confirming that the outlets all agree with each other.  i.e. if there is no significant voltage between the sundry grounds, they are probably all OK, and if they are all bad the same way it's pretty much relative so less of a threat.

A voltmeter and a very long extension cord can generally allow you to confirm that you don't have any killer outlets lurking around. If you can plug in close to the electrical panel, that is likely to be properly grounded..

JR
Title: Re: freak accident? help!
Post by: Mike Sokol on May 03, 2012, 10:30:58 pm
If we're done scaring each other, to offer some practical advice, you need a reliable external ground reference to determine which outlets are trying to kill you, BUT you can safely make it through 99.9% of your gigs, plugging into random outlets that are available to you, just by confirming that the outlets all agree with each other.  i.e. if there is no significant voltage between the sundry grounds, they are probably all OK, and if they are all bad the same way it's pretty much relative so less of a threat.

A voltmeter and a very long extension cord can generally allow you to confirm that you don't have any killer outlets lurking around. If you can plug in close to the electrical panel, that is likely to be properly grounded..

JR
I've actually built that exact rig you describe, but its best use is detecting ground loop differential voltages. Be aware that your test could in fact protect the gear, but kill the artist or engineer who touches a mic or guitar and a real ground such as a metal rail at the same time.

I've used a Fluke VoltAlert on hundreds of ground testing situations both real-world and test-bed, and find that it's close to 100% accurate at finding elevated voltages on sound gear. Even though a Fluke VoltAlert is rated for 90 to 1,000 volts, it will reliably trigger on as little as 40 volts AC. Since any "hot" ground is likely to be at a full 120 volts, you can detect a dangerous ground voltage from inches to a few feet away from electrified gear. It's as simple as pointing a non-contact AC tester in front of the outlet to know if the ground is energized, and using a standard voltmeter to make sure it's really 120 volts. I simply walk around the stage an point a VoltAlert at every mic and guitar amp on stage, and in a few minutes KNOW that the stage is safe from shock. Using a GLIT (Ground Loop Impedance Tester) such as the Amprobe INSP-3 is the final test that will confirm if the ground will actually carry sufficient current to trip a 20 amp circuit breaker if things short out to the chassis.

Since I built a test rig that will let me create these hot-ground and ground-loop situations at will, both on the bench and in the field, I've done a lot of experimenting with the best ways to find and eliminate ground loop hums and detect hot chassis situations. I would like the opportunity to demonstrate how this works to the pro-audio industry, and will turn over my design data and field notes to any manufacturer who wants to replicate my experiments for peer review.

Another thing I've been promoting in my ASSIST sound installer classes is that venues that will be installing new sound systems should have an assay done on their entire electrical system to determine if there's any hot or open grounds as well as limit ground loop differential voltage due to double-bonded G-N connections in sub panels. Of course, a standard electrician or inspector doesn't know enough about grounds to detect a RPBG or Ground Loop differential, so they need to be educated on sound system grounding. But once I show them how it works, they understand completely.

Mike Sokol
Title: Re: freak accident? help!
Post by: John Hyun on May 03, 2012, 10:39:17 pm
thanks Mike for the info.  they do help.

but I still have some unanswered questions a few posts back.  if anyone can chime in, please advise!

thanks.
Title: Re: freak accident? help!
Post by: Samuel Rees on May 03, 2012, 10:41:43 pm
The obviously logical and responsible solution is to get a magnetic implant in your hand so you can "sense" live gear. Haha.
Title: Re: freak accident? help!
Post by: Mike Sokol on May 03, 2012, 10:58:04 pm
thanks Mike for the info.  they do help.

but I still have some unanswered questions a few posts back.  if anyone can chime in, please advise!

thanks.

To answer your questions:

Quote
1) I definitely feel obligated to inform the reception hall.  Our liaison came in when she saw all the smoke and asked if everything was okay.  She didn't seem to think it was serious since nobody spoke to me afterwards.  If I'm the first to get this problem, I won't be the last.  How should I go about this?  I have a feeling it might just get brushed by.

Yes, I would ask the hall to contact an electrician to verify grounds. And show him my video on RPBGs and have them run a test wire from a real ground such as the main box. Not to do so could cause some future sound tech to face a similar or worse situation. 

Quote
2) I didn't get to test my gear yet (the EQ which the heated cable was connected to).  Is there any damage I should inspect?  My snake seemed to work fine, but would the heat cause any damage (other channels, the box, etc.)

You need to open up everything that was connected to another piece of gear and visually inspect the ground wires for signs of melting. Note that you could blow the ground wires off of gear with audio transformer outputs which many not show up until you hook it up to something else.

Quote
3) So seeing that the channel with the EQ was the only one frying, it was probably the outlet the rack was connected to?  If that's the case, how come none of the other stuff connected heated? the mixer and rack was connected to same outlet.

If you're lucky the damage will be confined to a single current path as you describe.

Quote
4) the rack has a power conditioner.  guess that doesnt help in this situation does it?

Nope, they don't do squat to protect you from RPBG outlets. There's no way for them to know the ground is hot if the neutral is also hot.

Quote
5) what exactly do I need to test?  seem's like people are posting different things.
I would open up and visually inspect everything you can for blown traces and wires, and try out each piece of gear separately. You can certainly damage transistors and IC's as well, so the damage may not be confined to just burnt grounds.

Hope this helps.... Mike
Title: Re: freak accident? help!
Post by: John Roberts {JR} on May 03, 2012, 11:34:00 pm
I've actually built that exact rig you describe, but its best use is detecting ground loop differential voltages. Be aware that your test could in fact protect the gear, but kill the artist or engineer who touches a mic or guitar and a real ground such as a metal rail at the same time.

I've used a Fluke VoltAlert on hundreds of ground testing situations both real-world and test-bed, and find that it's close to 100% accurate at finding elevated voltages on sound gear. Even though a Fluke VoltAlert is rated for 90 to 1,000 volts, it will reliably trigger on as little as 40 volts AC. Since any "hot" ground is likely to be at a full 120 volts, you can detect a dangerous ground voltage from inches to a few feet away from electrified gear. It's as simple as pointing a non-contact AC tester in front of the outlet to know if the ground is energized, and using a standard voltmeter to make sure it's really 120 volts. I simply walk around the stage an point a VoltAlert at every mic and guitar amp on stage, and in a few minutes KNOW that the stage is safe from shock. Using a GLIT (Ground Loop Impedance Tester) such as the Amprobe INSP-3 is the final test that will confirm if the ground will actually carry sufficient current to trip a 20 amp circuit breaker if things short out to the chassis.

Mike Sokol

Going into my way-back machine I vaguely recall a device based on old neon lamps that IIRC took 90V to light. But the one I recall had two metal probes so required direct conduction.

That Fluke Volt alert(tm) sounds like a useful tool.

If you have a metal superstructure... measure it...  This isn't rocket science.  I fear the trusty plug in receptacle tester is giving people a false sense of security.

JR


 

Title: Re: freak accident? help!
Post by: Rob Spence on May 04, 2012, 12:00:42 am
I don't have anything to add to the safety discussion but I do have a question for the OP on his configuration.

Why is there an EQ wired into the output of the DSP?
Title: Re: freak accident? help!
Post by: Mike Sokol on May 04, 2012, 07:46:30 am
Going into my way-back machine I vaguely recall a device based on old neon lamps that IIRC took 90V to light. But the one I recall had two metal probes so required direct conduction.

That Fluke Volt alert(tm) sounds like a useful tool.

If you have a metal superstructure... measure it...  This isn't rocket science.  I fear the trusty plug in receptacle tester is giving people a false sense of security.

JR
You are correct.... a 3-light receptacle tester misses a lot of faults, and can indicate the outlet is wired correctly, when in fact it could be wired with a hot ground which can cause equipment damage and electrocution. If your mixing console or stage amps are plugged into a hot-ground, a non-contact tester will beep at you if you point it at a mic, amp, or metal surface. It will even beep if you point it at the outside of any XLR cable connected to the gear. The larger the charged surface, the further away the VoltAlert will detect. So a microphone at 120 volts will detect from 4 inches away, while a mixing console at 120 volts with a few square feet of area will detect at 12 inches away. An RV or tour bus at 120 volts will detect at perhaps 2 to 3 feet away.   

See http://www.youtube.com/watch?v=jF3Ntoa8ab8 for my video on how to use a VoltAlert to test outlet polarity.

See http://www.youtube.com/watch?v=obeh9m4OMv4 for a Q&A on the VoltAlert itself.

If you have any questions on how this works, my email is mike@fitsandstarts.com

Mike Sokol
Title: Re: freak accident? help!
Post by: Jay Barracato on May 04, 2012, 07:58:19 am
You are correct.... a 3-light receptacle tester misses a lot of faults, and can indicate the outlet is wired correctly, when in fact it could be wired with a hot ground which can cause equipment damage and electrocution. If your mixing console or stage amps are plugged into a hot-ground, a non-contact tester will beep at you if you point it at a mic, amp, or metal surface. It will even beep if you point it at the outside of any XLR cable connected to the gear. The larger the charged surface, the further away the VoltAlert will detect. So a microphone at 120 volts will detect from 4 inches away, while a mixing console at 120 volts with a few square feet of area will detect at 12 inches away. An RV or tour bus at 120 volts will detect at perhaps 2 to 3 feet away.   

See http://www.youtube.com/watch?v=jF3Ntoa8ab8 for my video on how to use a VoltAlert to test outlet polarity.

See http://www.youtube.com/watch?v=obeh9m4OMv4 for a Q&A on the VoltAlert itself.

If you have any questions on how this works, my email is mike@fitsandstarts.com

Mike Sokol

Mike,

Just to confirm, you are talking about the $25 gadget shaped like a pen? At that price, why wouldn't everyone carry one? I will be adding it to the tool box this weekend.
Title: Re: freak accident? help!
Post by: Mike Sokol on May 04, 2012, 08:27:11 am
Mike,

Just to confirm, you are talking about the $25 gadget shaped like a pen? At that price, why wouldn't everyone carry one? I will be adding it to the tool box this weekend.
Yup, here's the model I use in all my tests and demonstrations:

Fluke 1AC A1 II   ACV Detector 90-1000V, ENGL/LASPN/CFR Flat-tip

You can get one at Sears for $27 http://www.sears.com/shc/s/p_10153_12605_03470489000P and it's also available on Amazon for a few bucks less (but you pay shipping). http://www.amazon.com/dp/B000EJ332O/ref=asc_df_B000EJ332O2002345?smid=A1CTV5F7AC4RR1&tag=dealtmp3938-20&linkCode=asn&creative=395105&creativeASIN=B000EJ332O

Make sure you get the standard 90 to 1,000 volt model (Fluke 1AC A1 II) which will detect hot grounded chassis down to 40 volts. Fluke also makes a 24-volt model for alarm and machine control testing, but it's too sensitive to differentiate between hot and neutral contacts on an outlet. Basically, the low-voltage version will beep on ANYTHING, so get the standard 90-1,000 volt model.

And yes, I think that everyone who plugs their expensive sound gear into unknown outlets every week should keep one in their tool bag. It takes literally a few seconds to confirm that the grounds are not electrified and safe to plug in. It will also let you know if a guitar player has broken off the ground pin on his power cord which will electrify his stage amp and the strings on his guitar as well. Hey, I don't want musicians telling me they got shocked from one of my mics, when they really got shocked from their own guitar....

Mike
Title: Re: freak accident? help!
Post by: John Halliburton on May 04, 2012, 08:38:13 am
The gold standard method for testing is to run a wire to the ground stake ouside the building, but NOBODY will do that. The easiest way is to use a $20 non-contact tester such as a Fluke VoltAlert to confirm if the ground pin is indeed at earth potential. As for a safety issue, it will indeed kill you if you plug a guitar amp into a RPBG (Reverse Polarity Bootleg Ground) and your PA system is connected to a properly grounded outlet.

Mike,

Looks like Fluke has a new Volt Alert in the family:

http://www.fluke.com/Fluke/usen/Electrical-Testers/Electrical-Testers/2AC.htm?PID=70668

Appears to be designed to be always on.

Best regards,

John
Title: Re: freak accident? help!
Post by: John Hyun on May 04, 2012, 08:44:00 am
I don't have anything to add to the safety discussion but I do have a question for the OP on his configuration.

Why is there an EQ wired into the output of the DSP?

hey rob, where's the DSP?  I don't have a dsp anywhere.  maybe you read something wrong, or I wrote up something wrong/vaguely?
Title: Re: freak accident? help!
Post by: John Hyun on May 04, 2012, 08:54:34 am
hey mike, thanks again for you input and the links.

when you wrote "If you're lucky the damage will be confined to a single current path as you describe", is there a scientific explanation why it would be confined to a single current path? I'm just really perplexed why the other cables didnt burn up.  Also, I have no experience opening up gear, expect poweramps to dust them.  I did have the system back up (at the time I had no idea what was wrong, or the potential damage I may have incurred, but I had no backup system and the event was starting in 30 minutes) and it was running smoothly for the next 3-4 hours including high levels just before clipping for dance music for 2 hours.  A visual inspection should still be performed correct?  Even though it may be passing signal, something may have happened inside.

Just another question...what would happen if a plug without a ground is plugged into an RPBG?  is that the example what you wrote previously about the electric guitarist's amp?

sorry if i'm repeating stuff, but I'm trying to learn and make sure I understand everything!

thanks again.
Title: Re: freak accident? help!
Post by: Mike Sokol on May 04, 2012, 09:08:51 am
Mike,

Looks like Fluke has a new Volt Alert in the family:

http://www.fluke.com/Fluke/usen/Electrical-Testers/Electrical-Testers/2AC.htm?PID=70668

Appears to be designed to be always on.

Best regards,

John
That could work. I'll get Fluke to send me a unit for testing. One of the biggest errors when using test gear is simply not turning it on. So, for instance you could have a dead battery in a VoltAlert and the fact that you get no beep or light could fool you into thinking a circuit was inactive. But I always confirm the tester is working by plugging it into a Hot contact to begin with to make sure it beeps, then checking the ground for no beeping. That way a failed tester won't give you a false sense of security.

Of course, something that's always on gets around the legal problems of consumers who are too stupid to hit the on switch. But I've got to assume all sound guys know how to turn on a switch, correct? 
Title: Re: freak accident? help!
Post by: Mike Sokol on May 04, 2012, 09:28:16 am
hey mike, thanks again for you input and the links.

when you wrote "If you're lucky the damage will be confined to a single current path as you describe", is there a scientific explanation why it would be confined to a single current path? I'm just really perplexed why the other cables didnt burn up.  Also, I have no experience opening up gear, expect poweramps to dust them.  I did have the system back up (at the time I had no idea what was wrong, or the potential damage I may have incurred, but I had no backup system and the event was starting in 30 minutes) and it was running smoothly for the next 3-4 hours including high levels just before clipping for dance music for 2 hours.  A visual inspection should still be performed correct?  Even though it may be passing signal, something may have happened inside.

Just another question...what would happen if a plug without a ground is plugged into an RPBG?  is that the example what you wrote previously about the electric guitarist's amp?

sorry if i'm repeating stuff, but I'm trying to learn and make sure I understand everything!

thanks again.
If you're not comfortable opening up gear, get somebody who is to help. But typically over-current ground damage is quite visible. See attached for an example. Also, smell is a good indicator. So if you sniff around your gear you can usually smell something "burnt", which means you need to open it up and inspect. Be sure to unplug power before opening any panels since you don't want to do more damage or get shocked.

And yes, even if the gear is still passing signal, you can still have damage. What can happen with transformer output gear is that when the ground trace vaporizes due to high current, the ground is then "lifted" and will work in that circuit. But now you have a piece of gear that may not work properly if it's feed something that NEEDS to be grounded. I would at least do a visual inspection of the circuit boards and test for operation. Probably only the gear in the direct current path was affected, so that's where to start looking. And anything that heated up or sparked is the prime suspect.

Mike
Title: Re: freak accident? help!
Post by: John Roberts {JR} on May 04, 2012, 09:30:24 am
hey mike, thanks again for you input and the links.

when you wrote "If you're lucky the damage will be confined to a single current path as you describe", is there a scientific explanation why it would be confined to a single current path? I'm just really perplexed why the other cables didnt burn up.

I'm no Mike, but unless the wiring mistake occurred in the fuse box and I'm not sure if that is even possible, the hot ground will still be fused at something like 15a trip current, so the fault will generate heat and chaos until the fuse gives up it's calibrated smoke.

Quote
Also, I have no experience opening up gear, expect poweramps to dust them.  I did have the system back up (at the time I had no idea what was wrong, or the potential damage I may have incurred, but I had no backup system and the event was starting in 30 minutes) and it was running smoothly for the next 3-4 hours including high levels just before clipping for dance music for 2 hours.  A visual inspection should still be performed correct?  Even though it may be passing signal, something may have happened inside.
Generally a ground to ground fault path like that will not damage the gear while it could open a puny ground lead. 

Safety grounds inside gear that connect to labelled panel grounds must pass a UL ground bonding test, where it must survive a few tens of amps with only a few volts of IxR drop. This insures that no meat puppets will get exposed to dangerous fault voltage and the service fuse or circuit breaker will draw enough current to trip. 

I recall having to redesign one small fixed install amp, where this ground fault test, vaporized a too small PCB ground trace.
Quote
Just another question...what would happen if a plug without a ground is plugged into an RPBG?  is that the example what you wrote previously about the electric guitarist's amp?
Probably not exactly...  Consumer gear with 2 wire line cords is double insulated so still safe, while it may make inferences about hot and neutral that could affect noise floor.

I suspect Mike is referring to the old guitar amp practice of not grounding the guitar amp chassis to safety ground but instead to cap couple it to one side or the other of the mains for lowest hum level.  This was just a bad idea and is no longer done. This arrangement was not generally life threatening, but could give musos a decent shock between a guitar chassis cap coupled to hot and a grounded mic.

Quote
sorry if i'm repeating stuff, but I'm trying to learn and make sure I understand everything!

thanks again.

This is important stuff to understand, so keep asking until you get it. 

JR
Title: Re: freak accident? help!
Post by: Bob Charest on May 04, 2012, 11:01:15 am
Mike,

Just to confirm, you are talking about the $25 gadget shaped like a pen? At that price, why wouldn't everyone carry one? I will be adding it to the tool box this weekend.
I just did... Thanks Mike!
Title: Re: freak accident? help!
Post by: Mike Sokol on May 04, 2012, 03:36:08 pm
I just did... Thanks Mike!

You're most welcome.

Please post pictures of any blown up gear or hot-ground outlets you all find. I'm starting a collection....

Mike
Title: Re: freak accident? help!
Post by: Guy Luckert on May 04, 2012, 07:15:17 pm

You're most welcome.

Please post pictures of any blown up gear or hot-ground outlets you all find. I'm starting a collection....

Mike

I'll send you a pic of a Kustom guitar amp with a factory 3 prong plug that when plugged in a correctly wired outlet has a hot chassis !!
Title: Re: freak accident? help!
Post by: Mike Sokol on May 04, 2012, 09:22:54 pm
I'll send you a pic of a Kustom guitar amp with a factory 3 prong plug that when plugged in a correctly wired outlet has a hot chassis !!
I gotta see this!!!! Yes please send pics. The only simple explanation would be the ground is disconnected inside the amp, and the power transformer is leaking to the chassis. That would do it.... Have you metered the ground pin on the power plug to the chassis for resistance (while unplugged)? Should be very close to zero ohms. If it's open (hi-z) that would splain it....
Title: Re: freak accident? help!
Post by: Mike Sokol on May 07, 2012, 01:34:21 pm
I just did a consulting gig at a medium sized church in New Jersey, and EVERY outlet I looked at was bootleg grounded, but none that I tested was reversed polarity. Of course, bootleg grounded outlets by themselves are not dangerous to humans or sound gear, but there was ground loop hum galore that was modulated by the current draw of the amps. The sound techs in the church had attended one of my training classes last year, so they knew better than to cut off ground pins on the extension cords. And they had put in a few HumX ground lifts which cut the hum considerably, but I don't think there was a single properly grounded outlet in the entire place. Yikes!!!!!

Mike
Title: Re: freak accident? help!
Post by: Paul Dershem on May 07, 2012, 03:49:32 pm
Thanks for the info, Mike.

After reading your posts, I started researching non-contact units like the one you recommend. One article said some are powered by small disk batteries (like watch or hearing aid batteries), while others are powered by AAA cells, which can be purchased just about anywhere. Pragmatism draws me to AAA batteries.

Sadly, it's hard to determine from websites like Amazon what kind of batteries a specific unit takes. Can you shed any light on this? How often do yo have to replace batteries on units that are always on vs. units that have to be switched on?
Title: freak accident? help!
Post by: Samuel Rees on May 07, 2012, 04:28:33 pm
This has been a great thread. I learned a good bit about a serious safety issue that I was apparently ignorant of previously, and I'm going to pick up the recommended gear to test with ASAP.

OP - Thanks for the post, glad no one was hurt.
Title: Re: freak accident? help!
Post by: Chris Hindle on May 07, 2012, 04:47:39 pm

Sadly, it's hard to determine from websites like Amazon what kind of batteries a specific unit takes. Can you shed any light on this? How often do yo have to replace batteries on units that are always on vs. units that have to be switched on?
I have had the Fluke <no tone> model for 10 or 12 years now. It is always on, and the AA (or AAA - I forget) lasts me 4 or 5 years. I probably use it at least a dozen times each show, and whenever I am screwing with the house or office electrical.
A little tap on the sensor prong makes it blink - easy test before each use
Title: Re: freak accident? help!
Post by: Mike Sokol on May 07, 2012, 04:56:57 pm
I have had the Fluke <no tone> model for 10 or 12 years now. It is always on, and the AA (or AAA - I forget) lasts me 4 or 5 years. I probably use it at least a dozen times each show, and whenever I am screwing with the house or office electrical.
A little tap on the sensor prong makes it blink - easy test before each use
The Fluke VoltAlert takes AAA batteries which, of course, are available anywhere. I think that most any brand non-contact tester made by Amprobe, Sperry, Triplett, and others... would work for detecting a RPBG. You DO NOT want a low-voltage non-contact tester (24 volts), get one listed for 80  or 90 volts to 600 or 1,000 volts.. A low voltage tester will blink and beep anywhere NEAR a powered outlet, when you want a tester that will allow you to differentiate between the hot and neutral contacts in a wall outlet.

Mike

 
Title: Re: freak accident? help!
Post by: Paul Dershem on May 07, 2012, 05:46:23 pm
Thank you, gentlemen.
Title: Re: freak accident? help!
Post by: Mike Sokol on May 07, 2012, 06:18:15 pm
Thank you, gentlemen.
I'm glad this info is helpful... but realize that most of the pro-audio and electrical test gear manufacturers aren't aware of how dangerous RPBG outlets can be, or that their own test gear won't identify the condition. 

So to promote this issue to the various manufacturers and agencies I need pictures of any examples you've come across. If you have pictures of any burned up audio gear or hot electrical outlet grounds, please post them here or email them to me directly at mike@fitsandstarts.com   

Thanks.... Mike
Title: Re: freak accident? help!
Post by: John Roberts {JR} on May 07, 2012, 07:02:46 pm
I'm glad this info is helpful... but realize that most of the pro-audio and electrical test gear manufacturers aren't aware of how dangerous RPBG outlets can be, or that their own test gear won't identify the condition. 

So to promote this issue to the various manufacturers and agencies I need pictures of any examples you've come across. If you have pictures of any burned up audio gear or hot electrical outlet grounds, please post them here or email them to me directly at mike@fitsandstarts.com   

Thanks.... Mike

I wouldn't say this problem is exactly unknown to manufacturers and safety agencies...

As i posted UL has a ground bonding test that confirms properly designed "safety" grounds will not vaporize or allow dangerous voltage rise while passing several tens of amps, long enough to trip the primary breaker.

There is a gray area perhaps regarding non-"safety" grounds, like your sundry examples that inadvertently got in the primary current path and did not hold the current as is UL's plan for safe shutdown of the fault.  IIRC there is some fine print in the UL regulations that qualify bonding only for external connections labelled as "Ground".  As i recall my in house guy told me I could relabel my barrier strip ground screw as 0V instead of ground and wouldn't need to pass the ground bond test, but i redesigned the board to pass because it was the right thing to do (damn I can't imagine not doing that). 

I am not suggesting that all these companies are skirting the safety regulations, because XLR pin 1 is not formerly labelled as a "safety ground", and many legacy designs (and probably some current ones) wouldn't pass the safety bond test. Maybe pin 1 should be formally labelled "ground" and safety tested... ? 

For awareness, how about being sued, when a customer got killed between one rouge power outlet and one correct one, while using two very UL ground correct guitar amps. UL is aware of this also since they were right there in court too.

I agree this is poorly understood by many and you need to keep raising a stink... but this isn't exactly a secret...

JR
Title: Re: freak accident? help!
Post by: Mike Sokol on May 07, 2012, 07:28:00 pm

I agree this is poorly understood by many and you need to keep raising a stink... but this isn't exactly a secret...

JR
I agree it's not exactly a secret. But testing for raised potential on grounds isn't known by most electricians and inspectors. The standard check by inspectors is still the 3-light testers, and as you can see from the beginning of this thread most everybody will assume that a 3-light tester or metering between H-N-G will guarantee a safe electrical outlet, when in fact it will miss this dangerous condition. I've also raised the idea of hot grounds with the RVIA and RVDA (Recreational Vehicle manufacturers associations) and they didn't want to promote hot-skin testing of RVs since it would "scare" consumers into not purchasing an RV or trailer.  Perhaps there's a little of this with audio manufacturers as well, since their own equipment is not at fault, but plugging into a mis-wired outlet can certainly wreak havoc.

Another really interesting tangent to the idea of RPBG is that lightning strikes on buildings can burn out the exterior grounding rod. In one case the ground rod fused the sandy soil into a ground rod Popsicle made of glass, essentially insulating it from the earth. I found this out when the facility noted that the lighting board operator could feel tingles in his fingertips that matched the lightning flashes outside. The ground-neutral bond in the main panel was floating above the earth by 40 volts or so, but when a lightning cloud went overhead it would pulse to a few hundred volts. The entire building's electrical system had lost its ground reference, which of course is what the ground rod is supposed to supply in the first place. From my conversations with several power station operators, there is no code requirement for a building ground to be retested after a lightning strike. So the first strike can take out the ground rod connection, leaving everything inside the building susceptible to side flashes from the next near lightning strike.

The point is that while the majority of building electrical systems are safe, there are a few of them with incorrect wiring that never seems to get fixed. That's largely due to the fact that when a piece of gear blows up, very few people perform a system test to REALLY see what happened. They mostly assume they did something themselves to get shocked or blow up gear. And that's often not the case.

Mike
Title: Re: freak accident? help!
Post by: Kellen Tyburski on May 07, 2012, 08:08:58 pm

Just to confirm, you are talking about the $25 gadget shaped like a pen? At that price, why wouldn't everyone carry one? I will be adding it to the tool box this weekend.

I have one of these (and the GFCI tester too) and I will def be carrying them with me on gigs from now on. Just so I am clear, I can quickly test for the RPBG outlet by just putting the tip of my Fluke Voltalert into the ground socket to test if its electrified?

This is a very cool thread, I will definitely be reading and re-reading it and I'm sure it will be the basis for further research...
Title: Re: freak accident? help!
Post by: Mike Sokol on May 07, 2012, 08:22:05 pm
I have one of these (and the GFCI tester too) and I will def be carrying them with me on gigs from now on. Just so I am clear, I can quickly test for the RPBG outlet by just putting the tip of my Fluke Voltalert into the ground socket to test if its electrified?

This is a very cool thread, I will definitely be reading and re-reading it and I'm sure it will be the basis for further research...
Yes, if you jab the plastic tip of the tester into the ground contact on the outlet, there should NOT be a beep/light. If it lights up, then you have a hot ground for some reason. In fact if you get anywhere near the front of a hot-ground outlet, the tester will light up an inch or more away. See http://www.youtube.com/watch?v=_pwCY4_LwJo&feature=youtu.be&noredirect=1 for the proof on concept video I did a few weeks ago.

This assumes you're using a standard sensitivity 90-1,000 volt tester, not a low-voltage (high sensitivity) tester.   
Title: freak accident? help!
Post by: Rob Spence on May 07, 2012, 08:34:18 pm
To the OP
I was referring to the unity processor ( not digital ? ).


Sent from my iPhone using Tapatalk
Title: Re: freak accident? help!
Post by: Vic Snyder on May 07, 2012, 08:42:19 pm
First let me say that I have little experience with professional sound equipment other than some repairs to friends electronic equipment and providing power to a church tent carnival (and thus sound equipment) over too many years decades. You can blame Mike Sokol for making me aware of this forum even existing.

Working with electrical power distribution and instrumentation over the years I have fought the "No ground, no problem just tie the ground to neutral" dangerous misconception. The theory that the neutral and the ground are bonded back at the panel doesn't have any value. It is a dangerous practice to tie the neutral to the ground connection at a receptacle or any other place other than the main panel.

Even without the apparently common (from experience and what I have read here) practice of supplying equipment from multiple receptacle sources, the neutral/ground receptacle bond can set up a very unsafe condition. If the neutral path back to the panel is lost (corroded,, burned off, otherwise open) then the 120 volt supply to the equipment will be looking for a path to the ground. With the improper receptacle neutral/ground bond established the quickest path in grounded drill motors or other appliances  is via that grounded equipment case, through your body to ground. Not good.

Anyway, as Mike points out, there isn't a commercially available tester which I'm aware of to highlight the receptacle neutral/ground bond problem. Fortunately there is a relatively cheap solution for building owners should it be found that 3 prong receptacles have been installed with neutral/ground receptacle bonding on 2 wire (non-ground green/bare wire) systems. It is allowed by the National Electric Code to install GFI receptacles on a 2 wire system to provide 3 prong receptacles on old ungrounded systems. GFI's need no ground to do the job of monitoring current imbalance. Commercial packs of GFI receptacles are generally pretty reasonable in cost these days at big box stores.

That said, I relalize that although a GFI may be safe on a 2 wire ungrounded system, it may set up problems with hum in some of your audio equipment.

Sorry to butt in uninvited. vic Snyder

I agree it's not exactly a secret. But testing for raised potential on grounds isn't known by most electricians and inspectors. The standard check by inspectors is still the 3-light testers, and as you can see from the beginning of this thread most everybody will assume that a 3-light tester or metering between H-N-G will guarantee a safe electrical outlet, when in fact it will miss this dangerous condition. I've also raised the idea of hot grounds with the RVIA and RVDA (Recreational Vehicle manufacturers associations) and they didn't want to promote hot-skin testing of RVs since it would "scare" consumers into not purchasing an RV or trailer.  Perhaps there's a little of this with audio manufacturers as well, since their own equipment is not at fault, but plugging into a mis-wired outlet can certainly wreak havoc.

Another really interesting tangent to the idea of RPBG is that lightning strikes on buildings can burn out the exterior grounding rod. In one case the ground rod fused the sandy soil into a ground rod Popsicle made of glass, essentially insulating it from the earth. I found this out when the facility noted that the lighting board operator could feel tingles in his fingertips that matched the lightning flashes outside. The ground-neutral bond in the main panel was floating above the earth by 40 volts or so, but when a lightning cloud went overhead it would pulse to a few hundred volts. The entire building's electrical system had lost its ground reference, which of course is what the ground rod is supposed to supply in the first place. From my conversations with several power station operators, there is no code requirement for a building ground to be retested after a lightning strike. So the first strike can take out the ground rod connection, leaving everything inside the building susceptible to side flashes from the next near lightning strike.

The point is that while the majority of building electrical systems are safe, there are a few of them with incorrect wiring that never seems to get fixed. That's largely due to the fact that when a piece of gear blows up, very few people perform a system test to REALLY see what happened. They mostly assume they did something themselves to get shocked or blow up gear. And that's simply not the case.

Mike
Title: Re: freak accident? help!
Post by: Mac Kerr on May 07, 2012, 09:37:07 pm
First let me say that I have little experience with professional sound equipment other than some repairs to friends electronic equipment and providing power to a church tent carnival (and thus sound equipment) over too many years decades. You can blame Mike Sokol for making me aware of this forum even existing.

Please go to your profile and change the "Name" field to your real first and last name as required by the posting rules clearly displayed in the header at the top of the section, and in the Site Rules and Suggestions (http://forums.prosoundweb.com/index.php/board,36.0.html) in the Forum Announcements section, and on the registration page when you registered.

Mac
Title: Re: freak accident? help!
Post by: Mike Sokol on May 07, 2012, 10:02:57 pm

That said, I relalize that although a GFI may be safe on a 2 wire ungrounded system, it may set up problems with hum in some of your audio equipment.

Vic, good to see you here. And yes, sound systems don't do very well without at least one solid ground point. Too much risk of buzzes and stray RF pickup.
 
The other problem is that older sound gear tends to have a lot of leakage to ground via power transformer insulation breaking down from years of running hot and being vibrated on the truck. I think that's one of the reasons that GFCI's are not very well accepted in the pro-audio industry. We can't take the chance of a GFCI tripping on a small leakage to ground since that would shut down the show. I know guitar players who would rather accept the risk of getting shocked to having their amp shut down.

Also, GFCI's will most likely trip in sound systems due to ground loop currents between remote pieces of gear. I need to set up an experiment to prove that hypothesis, but I do know from experimentation that ground loop currents in sound systems are often on the order of several amperes of current, which will certainly trip a GFCI that's set for 6 ma.

That being said, your comment on GFCI's being installed in ungrounded residence systems is spot on. However, I would guess that GFCI's on stage would be swapped out for standard, non-GFCI outlets after tripping once or twice during a show. 

But certainly, educating ourselves about what can go wrong with electrical systems in general is a good thing. Your input is greatly appreciated.

Mike
Title: Re: freak accident? help!
Post by: Mike Sokol on May 08, 2012, 08:22:56 am
That being said, your comment on GFCI's being installed in ungrounded residence systems is spot on. However, I would guess that GFCI's on stage would be swapped out for standard, non-GFCI outlets after tripping once or twice during a show. 

I believe that another reason GFCI's trip in pro-sound applications, is that we often cascade power strips with surge protectors. First, take a look at how a GFCI (Ground Fault Circuit Interruptor) works: http://www.noshockzone.org/rv-electrical-safety-part-viii-gfci/

A typical consumer GFCI is set to trip around 5ma to 6ma, which is a small enough current to protect humans from electrocution. However, surge strips often use MOV devices (Metal Oxide Varistors) to limit voltage spikes. And those MOV devices will leak up to 4ma current to the ground. So plugging a single surge strip into a GFCI protected outlet won't cause it to trip, but plugging TWO surge strips into a GFCI outlet will cause those leakage currents to add up to more then the 6ma trip setting. There are high-threshold GFCI's for industrial and mining applications that trip around 30ma, if memory serves, but I don't think they're available for residential applications since you could still be electrocuted by 30ma shocking current.

I'm going to contact Furman for clarification on their rack-mount power distros to see if they can shed on any light on using them on GFCI outlets. 

Mike
Title: Re: freak accident? help!
Post by: John Roberts {JR} on May 08, 2012, 10:03:36 am
The lightning turning sand into a glass insulator is wild... back in the good old days there were plenty of steel cold water pipes running underground to tie to, but nowadays so much plumbing is PVC so that robust ground is no longer commonly available.

Keep up the good fight...

JR
Title: Re: freak accident? help!
Post by: J. Taylor Webb on May 08, 2012, 10:15:15 am
Hi Victor, and welcome;

You reinforce a very good point,
Having experience in both electrical and sound, I've seen this kind of thing before.
I have to admit, I haven't given too much thought to the 'RPBG' situation as Mike has described it, however, it's not only potentially unsafe, it is A VIOLATION of the NEC( now IEC) to connect a neutral to ground anywhere other than the 'first means of disconnect', or the main panel.
The whole idea is to have redundant paths for the neutral and ground from point of use or
IE- outlet, to the panel. Redundant paths being two wires running the entire distance.
(There is an allowance for using rigid(as opposed to flex)conduit as a ground, if it meets certain criteria.)
This is commonly referred to as a 'three wire circuit', to indicate the presence of three separate   
conductors, - hot, neutral, and ground, none of which connect to each other.
Bonding the ground prong on an outlet, to neutral anywhere is a clear code violation( though that doesn't mean it doesn't happen, of course.) 
It was a more common practice many years ago, and old habits die hard.
I have used the tester described in this thread, for many years, and won't go anywhere with out it.
Once you get used to it, like many other tools, you won't know how you got by with out one.
You can check power on cords, even heavy jacket ( SO & such) right through the jacket,
romex, and many other power conductors and cables. IF I'm correct it works off of the
electrostatic fields of the conductor, and thus requires no contact. A real jewel in the JIC box.
This thread is real food for thought as far as safety goes. Like I said, even as a licensed
electrician, I would not have given enough thought for the potential for damage and injury
to this RPBG thing. I admit. I have been guilty of just checking recep's with the three prong tester... :-\


First let me say that I have little experience with professional sound equipment other than some repairs to friends electronic equipment and providing power to a church tent carnival (and thus sound equipment) over too many years decades. You can blame Mike Sokol for making me aware of this forum even existing.

Working with electrical power distribution and instrumentation over the years I have fought the "No ground, no problem just tie the ground to neutral" dangerous misconception. The theory that the neutral and the ground are bonded back at the panel doesn't have any value. It is a dangerous practice to tie the neutral to the ground connection at a receptacle or any other place other than the main panel.

Even without the apparently common (from experience and what I have read here) practice of supplying equipment from multiple receptacle sources, the neutral/ground receptacle bond can set up a very unsafe condition. If the neutral path back to the panel is lost (corroded,, burned off, otherwise open) then the 120 volt supply to the equipment will be looking for a path to the ground. With the improper receptacle neutral/ground bond established the quickest path in grounded drill motors or other appliances  is via that grounded equipment case, through your body to ground. Not good.

Anyway, as Mike points out, there isn't a commercially available tester which I'm aware of to highlight the receptacle neutral/ground bond problem. Fortunately there is a relatively cheap solution for building owners should it be found that 3 prong receptacles have been installed with neutral/ground receptacle bonding on 2 wire (non-ground green/bare wire) systems. It is allowed by the National Electric Code to install GFI receptacles on a 2 wire system to provide 3 prong receptacles on old ungrounded systems. GFI's need no ground to do the job of monitoring current imbalance. Commercial packs of GFI receptacles are generally pretty reasonable in cost these days at big box stores.

That said, I relalize that although a GFI may be safe on a 2 wire ungrounded system, it may set up problems with hum in some of your audio equipment.

Sorry to butt in uninvited. vic Snyder
Title: Re: freak accident? help!
Post by: Mike Sokol on May 08, 2012, 06:17:28 pm
This is commonly referred to as a 'three wire circuit', to indicate the presence of three separate  conductors, - hot, neutral, and ground, none of which connect to each other.

Yes, you are correct. And modern main and sub-panel boxes are designed with a separate neutral bus to keep the neutrals and grounds wire separated except for the incoming main panel, where the Neutral, Ground, and Ground Rod is to be bonded together. However, I've seen a number of modern installations (last year) where the electrician also bonded the Neutral and Ground buss together AT EACH SUB-PANEL. This is not only in clear violation of the code, but I believe it's the primary reason we see (and hear) ground loop hum in so many sound systems. When I asked the inspector why he allowed this secondary G-N bonding to happen, he said they did that in all new installations, since he felt it gave a better ground. That's a bunch of horse hooey since now any voltage drop in the neutral buss will be reflected in the safety ground wire. And that causes the receptacle grounds in various parts of the room to have different voltages, sometimes as much as 3 or 4 volts difference.

I have an entire test bench setup where I can inject as much ground-loop current as I like into any audio circuit (up to 40 amps) and find that common XLR mic cable will have about 1 ampere of current flow per volt of ground loop differential. That's why you can use a common clamp-on ammeter to find where the hum currents are flowing. See diagram below. Note that this current is not flowing back through the twisted pair inside the shield, so you don't need to peel back the shield and clamp around a single conductor, like you would have to do with an extension cord. Nope, you just clamp a good old ammeter right around the mic cable, and if you see significant amps (1/10 or more) then you have a problem. Some powered speakers will hum at even 100 mA current (they have the pin 1 problem where the XLR shield is routed through a trace on the circuit board), while others will be hum free with 5 or 6 amps of ground loop current. Since my ground loop rig lets me create any level of ground loop I feel like, just for grins I've tested all sorts of power amps and powered speakers to see how sensitive they were to ground loop induced hum. Many will audibly hum with as little as 1/4 volt of ground loop differential, and when you inject 1 to 2 volts ground loop voltage (which turns into 1 to 2 amps ground loop current) they will sound like a swarm of angry hummingbirds.

And it's all caused by incorrectly bonded safety grounds in buildings, I'm sure of it. What this makes sound techs do is cut off the ground pins on power cords or lift pin-1 on XLR cables. Done correctly, this may not cause an immediate safety issue, but if later you plug in your amp with the broken off ground in a solo situation (not grounded via the XLR cable to another mixer with a real ground) you've created a chassis with a floating ground. Then it's just one tiny step to have hot-to-chassis leakage via an old cooked power transformer or wire trapped under a panel screw, and BAMM.... you (or a musician) are electrocuted. That's why I think it's so important to take stage grounding seriously, and why I've been studying this phenomenon for the last two years.   

This is getting really interesting.... Mike
Title: Re: freak accident? help!
Post by: Jonathan Johnson on May 08, 2012, 06:55:09 pm
Going into my way-back machine I vaguely recall a device based on old neon lamps that IIRC took 90V to light. But the one I recall had two metal probes so required direct conduction.

(Edit: removed potentially unsafe information.)

If you plug one end of a neon voltage tester into the hot side of a 120V outlet and leave the other lead dangling in free air, it will glow VERY dimly.

I don't think it's as accurate as the VoltAlert, but it may be useful in a pinch.
Title: Re: freak accident? help!
Post by: Keith Humphrey on May 08, 2012, 08:18:56 pm
Mike,
Just would like to thank you for the additional use of the non-contact voltage tester. I have used one for a fairly long time to trace wiring circuits. Like most folks I've always relied on the 3 light tester and a voltmeter when going into unknown places.

Not to take too much of a tangent but I have run into situations in some older buildings where circuits are on separate building grounds that are not bonded. Is there a fairly easy way to determine this without tracing the circuits and opening up the breaker boxes? 
Title: Re: freak accident? help!
Post by: Mike Sokol on May 08, 2012, 09:54:35 pm
Mike,
Just would like to thank you for the additional use of the non-contact voltage tester. I have used one for a fairly long time to trace wiring circuits. Like most folks I've always relied on the 3 light tester and a voltmeter when going into unknown places.

Not to take too much of a tangent but I have run into situations in some older buildings where circuits are on separate building grounds that are not bonded. Is there a fairly easy way to determine this without tracing the circuits and opening up the breaker boxes?

I've had something similar happen at a University graduation where they pulled power from multiple buildings, each of which had its own ground rod. They had the GFCI breakers on the building exteriors trip randomly, which spoiled the ceremony. I identified the primary reason for the GFCI tripping was a TRS to XLR adapter feeding 500 feet of mic cable to the video trailer, which was powered from the other building. Since there was a direct shield connection between the two mixers that were powered from different buildings, there were all kinds of ground loop currents flowing, which tripped the GFCI's at the worst possible time. I added a Whirlwind IMP-2 DI box on the output of the first mixer feeding the mic input on the video sound mixer. By lifting the ground switch on the IMP-2, I was able to disconnect the shield ground and stop the ground loops. That has eliminated the GFCI tripping for the past 3 years I've done the graduation. 

So let's consider your situation. You want to know how to identify which outlets are being fed by separate building grounds, probably to stop hums or other artifacts. You can either choose to live with the different ground potentials and use audio isolation transformers between all gear. Or you could get someone to run a very heavy grounding strap between the different power panels (but check with local and national electrical codes first, since I don't know the legal aspects of this sort of bond, but I suspect it's OK to do).

As far as testing for separate ground rod connections in various outlets, it could be as simple as running a test wire between the ground contacts on various outlets and monitoring the voltage, which should be a fraction of a volt at most. I have a test setup using ProStat Ground Qubes and 100 ft wires with banana plugs hooked into a digital voltmeter. See http://www.prweb.com/releases/2011/03/prweb5140894.htm The Qubes are only $5 each and protect you from accidentally hooking into a hot wire. But remember to check the outlet grounds for line voltage using VoltAlert to be sure nothing has been RPBG connected.

Let me think on this a bit more, but that's what I can think of off the top of my head.

Mike
Title: Re: freak accident? help!
Post by: Steve Hurt on May 08, 2012, 10:29:50 pm
Trying to boil this down and make it simple enough to retain....

I have a Greenlee GT-11 voltage detector which lights up only when held next to the hot wire.
I assume it is Greenlee's version of the tester you mention.  I also have a number of the little yellow 3-light testers.

If I:
1) Test all outlets with the Greenlee tester and on all sockets, the only hot wire is the correct one (the smaller of the 2 blades),
and
2) Test each outlet with my yellow 3 light tester that tests for "open ground/open neutral/hotground reverse/hot neutral reverse/wired correct" and get the "wired correct" lights

Then, I'm good to go?

(Actually, it sounds like I need only to check the ground leg with the Greenlee tester)
Title: Re: freak accident? help!
Post by: Mike Sokol on May 08, 2012, 11:14:30 pm
Trying to boil this down and make it simple enough to retain....

I have a Greenlee GT-11 voltage detector which lights up only when held next to the hot wire.
I assume it is Greenlee's version of the tester you mention.  I also have a number of the little yellow 3-light testers.

If I:
1) Test all outlets with the Greenlee tester and on all sockets, the only hot wire is the correct one (the smaller of the 2 blades),
and
2) Test each outlet with my yellow 3 light tester that tests for "open ground/open neutral/hotground reverse/hot neutral reverse/wired correct" and get the "wired correct" lights

Then, I'm good to go?

(Actually, it sounds like I need only to check the ground leg with the Greenlee tester)


You are correct, and even though in theory you only need to check the U-shaped ground with a non-contact tester, you should really confirm that line level voltage lights up the tester every time. That's the best way to confirm that your battery hasn't just gone dead in your non-contact tester, which would lull you into a false sense of security. So I generally test from Hot to Neutral to Ground to Hot, in a circle. Which should of course be On - Off - Off - On. You will find that if the Ground is wired at line level due to a RPBG, your tester will light up anywhere near the front of the afflicted outlet. That's because there's a metal strap going up the center of the outlet which will indeed radiate voltage and be capacitively coupled to the tester (yes, that's how they work). Then just plug in your 3-light ground tester to confirm everything else, and you're good to go.

I would be remiss if I didn't mention GLITs (Ground Loop Impedance Testers), which actually fire a single-cycle 20-amp pulse between the Hot and Ground connections. It then measures the voltage drop on the ground line and comes up with an impedance number in ohms. A solid ground connection should be less than 1 ohms, while a corroded connection will open up and show high impedance. I bought my first Woodhead GLIT over 30 years ago when I was an IE for Corning Glass, but now use an Amprobe INSP-3 which provides all sorts of additional testing.

I think there's some money to be made by electricians who could check clubs, performance stages, and churches for proper electrical outlets. As you now know from this thread, proper power for sound systems goes WAY beyond just plugging in a 3-light tester. I think all electrical outlets should assayed for proper voltage, ground impedance, ground loops, voltage drop, and a bunch of other things.

So you see, I've been worrying about this sort of thing for over 30 years, but really started studying it in detail when I started teaching my sound installer classes. If the moderator doesn't mind me plugging my seminars, here's the link to my ASSIST seminar at www.howtosound.com/ASSIST which includes everything I've been writing about here on sound system electrical safety using live demonstrations before your very eyes.

Mike Sokol
Title: Re: freak accident? help!
Post by: info@travelingmonkeysound.com on May 09, 2012, 01:32:22 am
Mike thanks for hammering this home... I hope you get royalties on all the testers that were bought in the past few days. But I'm curious - how do you ever put the pen tester down, with so much danger at every outlet?

Title: Re: freak accident? help!
Post by: Mike Sokol on May 09, 2012, 02:16:16 am
Mike thanks for hammering this home... I hope you get royalties on all the testers that were bought in the past few days. But I'm curious - how do you ever put the pen tester down, with so much danger at every outlet?
No, sorry to say I can't get a single test gear manufacturer (or even audio gear manufacturer) to support my electrical safety experiments or training in any way. I do get a few free testers from them, but I think it's mostly out of morbid curiosity to see what I'll come up with for tests. That said, I'm still working away at this with my blog at www.noshockzone.org and video channel at www.youtube.com/howtoseminars but really need sponsorship support so I can do this seriously.

Of course, I don't walk around with a non-contact tester wherever I go, but do check any new situation I'm in to protect myself and my seminar gear. And of course you want to use common sense. If you blow up a piece of gear or feel a shock, then it's time to bring out the big guns and really figure out what's wrong. I think many of these shock hazards have existed for decades but it wasn't until we started pulling power from multiple outlets with modern sound systems that the extent of the problem has become evident.

If anybody has any sponsor ideas, I'm all ears....

Mike
Title: Re: freak accident? help!
Post by: Jonathan Johnson on May 09, 2012, 10:52:00 am
I think many of these shock hazards have existed for decades but it wasn't until we started pulling power from multiple outlets with modern sound systems that the extent of the problem has become evident.

I think the big obstacle here is that professional audio & video is about the only application where you would pull power from multiple sources and interconnect the grounds from those sources -- without even realizing that you are interconnecting the grounds. As a result of being the only application, most electrical device manufacturers will consider it to be a "minor" problem that doesn't apply to most commercial and residential applications. Further, it is so ingrained in electricians' and engineers' minds that all electrical systems should be set up in a star pattern, that they fail to consider that the end user might create a cross connection (loop) outside the installed system.

Even in the audio world, we've long known that a difference in potential between grounds will cause hum, but we have always assumed that the ground isn't hot. We've failed to realize the potential danger of a hot ground, and we've failed to realize the real possibility that a miswired receptacle or cord could result in that condition.

I sense some "not my problem" going on here. The AV manufacturers suggest that it's a problem best dealt with by the house electrician, and the electricians say the AV manufacturers shouldn't create a ground loop (star only).

Someone earlier mentioned power conditioners. Manufacturers could integrate GFCI technology (or even non-contact voltage sensor technology) to detect current on the ground. If detected, it would disconnect all three conductors and indicate the fault. (To avoid nuisance tripping due to small leakages it could have a higher fault current limit than typical residential GFCIs.

You're far less likely to see Reverse Polarity / Bootleg Ground in new construction. It seems to me that you're more likely to see it in remodels, where a receptacle may be fed by an older two-wire circuit, or where installations may be done by poorly trained volunteers.

Again, RPBG is the sort of thing that most people won't even detect, because most appliances are single-source and won't share a common safety ground with another appliance, and most small appliances now don't even require a safety ground since they have nonconductive shells. As you talk to manufacturers, you need to highlight that interconnected grounds are a common (and necessary) occurrence, that RPBG is a very real problem, and that ground loop hum is not just an annoyance but an indicator of a dangerous situation.

In other words, "if it hums, it could kill you."

We need to educate the professional audio folks to make them aware of this issue and teach them to test for and recognize it. One voice in a crowd doesn't count for much, but if you get thousands of AV professionals talking about it, the manufacturers will take it seriously.
Title: Re: freak accident? help!
Post by: Geoff Doane on May 09, 2012, 02:05:42 pm
I've had something similar happen at a University graduation where they pulled power from multiple buildings, each of which had its own ground rod. They had the GFCI breakers on the building exteriors trip randomly, which spoiled the ceremony. I identified the primary reason for the GFCI tripping was a TRS to XLR adapter feeding 500 feet of mic cable to the video trailer, which was powered from the other building. Since there was a direct shield connection between the two mixers that were powered from different buildings, there were all kinds of ground loop currents flowing, which tripped the GFCI's at the worst possible time.

The GFCIs don't trip from ground current, they trip because of an imbalance in current between the hot and neutral. (A GFCI doesn't even need a ground to work, and it's one way to deal with 2-wire outlets that can't easily have a ground added, rather than making a bootleg ground.)  In your situation, I suspect there was more going on than simply a difference in ground potentials.  My guess is that there was already slight leakage downstream from the GFCIs, and either the ground wasn't good enough to draw enough current to exceed the GFCI's threshold, or the two grounds were in opposite polarity, thereby going past the trip point.

Pulling power from different services for the same system is fraught with danger.  I've heard some horror stories of extension cords run between houses.  All this discussion makes me even more determined to use my own distro as much as possible, since there will be only one tie-in point, and I know that no shortcuts have been taken with the construction.

Thanks Mike, for bringing this RPBG subject to the forum.  It's not a scenario I had considered before, although it's all too easy to see how it could happen once you pointed it out.  I did a bit of googling after you brought it up, and it is all too common in renovations it seems.  Apparently the NEC bans it (the bootleg ground, even with the correct polarity), but it's a cheap way around the problem that isn't easily detected by inspectors.  From what I could find, if a ground cannot be added to the circuit, a GFCI is an acceptable substitute, but the GFCI and all outlets downstream must be labeled "NO GROUND".

GTD
Title: Re: freak accident? help!
Post by: Mike Sokol on May 09, 2012, 05:19:27 pm
The GFCIs don't trip from ground current, they trip because of an imbalance in current between the hot and neutral. (A GFCI doesn't even need a ground to work, and it's one way to deal with 2-wire outlets that can't easily have a ground added, rather than making a bootleg ground.)  In your situation, I suspect there was more going on than simply a difference in ground potentials.  My guess is that there was already slight leakage downstream from the GFCIs, and either the ground wasn't good enough to draw enough current to exceed the GFCI's threshold, or the two grounds were in opposite polarity, thereby going past the trip point.

Perhaps you're correct.... but to top it off I believe that at least one of the GFCI's in question was wired with a bootleg ground (proper polarity). In that case the make-up ground loop current from the remote circuit would unbalance the H-N circuit at the GFCI and cause it to trip.  I didn't get a chance to analyze the actual failure since much of it was here-say and there was no way to duplicate the failure. As long as it worked properly the following year, everybody was happy. 

Now why in the world they would bootleg ground a GFCI I just don't know. But since all the other outlets in the old building were also bootleg grounded, I think it was inertia. It's just one of those things with old buildings and maintenance personal. I think they've now laid new conduit and proper 3-wire circuits in most of the buildings, but I'm still looking for trouble. 

Get's the old brain going, doesn't it???

Mike
Title: Re: freak accident? help!
Post by: Mike Sokol on May 09, 2012, 09:00:41 pm
I think the big obstacle here is that professional audio & video is about the only application where you would pull power from multiple sources and interconnect the grounds from those sources -- without even realizing that you are interconnecting the grounds. As a result of being the only application, most electrical device manufacturers will consider it to be a "minor" problem that doesn't apply to most commercial and residential applications. Further, it is so ingrained in electricians' and engineers' minds that all electrical systems should be set up in a star pattern, that they fail to consider that the end user might create a cross connection (loop) outside the installed system.

Even in the audio world, we've long known that a difference in potential between grounds will cause hum, but we have always assumed that the ground isn't hot. We've failed to realize the potential danger of a hot ground, and we've failed to realize the real possibility that a miswired receptacle or cord could result in that condition.

I sense some "not my problem" going on here. The AV manufacturers suggest that it's a problem best dealt with by the house electrician, and the electricians say the AV manufacturers shouldn't create a ground loop (star only).

Someone earlier mentioned power conditioners. Manufacturers could integrate GFCI technology (or even non-contact voltage sensor technology) to detect current on the ground. If detected, it would disconnect all three conductors and indicate the fault. (To avoid nuisance tripping due to small leakages it could have a higher fault current limit than typical residential GFCIs.

You're far less likely to see Reverse Polarity / Bootleg Ground in new construction. It seems to me that you're more likely to see it in remodels, where a receptacle may be fed by an older two-wire circuit, or where installations may be done by poorly trained volunteers.

Again, RPBG is the sort of thing that most people won't even detect, because most appliances are single-source and won't share a common safety ground with another appliance, and most small appliances now don't even require a safety ground since they have nonconductive shells. As you talk to manufacturers, you need to highlight that interconnected grounds are a common (and necessary) occurrence, that RPBG is a very real problem, and that ground loop hum is not just an annoyance but an indicator of a dangerous situation.

In other words, "if it hums, it could kill you."

We need to educate the professional audio folks to make them aware of this issue and teach them to test for and recognize it. One voice in a crowd doesn't count for much, but if you get thousands of AV professionals talking about it, the manufacturers will take it seriously.

All good points. As far as the last one, I'm looking for any potential soapboxes both in the pro-audio community and the renovators community. If anyone has suggestions for either, I'm interested.

Mike Sokol 
Title: Re: freak accident? help!
Post by: Mike Sokol on May 10, 2012, 03:04:45 pm

Again, RPBG is the sort of thing that most people won't even detect, because most appliances are single-source and won't share a common safety ground with another appliance, and most small appliances now don't even require a safety ground since they have nonconductive shells.

But there is one really big danger from plugging appliance into hot-grounded outlet. ELECTROCUTION!!!  I was teaching a NoShockZone seminar last year at Ball State University, and one of the students in the class remarked that the outlet in her family basement was "messed up" and the refrigerator would shock you if you stood on the damp concrete floor and touched the door handle. The solution was that everybody in her family "knew" to jump in the air while opening the refrigerator door to avoid a shock!!!   When I told her about the potential lawsuit her family would face if a repairman or neighbor went to the basement and leaned on the fridge, she was unimpressed. She had been doing this for years and assumed it was no big deal. Of course, that's death by electrocution just waiting to happen, but consumers seem oblivious to just how easy it would be to die from that scenario.

BTW: A non-contact tester (like a VoltAlert) will also tell you if a chassis is electrically hot,  but may false trigger as hot on a double-insulated "ungrounded" appliance. In that case, the chassis is actually at 60 volts (half the line voltage) just due to capacitive coupling in the power supply, but there's less than 1 milliamps of current flow, so your body discharges the voltage to whatever is grounding you. Yeah, that makes me a little uneasy too....

This whole business of double-insulated, ungrounded guitar amps also makes me feel a little squeamish. I saw a modern Roland guitar amp at a church last summer that had a factory "ungrounded" plug, but the chassis was a full 120 volts, and they said a lead singer had picked up a guitar plugged into it a few weeks before and got a nasty shock from it when he also touched the mic. Of course, that will teach lead singers to keep their hands off the instruments (just kidding) but highlights the fact that double-insulated "ungrounded" appliances can, in fact, internally short their chassis to the line voltage. And, of course, your guitar strings are tied to that same chassis. I have a really terrifying demonstration of that particular situation.

Mike     
Title: Re: freak accident? help!
Post by: John Roberts {JR} on May 10, 2012, 04:16:20 pm
A lot of very smart people have put much time and effort into human safety but it is hard to get much movement without dead bodies, and/or profit motive.

I've already suggested this, but IMO UL "should" expand ground bond current testing to include audio common connections even if not labelled as ground.

To extrapolate out, this means even XLR cable shields should pass ground bond testing, to fully protect the meat puppets.  It's is easy to see that will never happen, but we can ask.

I have seen houses with miswired outlets declared uninhabitable, until the killer outlets are corrected. But that only happens after a death. 

How do we inform DIY electricians?

I submit that new outlets when purchased need warnings and instructions, but this alone would not prevent idiots from putting themselves at risk.

No easy answer...

Keep tilting at those windmills... but look out you don't get a shock if the windmill is making electricity.

JR

Title: Re: freak accident? help!
Post by: info@travelingmonkeysound.com on May 10, 2012, 04:37:55 pm
A lot of very smart people have put much time and effort into human safety but it is hard to get much movement without dead bodies, and/or profit motive.

I've already suggested this, but IMO UL "should" expand ground bond current testing to include audio common connections even if not labelled as ground.

To extrapolate out, this means even XLR cable shields should pass ground bond testing, to fully protect the meat puppets.  It's is easy to see that will never happen, but we can ask.

I have seen houses with miswired outlets declared uninhabitable, until the killer outlets are corrected. But that only happens after a death. 

How do we inform DIY electricians?

I submit that new outlets when purchased need warnings and instructions, but this alone would not prevent idiots from putting themselves at risk.

No easy answer...

Keep tilting at those windmills... but look out you don't get a shock if the windmill is making electricity.

JR

And check that chassis is not a live wire!
Title: Re: freak accident? help!
Post by: Mike Sokol on May 10, 2012, 04:40:05 pm
How do we inform DIY electricians?

I submit that new outlets when purchased need warnings and instructions, but this alone would not prevent idiots from putting themselves at risk.

Well, one way to do this is hit them at the point of sales. We already have proposals into Lowe's and Home Depot to produce a series of in-store videos and on-line articles about properly installing outlets and testing them for RPBG's. Amprobe and Fluke have been contacted about supporting that sort of training, both for consumers at big box stores as well as local electricians and inspectors. And of course, we're already pitching the idea of these training videos and articles to a variety of pro-sound manufacturers.

And as you've predicted, we've only gotten some lukewarm response so far. It seems that nobody want to talk about this until the body count piles up. But all is not lost.... your feedback on this forum is valuable in convincing magazines, websites, and manufacturers that it's an important topic that shouldn't be dismissed so lightly.

Plus I hope that everyone who reads this thread understands the importance of checking your outlet power BEFORE plugging in your expensive gear....

Mike
Title: Re: freak accident? help!
Post by: Mike Sokol on May 10, 2012, 04:44:47 pm
And check that chassis is not a live wire!

Nils.... not to be critical about hair, but your avatar looks like it got a big shock....  ;)

Mike
Title: Re: freak accident? help!
Post by: Greg_Cameron on May 10, 2012, 04:46:22 pm
I have seen houses with miswired outlets declared uninhabitable, until the killer outlets are corrected. But that only happens after a death.

The house I now own with my wife had an interesting issue in the laundry room. We had a brand new Kenmore washer and dryer set put in there. One day whilst doing laundry my wife was reaching into the dryer while resting her hand on the locking nub of the front loading washer right next to the dryer. She got a good jolt. I whipped out the DMM and check between the dryer drum and washer enclosure. a bit over 100V. I checked the outlet the dryer was plugged into, it had a hot neutral swap. Now even so, it should not have been an issue. For some reason, the drum of the dryer was tied to neutral instead of ground. The ground wiring on both receptacles was correct. AFAIK, any metal parts that a person could come into contact with should only be tied to ground. I replaced both outlets with GFCIs when correcting the wiring issue and sent a message to Sears that I believe they might have a design or assembly issue. They didn't seem very concerned, which of course concerned me since it's a potentially fatal issue. Not good policy.
Title: Re: freak accident? help!
Post by: info@travelingmonkeysound.com on May 10, 2012, 04:56:46 pm
Nils.... not to be critical about hair, but your avatar looks like it got a big shock....  ;)

Mike

Why do you think I'm following this thread?
Title: Re: freak accident? help!
Post by: Mike Sokol on May 10, 2012, 04:58:57 pm
The house I now own with my wife had an interesting issue in the laundry room. We had a brand new Kenmore washer and dryer set put in there. One day whilst doing laundry my wife was reaching into the dryer while resting her hand on the locking nub of the front loading washer right next to the dryer. She got a good jolt. I whipped out the DMM and check between the dryer drum and washer enclosure. a bit over 100V. I checked the outlet the dryer was plugged into, it had a hot neutral swap. Now even so, it should not have been an issue. For some reason, the drum of the dryer was tied to neutral instead of ground. The ground wiring on both receptacles was correct. AFAIK, any metal parts that a person could come into contact with should only be tied to ground. I replaced both outlets with GFCIs when correcting the wiring issue and sent a message to Sears that I believe they might have a design or assembly issue. They didn't seem very concerned, which of course concerned me since it's a potentially fatal issue. Not good policy.

The proper way to test for this sort of situation is to use GLIT (Ground Loop Impedance Tester) with a kludge cable probe that let's you check the ground impedance of every part of the appliance. Perhaps something wasn't bonded properly inside the washer or dryer.

And yes, it would be nice if Sears was a little more concerned, but big companies generally don't admit to anything with potential liability.

Mike
Title: Re: freak accident? help!
Post by: Paul Dershem on May 10, 2012, 09:03:43 pm
Well, one way to do this is hit them at the point of sales. We already have proposals into Lowe's and Home Depot to produce a series of in-store videos and on-line articles about properly installing outlets and testing them for RPBG's. Amprobe and Fluke have been contacted about supporting that sort of training, both for consumers at big box stores as well as local electricians and inspectors. And of course, we're already pitching the idea of these training videos and articles to a variety of pro-sound manufacturers.

And as you've predicted, we've only gotten some lukewarm response so far. It seems that nobody want to talk about this until the body count piles up. But all is not lost.... your feedback on this forum is valuable in convincing magazines, websites, and manufacturers that it's an important topic that shouldn't be dismissed so lightly.

Plus I hope that everyone who reads this thread understands the importance of checking your outlet power BEFORE plugging in your expensive gear....

Mike

My VoltAlert arrived today, and I want to thank you for your guidance.

If you REALLY want to get the word out, you may want to contact Tom Silva, of television's This Old House, or Mike Holmes, of Holmes on Homes and Holmes Inspection. Both of these guys are considered highly credible and enjoy an ardent following.
Title: Re: freak accident? help!
Post by: Tim Perry on May 11, 2012, 12:33:47 am

No easy answer...

Keep tilting at those windmills... but look out you don't get a shock if the windmill is making electricity.

JR

i was at a recording gig last year and raised a fuss about the band with a ratty PA who INSISTED on using a ground lift adapter on the while PA.  Tempers flared, I threatened to leave, an accommodation was reached.

I recorded by all separate mics (no splits from their mics). I later provided system for the band. 

Point is, no ground is a much more common occurrence then a hot ground pin. And arguing with someone who is certain that the way hes been doing it for years is just fine is nearly futile.
Title: Re: freak accident? help!
Post by: Jonathan Johnson on May 11, 2012, 02:32:10 am
The house I now own with my wife had an interesting issue in the laundry room. We had a brand new Kenmore washer and dryer set put in there. One day whilst doing laundry my wife was reaching into the dryer while resting her hand on the locking nub of the front loading washer right next to the dryer. She got a good jolt. I whipped out the DMM and check between the dryer drum and washer enclosure. a bit over 100V. I checked the outlet the dryer was plugged into, it had a hot neutral swap. Now even so, it should not have been an issue. For some reason, the drum of the dryer was tied to neutral instead of ground. The ground wiring on both receptacles was correct. AFAIK, any metal parts that a person could come into contact with should only be tied to ground. I replaced both outlets with GFCIs when correcting the wiring issue and sent a message to Sears that I believe they might have a design or assembly issue. They didn't seem very concerned, which of course concerned me since it's a potentially fatal issue. Not good policy.

This is not Sears' (the corporation) fault and not necessarily the manufacturer's fault. It is the fault of the installer (which may be an employee of the local Sears store). It may be the fault of the local Sears store for failing to properly train the installer.

Dryers and ranges do not ship with cords. The cordset is attached by the installer. Because 3-prong 120/240V receptacles (hot-hot-neutral) are common in older homes while newer homes have 4-prong 120/140V receptacles (hot-hot-neutral-ground), the manufacturer leaves it up to the installer to choose and install the cordset that matches the existing receptacle.

However, the manufacturer ships the appliance with the neutral and ground bonded in the appliance. The installer must unbond these when connecting a 4-prong cordset, or leave them bonded when connecting a 3-prong cordset (essentially creating a bootleg ground).

In your case, it appears the installer left it bonded and probably left the ground lead in the cordset disconnected in the appliance.

As for myself, when I install such an appliance I will change the receptacle (rewiring if necessary) to provide a properly connected 4-prong 120/240V receptacle rather than installing a 3-prong 120/140V cordset with a bootleg ground. Of course, I can't expect appliance installers to do this, but for liability reasons, I would suggest that appliance installers should refuse to install an appliance using a 3-prong cord with bootleg ground. The homeowner then has two choices: call an electrician to rewire the receptacle, or install the cordset themselves.
Title: Re: freak accident? help!
Post by: Mike Sokol on May 11, 2012, 08:10:02 am

Point is, no ground is a much more common occurrence then a hot ground pin. And arguing with someone who is certain that the way hes been doing it for years is just fine is nearly futile.
Very true.... and most of you know that an "ungrounded" sound system can go instantly hot at any time if a single piece of gear develops a short or leakage to actual earth ground. The best you can do is protect yourself from getting shocked and hope that nobody in the band kills themselves on your watch.

I really think that bringing and using your own power distro is the safest thing to do. I built my first distro for my band back in the late 70's after a particularly nasty shock from a club. We were all plugged into what appeared to be grounded stage outlets, but I later discovered that while all the outlet grounds were connected together, NONE of these "grounds" were connected back to the ground buss in the main panel. So EVERYTHING was floating together. I built a simple circuit breaker box with Edison outlets on a piece of plywood and had a pair of vice-grips on a 100 ft piece of 10 gauge wire I could clamp onto a cold water pipe in the basement.  Not exactly code, but it worked perfectly. That's also when I bought my first Ground Loop Impedance Tester (GLIT) since I really don't like getting shocked on stage (or anywhere else for that matter).

Mike
Title: Re: freak accident? help!
Post by: John Livings on May 12, 2012, 02:52:47 am
However, the manufacturer ships the appliance with the neutral and ground bonded in the appliance. The installer must unbond these when connecting a 4-prong cordset, or leave them bonded when connecting a 3-prong cordset (essentially creating a bootleg ground).

+1,  The actual cord set may also have the ground nico pressed (swaged) to the neutral when you purchase it.

Also common is have installers run out of space in the neutral bar in a sub panel and run the neutral to the grounding bar.
Then installing a jumper between N and G. (or vice versa)

Some have been told not to add more than one wire to the same connecting screw hole in the neutral or ground buss connection.

The solution would be to install another (or more spaces) neural bar.

One other thing is very common; Correction of a "Dropped Neutral," (or loose connection upstream) In a box all the neutral are tied together and the neutral going to your receptacle is "Open" (Dead). Many times the installer can get his 120 Volts by testing P to G = 120 V,  P to N = 0 V.

So he connects the ground to the neutral screw of the of the plug,There by energizing (when something is running)  all the Grounds and all metal connected to them.

In the older systems (2 wire) the N is dead and the installer jumps the Neutral screw of the receptacle  to the metal box as to complete the circuit (120 V)

Now all the metal conduit and grounds could be connected to neutrals.

Also common, is on the Duplex Receptacle is to break off the brass tab connecting the two plugs so each can be used as a separate circuit (no problem), Then on the neutral side of the plug, that tab is broken off, opening the N run to the next plug.
Now the "Jumping" starts again resulting in the grounds being connected to the Neutrals.

Use the screws for wire connections on receptacles not "Back Stabbing". Some folks are lucky, however with lots of noise and movement, back stabbing is a bad idea.

Another issue is an installer Jumping the neutral and ground bar or engaging a bonding screw. This is done sometimes when the installers looks at the Main Panel set up and then copies it.

Just my opinions

Also Mike and others , thanks for posting all this information, Most useful.

Regards,  John
Title: Re: freak accident? help!
Post by: Mike Sokol on May 12, 2012, 08:25:28 am
However, the manufacturer ships the appliance with the neutral and ground bonded in the appliance. The installer must unbond these when connecting a 4-prong cordset, or leave them bonded when connecting a 3-prong cordset (essentially creating a bootleg ground).

+1,  The actual cord set may also have the ground nico pressed (swaged) to the neutral when you purchase it.

Also common is have installers run out of space in the neutral bar in a sub panel and run the neutral to the grounding bar.
Then installing a jumper between N and G. (or vice versa)

Some have been told not to add more than one wire to the same connecting screw hole in the neutral or ground buss connection.

The solution would be to install another (or more spaces) neural bar.

One other thing is very common; Correction of a "Dropped Neutral," (or loose connection upstream) In a box all the neutral are tied together and the neutral going to your receptacle is "Open" (Dead). Many times the installer can get his 120 Volts by testing P to G = 120 V,  P to N = 0 V.

So he connects the ground to the neutral screw of the of the plug,There by energizing (when something is running)  all the Grounds and all metal connected to them.

In the older systems (2 wire) the N is dead and the installer jumps the Neutral screw of the receptacle  to the metal box as to complete the circuit (120 V)

Now all the metal conduit and grounds could be connected to neutrals.

Also common, is on the Duplex Receptacle is to break off the brass tab connecting the two plugs so each can be used as a separate circuit (no problem), Then on the neutral side of the plug, that tab is broken off, opening the N run to the next plug.
Now the "Jumping" starts again resulting in the grounds being connected to the Neutrals.

Use the screws for wire connections on receptacles not "Back Stabbing". Some folks are lucky, however with lots of noise and movement, back stabbing is a bad idea.

Another issue is an installer Jumping the neutral and ground bar or engaging a bonding screw. This is done sometimes when the installers looks at the Main Panel set up and then copies it.

Just my opinions

Also Mike and others , thanks for posting all this information, Most useful.

Regards,  John

All seem like real life scenarios to me. The problem is that sound gear manufacturers and sound technicians ASSUME that perfect wiring practices will always be used in buildings. And this is simply not the case. I've been on enough house wiring jobs to know that the electricians and inspectors often have a "good old boy" relationship where the inspectors stops by the job site for coffee and donuts, then signs-off on the job without even pulling out a meter.

The problem with all this extra N-G bonding is that any voltage drop in a hot wire(s) will be matched by an equal (and inverted phase) drop in the neutral wire(s). If a ground wire is bonded at a secondary neutral point, then it too will move around electrically. Power two pieces of audio gear (a mixer and a powered speaker) on these different outlets with "dirty" grounds then cross connect them with an XLR cable. Ta da.... now you have a ground loop current flowing in the shield of the XLR cable which is trying to equalize the voltage between the two pieces of gear. My tests show about 1 ampere of current per volt of ground loop differential will flow through the XLR shield, and you can see why there can be several amps of current flowing in an XLR shield between your mixing console and amp racks. I won't go into the pin 1 problem here, but depending on the audio gear manufacturer you'll either get a tiny bit of hum (rejected by the CMRR) or a lot of hum (due to a pin-1 XLR trace on the circuit board). These ground loop hums can seem to come and go at random times, but in actuality are being modulated by electrical loads changing in other rooms. So when somebody starts a coffee maker in the kitchen, you get a large current draw on the neutral line in that box, which causes a corresponding voltage differential between the two different grounds that your gear is plugged into, which causes ground loop currents and hum.

This crazy coming and going of hum often causes sound technicians to break off the ground pins or use a ground lift adapter on the power plug in a futile attempt to stop the hum. Of course, bypassing the safety ground on sound gear can be dangerous since you have the possibility of a piece of gear's chassis "floating" above ground and possibly at 120 volts. Now touching that piece of gear (your guitar stings or mic) while leaning on the grounded metal rail around the stage can cause your heart to be in the current path, with the result being electrocution. Yikes.....

Getting the power right is the first step in running a great sounding and safe sound system. Incorrectly connected power can make the best sound system sonically challenged and dangerous to be around.

Mike
Title: Re: freak accident? help!
Post by: Scott Wagner on May 12, 2012, 11:07:32 am
This topic should become a "sticky" before it becomes a "shocky".

Scott Wagner
Big Nickel Audio
Title: Re: freak accident? help!
Post by: Tim McCulloch on May 12, 2012, 11:57:10 am
This topic should become a "sticky" before it becomes a "shocky".

Scott Wagner
Big Nickel Audio

I have suggested that to the mods.  With some editing it could also become a Study Hall article.
Title: Re: freak accident? help!
Post by: Bob Charest on May 12, 2012, 06:42:23 pm
I have suggested that to the mods.  With some editing it could also become a Study Hall article.
Good ideas, Scott & Tim. After reading the thread and getting the Fluke Volt-Alert I found that the outlet in our kitchen island is a bad actor. The outlet must be tapped off the electric oven. I'll check it out later, but that one raised my awareness. All the other outlets were by the numbers.

Best regards,
Bob
Title: Re: freak accident? help!
Post by: Mike Sokol on May 12, 2012, 07:14:11 pm
Good ideas, Scott & Tim. After reading the thread and getting the Fluke Volt-Alert I found that the outlet in our kitchen island is a bad actor. The outlet must be tapped off the electric oven. I'll check it out later, but that one raised my awareness. All the other outlets were by the numbers.

Best regards,
Bob
Once you start looking around, you can find all sorts of mis-wired outlets using a VoltAlert or other non-contact AC tester. Most aren't dangerous, but all of them should be repaired by a properly trained electrician.
Title: Re: freak accident? help!
Post by: Bob Charest on May 12, 2012, 08:28:18 pm
Once you start looking around, you can find all sorts of mis-wired outlets using a VoltAlert or other non-contact AC tester. Most aren't dangerous, but all of them should be repaired by a properly trained electrician.
Hi Mike,
Agreed. It goes to the point of expectations that things have been done correctly. The other outlets on the main floor were all good. When I got up the next day I wanted to check the various power distribution sources we use for the band, and was also trying to get used to either turning it on to beep or just flash. So strong was my expectation that all was well when I tested the rogue outlet (nearest to where I was standing when I turned the Fluke on and hadn't been tested yet) that at first I thought I had turned on the unit incorrectly! To me, aside from a proper repeatable method, my mind-set needs to be changed.... Maybe there's a tester for my brain - Nah, no market for that!
Best regards,
Bob
Title: Re: freak accident? help!
Post by: Mike Sokol on May 13, 2012, 08:36:37 am
Agreed. It goes to the point of expectations that things have been done correctly.

Exactly right. How many times have you fired up a big genny on an outside gig, and the first thing you do is check the line voltage and frequency. Why? Because we don't trust generators.

But how many wall outlets have you plugged into without thinking about it? Thousands? Tens of Thousands? While most wall outlets have been wired correctly, and some are wired incorrectly but not dangerously, some will be wired so wrong as to present a shock hazard. And as others have note on this thread, manufacturers don't think about gear being cross-connected between multiple power outlets. That's mostly reserved for the A/V industry with mixers, amps, lights and projectors positioned around the room, all interconnected by signal cables, each with their own shields.

I don't mean to scare everybody with my postings, just to make you aware of electrical situations that can put your gear and your life in jeopardy. I've been studying this a lot lately and have figured out all sorts of wiring situations that make sound systems sick.
Title: Re: freak accident? help!
Post by: Tim McCulloch on May 13, 2012, 09:31:58 am
Exactly right. How many times have you fired up a big genny on an outside gig, and the first thing you do is check the one voltage and frequency. Why? Because we don't trust generators.

I trust generators but I don't trust the last asshat that rented it and decided to compensate for voltage drop over his too-small cable by setting the output voltage at 146v.  (yes, I've found that).  We see things like that when a genset has been on a construction site, out in the oil patch or used for temporary building power.

That said, we've developed a good relationship with a primary generator shop in our area.  We tell them we need a unit that is "computer-friendly, event-quality."  We tend to get units with <1000 hours, and have crystal controlled frequency and the voltage is spot on when they deliver it.

And I still check line freq, output voltage and neutral bonding before the genny tech leaves the site.
Title: Re: freak accident? help!
Post by: Mike Sokol on May 13, 2012, 09:02:22 pm
And I still check line freq, output voltage and neutral bonding before the genny tech leaves the site.

Everyone... please post pictures of any incorrectly wired power outlets you find.

Thanks...
Title: Re: freak accident? help!
Post by: Jordan Wolf on May 13, 2012, 09:03:04 pm
Everyone... please post pictures of any incorrectly wired power outlets you find.
I'll take a look around the hotel... ::)
Title: Re: freak accident? help!
Post by: Jonathan Johnson on May 14, 2012, 01:27:08 am
Everyone... please post pictures of any incorrectly wired power outlets you find.

Thanks...

I wish I had a picture of the wiring in my grandpa's barn, before I rewired it. Talk about a rat's nest of wires! Carefully tracing the wires, I discovered that one outlet had no means of cutout except pulling the meter or cutting the wire. No fuses, no circuit breakers.

It seems his body did not conduct electricity well. He would work on the electric fence (>1000 volts) without shutting it off; he said he could feel a little tingle. 120V didn't faze him in the least.

It was after he died (natural causes, not electrocution) that I educated myself on proper wiring methods and rewired both the barn and the house.

Below are two junction boxes I recently removed from my church. Their condition is exactly as they were prior to removal. They had no covers, were fed by non-grounded circuits, no wire nuts (twist & tape), and the few knockouts that had clamps had loose clamps. These were located in the crawl space, inconveniently right above the most convenient place to crawl. It was a nerve-wracking experience to scrape one's back on these.

I do not believe these were installed by my grandpa, but by another member of the church who has passed on. (There is still some rewiring that needs to be done to ensure proper grounding at all outlets and to more evenly distribute loads, but I think we've got the most dangerous stuff remedied.)

The point is there is a lot of bad wiring out there. You can test the outlets and they may test OK, but you don't know what's hiding inside the walls. Connections like these can be a fire hazard at currents below the rating of the fuse or circuit breaker.

(http://www.webmoth.com/gallery2/d/557-1/jbox.jpg)
Title: Re: freak accident? help!
Post by: Mike Sokol on May 14, 2012, 07:53:51 am

The point is there is a lot of bad wiring out there. You can test the outlets and they may test OK, but you don't know what's hiding inside the walls. Connections like these can be a fire hazard at currents below the rating of the fuse or circuit breaker.


I think that churches are some of the worst offenders in terms of dangerous wiring. They're often on very limited budgets, use untrained volunteers to do renovations, and add way more sound, video and lighting than was ever intended for the original wiring.

For instance, I taught a seminar in an old church in New Jersey a few years back. My contact there said I was going to be in the "new" addition that was done in '95.... Oh, that's 1895! Someone had run zip cord inside the walls and poked it out of fist-sized holes with a combination of unboxed power outlets and Edison bulb sockets with plug adapters. Yikes...! I was running long extension cords to the next room trying to get grounded power that wouldn't kill me.

Another church's gym had what appeared to be a brand new, grounded outlet installed on the wall behind the projector screen. Seemed like a good place to plug in my demonstration rack... but as soon as I did I saw the voltage bar on my Furman power strip was "pegged" to the red. I didn't power up the rack and then metered the outlet, which turned out to be their "special outlet" for the floor buffer which needed 240 volts. That's right, a standard NEMA 5-15 outlet designed for 120 volts with 240 volts wired into it. Fortunately I didn't turn on the full rack, but unfortunately I had cheated a bit and had a Sennheiser RF receiver plugged into a power strip which fed the Furman. Of course, I burned up the RF receiver but saved the rest of the rack from electrical damage. I'm a lot more careful around "new" outlets now. 

I could go on and on, but the point is to never assume that the outlet wiring is correct. I generally don't have these power problems on big gigs where we bring our own distro. It's those simple little gigs where we just need to throw up a few speakers on sticks and run a mic or two that get us into trouble. 
Title: Re: freak accident? help!
Post by: Kemper Watson on May 14, 2012, 08:15:41 am


I've run into 220 on an Edison outlet before. Outside a hotel on a guest pavilion. Custodian finally admitted that there was a marker glued to the brick wall that "must have fallen off".
Title: Re: freak accident? help!
Post by: Mike Sokol on May 14, 2012, 08:46:38 am

I've run into 220 on an Edison outlet before. Outside a hotel on a guest pavilion. Custodian finally admitted that there was a marker glued to the brick wall that "must have fallen off".

An Edison NEMA 5-15 or 5-20 outlet can NEVER be legally wired to 240 volts, no matter how big the signage.

There's a really quick way to test for 240 volt wiring using a non-contact tester such as a Volt-Alert. If there's interest here I'll make a video of it and post.
Title: Re: freak accident? help!
Post by: Jonathan Johnson on May 15, 2012, 02:15:56 am
Below are two junction boxes I recently removed from my church. Their condition is exactly as they were prior to removal. They had no covers, were fed by non-grounded circuits, no wire nuts (twist & tape), and the few knockouts that had clamps had loose clamps.(http://www.webmoth.com/gallery2/d/557-1/jbox.jpg)

You'll notice that one box has a conduit fitting. This box actually was attached to conduit. But the conduit was on the load side... the line feeding this box was an old ungrounded 2-wire Romex. Somebody "grounded" it by running a copper wire from the conduit setscrew to a cold water pipe. The wire was wrapped around the cold water pipe, not clamped on. As for the cold water pipe, it was never bonded to the ground conductor at the service entrance, and a recent underground water line replacement had left about 16" of metal pipe buried... under the church where the soil was bone dry.

Did I mention ALL of the receptacles at the church have a grounding slot?
Title: Re: freak accident? help!
Post by: Mike Sokol on May 15, 2012, 07:20:18 am

Did I mention ALL of the receptacles at the church have a grounding slot?

All I can say is WOW!

I've seen similar "grounding" some 30 years in an old apartment kitchen from the 1950's. There were new "grounded" outlets all around the sink, powering the refrigerator, etc... but when I used a Ground Loop Impedance Tester (GLIT), which shoots a single-cycle 20-amp pulse down the ground, we heard a big "zap" under the sink and the test failed. I could see a spark under the sink every time I hit the test button on the GLIT. 

Investigation showed that all of the kitchen outlet grounds were tied to a single loop of wire twisted around the copper cold water pipe under the sink. Of course, there was copper oxide in the way and it was a loose contact, but it was good enough for a 3-light tester to say the outlets were OK. The landlord accused me of burning up his wires with my fancy tester, so we ran a new wire down to the basement and bonded it in the service panel ground, and all was well. But after that I wondered about every "ground" in the apartment building. 

Of course that high-z "ground" would not have been able to sustain enough current to trip the circuit breaker (or blow the fuse in an old house). If it was a GFCI outlet it could have protected you from shock by tripping itself. But there wasn't a GFCI in sight, and wouldn't be required by code for many years.

As a side note, do you think this thread should be also posted on the Church Sound Forums? There's nearly 400,000 churches in the USA alone, and I've got to wonder just how many of them have dangerous wiring.... 
Title: Re: freak accident? help!
Post by: Mike Sokol on May 15, 2012, 10:12:02 am
For those of you who are morbidly curious, here's a picture of my NoShockZone test bench that I bring along on NSZ and ASSIST seminars. Note there's a B&K AC power supply in the left with lets me bias the chassis of any audio gear up to 120 volts AC, a Glo-Melt soldering transformer which allows me to insert up to 3 volts and 40 amperes of ground-loop differential between any two pieces of audio gear, plus a lot of ammeters, voltmeters, non-contact testers and audio isolation transformers.

Kids... don't try this at home! 
Title: Re: freak accident? help!
Post by: Tim McCulloch on May 15, 2012, 10:58:19 am
An Edison NEMA 5-15 or 5-20 outlet can NEVER be legally wired to 240 volts, no matter how big the signage.

There's a really quick way to test for 240 volt wiring using a non-contact tester such as a Volt-Alert. If there's interest here I'll make a video of it and post.

Mike, the key word here is "legally."  Up thread I posted about an Edison outlet I found in a hotel ballroom that was wired for either 208v or 240v.  It was a 20 amp, 120v outlet.  After the squints blew 3 follow spot lamps (on a Saturday in a non-major metropolitan city I metered it for them.

The hotel insisted there was nothing wrong with the outlet.  If I could have gotten a city inspector there on Saturday, I'd have tried to prove them wrong... oh well.
Title: Re: freak accident? help!
Post by: Mike Sokol on May 15, 2012, 12:07:41 pm

The hotel insisted there was nothing wrong with the outlet.  If I could have gotten a city inspector there on Saturday, I'd have tried to prove them wrong... oh well.

The scary part is I'll bet that outlet is STILL wired for 208/240 volts, and it's waiting for the next piece of 120 volt A-V gear to destroy.
Title: Re: freak accident? help!
Post by: Greg_Cameron on May 15, 2012, 12:39:50 pm
Dryers and ranges do not ship with cords. The cordset is attached by the installer. Because 3-prong 120/240V receptacles (hot-hot-neutral) are common in older homes while newer homes have 4-prong 120/140V receptacles (hot-hot-neutral-ground), the manufacturer leaves it up to the installer to choose and install the cordset that matches the existing receptacle.

FWIW, this is a gas dryer with a standard 120V 15A Edison plug, not an "electric" dryer with a 240V plug. So the cord in this case was installed by the manufacturer.

Greg
Title: Re: freak accident? help!
Post by: Mike Sokol on May 15, 2012, 01:45:27 pm
FWIW, this is a gas dryer with a standard 120V 15A Edison plug, not an "electric" dryer with a 240V plug. So the cord in this case was installed by the manufacturer.

Greg
So is this fixed now?

Another interesting thing is that a non-contact tester (Fluke VoltAlert) will alert if pointed at a chassis that's energized to more than 40 volts. See my video of an energized VW micro/micro bus (yes, that's two micro's). http://www.youtube.com/watch?v=rtT3te_XNBM

If you touch a VoltAlert to any appliance that has a grounded cord, it should NOT beep at you. If it beeps, then you have a failed ground, and the mic or mixing console (or dryer, in this case) is electrically energized.

If you touch a VoltAlert to a double-insulated appliance (one without a ground pin on the factory cord), then the appliance chassis will float up to approximately 1/2 line voltage (around 60 volts) which will make a non-contact tester beep. But there's not enough current (way under 1 mA) to cause a shock. That's how manufacturers qualify to leave off a grounded cord on double-insulated appliances, by certifying there's minimal internal AC leakage to the chassis.

The reasons that a "grounded" appliance shows voltage on its chassis can vary from an improperly grounded outlet, to a broken ground pin on the power cord plug, to a disconnected green/ground wire inside the appliance (or guitar amp). I've seen all of them too many times to even count. 
Title: Re: freak accident? help!
Post by: Mike Sokol on May 15, 2012, 02:02:50 pm
If any of you tie directly into power panels in older industrial buildings (like I used to do for my band's 50KW lighting system in my mis-spent youth) you should be aware of something called a High-Leg or Red-Leg Delta panel. See http://en.wikipedia.org/wiki/High-leg_delta

Too much to go into within this thread, but quickly note that from Neutral to two of the phases it measures 120 volts, but from Neutral to the High-Leg phase it measures 208 volts. This was common practice in industrial buildings in the 50's and 60's since it saved the power company from installing a separate single-phase transformer for office power.

I first ran into this at the Old Mill Inn back in the late 70's. It was actually a mill converted into a club, and when an incoming group tapped into the main panel to run their lights, they fed 208 volts into their lighting system by mistake. Blew out dozens of bulbs in one shot. I had a meter with me and worked as an IE for Corning, so I knew to stay away from the High-Leg 208-volt buss with my own lighting distro.

Meters are cheap... replacement bulbs are not...!

Title: Re: freak accident? help!
Post by: John Livings on May 15, 2012, 03:46:51 pm
A source of good information is;

"Electricity for the Entertainment Electrician & Technician"

By  Richard Cadena.

http://www.amazon.com/Electricity-Entertainment-Electrician-Technician-Richard/dp/0240809955/ref=la_B001IO9WAG_1_1?ie=UTF8&qid=1337110400&sr=1-1

It really covers a lot different scenarios,  most helpful information.

Regards,  John
Title: Re: freak accident? help!
Post by: Mike Sokol on May 16, 2012, 10:00:56 pm
A source of good information is;

"Electricity for the Entertainment Electrician & Technician"

By  Richard Cadena.

http://www.amazon.com/Electricity-Entertainment-Electrician-Technician-Richard/dp/0240809955/ref=la_B001IO9WAG_1_1?ie=UTF8&qid=1337110400&sr=1-1

It really covers a lot different scenarios,  most helpful information.

Regards,  John

Looks like a good book. I'll order a copy for my library...
Title: Re: freak accident? help!
Post by: Mike Sokol on May 18, 2012, 06:53:53 pm
For those interested in grounding and ground-loop issues, the saga continues on this thread  8)

http://forums.prosoundweb.com/index.php/topic,138041.0.html
Title: Re: freak accident? help!
Post by: Mike Sokol on May 20, 2012, 04:25:02 pm
At the request of one of my meter manufacturer contacts, I drew up a diagram of what happens when you interconnect two pieces of audio gear that are powered from a correctly grounded outlet and an RPBG outlet. Note from the attached diagram that you don't even have to turn on the power switch for the mixer or speakers, short circuit currents will flow from the grounds. However, with 100' of XLR interconnecting the gear (typical) you'll probably have a max peak shorting current of 100 amps before the Circuit Breaker tips.

Also note that this type of connection will NOT trip a GFCI on the properly wired outlet because the fault current is from ground-to-ground, and GFCI outlets don't care about ground current, only a balance of the hot and neutral currents.

Of course, any sort of DI box in the XLR line will have its internal ground wires vaporized by this fault current, and Belfoil shielded wire will be subject to turning red hot since it doesn't have the wider current path of a braided shield. This means that the shield in your snake will melt down, while the XLR cable connecting it to the offending piece of gear won't be damaged. No AC currents will flow in the 22 gauge twisted-pair signal wires, only in the shield. Of course, a pin-1 XLR lift in this connection will dump 120 volts AC right into your speakers' balanced input circuitry.

Getting even more interesting, isn't it?   
Title: Re: freak accident? help!
Post by: John Roberts {JR} on May 20, 2012, 06:15:32 pm
While a minor quibble it's AC current so shouldn't arrow heads point both ways?


JR
Title: Re: freak accident? help!
Post by: Mike Sokol on May 20, 2012, 06:20:27 pm
While a minor quibble it's AC current so shouldn't arrow heads point both ways?
JR

Yeah, you're right.... It's a tough room, isn't it???

Done
Title: Re: freak accident? help!
Post by: Tim McCulloch on May 20, 2012, 07:38:44 pm
At the request of one of my meter manufacturer contacts, I drew up a diagram of what happens when you interconnect two pieces of audio gear that are powered from a correctly grounded outlet and an RPBG outlet. Note from the attached diagram that you don't even have to turn on the power switch for the mixer or speakers, short circuit currents will flow from the grounds. However, with 100' of XLR interconnecting the gear (typical) you'll probably have a max peak shorting current of 100 amps before the Circuit Breaker tips.

Also note that this type of connection will NOT trip a GFCI on the properly wired outlet because the fault current is from ground-to-ground, and GFCI outlets don't care about ground current, only a balance of the hot and neutral currents.

Of course, any sort of DI box in the XLR line will have its internal ground wires vaporized by this fault current, and Belfoil shielded wire will be subject to turning red hot since it doesn't have the wider current path of a braided shield. This means that the shield in your snake will melt down, while the XLR cable connecting it to the offending piece of gear won't be damaged. No AC currents will flow in the 22 gauge twisted-pair signal wires, only in the shield. Of course, a pin-1 XLR lift in this connection will dump 120 volts AC right into your speakers' balanced input circuitry.

Getting even more interesting, isn't it?

Nice diagram, Mike.  The picture does indeed replace 1000 words.

Now you need to add the heartache and trauma with another frame that shows the effects of heating on the cable or the electricity being delivered to a human.  Just enough blood and gore to get the point across: this can kill you and/or burn down a building.
Title: Re: freak accident? help!
Post by: Mike Sokol on May 20, 2012, 07:56:37 pm
Nice diagram, Mike.  The picture does indeed replace 1000 words.

Now you need to add the heartache and trauma with another frame that shows the effects of heating on the cable or the electricity being delivered to a human.  Just enough blood and gore to get the point across: this can kill you and/or burn down a building.

Something like this? 8)

Title: Re: freak accident? help!
Post by: Tim McCulloch on May 20, 2012, 08:15:10 pm
Something like this? 8)

Needs some zombie food or heart exploding from the chest, lightning bolts radiating from the guitar or microphone... I was serious about the blood and guts.
Title: Re: freak accident? help!
Post by: Mike Sokol on May 20, 2012, 08:29:52 pm
Needs some zombie food or heart exploding from the chest, lightning bolts radiating from the guitar or microphone... I was serious about the blood and guts.

When I went through my OSHA safety training back in the late 70's, they made us look at a bunch of autopsy photos of arc flash burns. I had nightmares for weeks after that.

One of things we're pitching to the PSW editors is a 12-part series on all aspects of sound system electrical hookups and safety, how to find and stop hum from ground loops, reasons for buzz, shock avoidance, grounding issues, etc... I have the ability to create all these situations at will on a bench using real instruments, amplifiers and speakers. If you think a picture is good, a video of each of these situations is great.

But you're correct, we should probably animate some of these shock situations to make a point. 
Title: Re: freak accident? help!
Post by: Marty McCann on May 22, 2012, 11:36:38 am

When I went through my OSHA safety training back in the late 70's, they made us look at a bunch of autopsy photos of arc flash burns. I had nightmares for weeks after that.

One of things we're pitching to the PSW editors is a 12-part series on all aspects of sound system electrical hookups and safety, how to find and stop hum from ground loops, reasons for buzz, shock avoidance, grounding issues, etc... I have the ability to create all these situations at will on a bench using real instruments, amplifiers and speakers. If you think a picture is good, a video of each of these situations is great.

But you're correct, we should probably animate some of these shock situations to make a point.

This attachment should make a point . . . .
Title: Re: freak accident? help!
Post by: Mike Sokol on May 22, 2012, 11:50:06 am
This attachment should make a point . . . .
Yes it does. I studied that particular fault last year, which was a pair of copper thieves who tried to cut through a live high-voltage cable in a sub-station. They were using "insulated" fiberglass handle bolt cutters, but the load current in the cable created a huge arc-flash, which is basically an explosion of super-heated copper plasma on fire. Nice, huh?

Circuit breaker panels and wiring impedance limit the peak currents possible inside a residence or on a stage with Edison plugs, but you can get a pretty serious fireball with cam-locks on a 400 amp service. Big voltage will kill you, but but amperage will kill you and cook you.

I feel like having tacos for lunch. Hmmmm..... wonder why 8)
Title: Re: freak accident? help!
Post by: Mike Sokol on May 23, 2012, 09:37:19 am
The fun continues....

I've been in contact with both Whirlwind and Ebtech engineering, who think their DI and Hum Eliminator box transformers will withstand a full 120-volts AC common-mode differential. With that in mind, I'm modifying an IMP-2 to have a neon bulb in parallel across the ground-lift switch. See diagram. I'll place this "Super-IMP" in a RPBG situation as previously described. Don't worry, I can do this safely... but kids, don't try this at home. I'm pretty sure the Super-IMP will work normally with the switch in the lift position, and there will be no audio indication that anything is wrong. But my Neon bulb will light up, indicating a BIG voltage differential between the two power outlets. Note that adding the Neon bulb isn't what makes the IMP tolerant of 120-VAC ground differentials, that's a natural part of the transformer design. So if I flipped the switch to Ground, it would short out and burn up just like any other DI box with a ground switch. However, I think that modifying your existing DI boxes with a Neon bulb, and ALWAYS starting with the switch in lift position for any double-ground situation (a grounded instrument amp on stage feeding a grounded console in the back of the room) you'll get a visual indication (big red light) that the stage (or console) ground is hot, and that you need to proceed with extreme caution. DON'T FLIP THE SWITCH TO GROUND IF YOU SEE THE RED LIGHT!

My buddies at Whirlwind are sending me a scratch-n-dent IMP-2 for this experiment (Thanks Al Keltz) so I should be able to mod the Imp and do the experiment next week. I'll report back then, but I'm 99% certain this will all work as I've envisioned.

Stay tuned...
Title: Re: freak accident? help!
Post by: John Roberts {JR} on May 23, 2012, 10:05:17 am
That should give indication for 90v (peak dc) or more.

For DI between two, three-wire line cord products, that should work, but for one rouge outlet that powers up a chassis ground, without a solid ground on the other end you may only get an indication "while" being shocked.

Sounds like a nice elegant inexpensive indication, while neon lamps are probably not as common as they once were for on/off indicia.

You have probably already had this conversation, but remind your product manufacturing friends that this ground path, when not lifted or open, needs to be robust enough to take out the fuse/circuit breaker (tens of amps) with no life threatening voltage rise.   

JR
Title: Re: freak accident? help!
Post by: Mike Sokol on May 23, 2012, 10:54:06 am
That should give indication for 90v (peak dc) or more.

For DI between two, three-wire line cord products, that should work, but for one rouge outlet that powers up a chassis ground, without a solid ground on the other end you may only get an indication "while" being shocked.

Sounds like a nice elegant inexpensive indication, while neon lamps are probably not as common as they once were for on/off indicia.

You have probably already had this conversation, but remind your product manufacturing friends that this ground path, when not lifted or open, needs to be robust enough to take out the fuse/circuit breaker (tens of amps) with no life threatening voltage rise.   

JR

Yes, to all of the above. And you can still buy Neon lamps with a panel-mount from Radio Shack for around $3 a pair. I have one sitting on my desk in front of me.

And I don't think this is the fix-everything solution. But it will indicate if you have one rogue outlet on stage that's been wired incorrectly as a RPBG. The real problem is that even if the DI's ground path wire is heavy enough to withstand the full short circuit current sufficient to trip a CB, it will likely never come to that. I'll have to calculate the resistance of 100 ft of BelFoil drain wire and do the calculation, but I'm pretty sure there's enough series resistance there to reduce the short circuit current into the 30 to 50 amp peak range, possibly even below 20 amps. In that case the CB won't trip, but your snake will simply heat up red hot and melt. Something's going to become the fuse, and Murphy's Law dictates it will be the most expensive piece of gear in the current path.... 

And remember that even if the "good" outlet is wired properly with a GFCI, it won't know there's a current imbalance (because all the shorting current is in the ground wires) and won't trip at 5 mA or whatever. This fault will need to trip the 20 amp panel circuit breaker in the panel feeding the rouge outlet. This fault won't even trip the circuit breaker in your power strip, since it's not in the ground fault path. 
Title: Re: freak accident? help!
Post by: Chuck Simon on May 23, 2012, 11:15:07 am
Well, I'm convinced.  I just ordered a Fluke Volt-Alert.  Thank you Mike!
Title: Re: freak accident? help!
Post by: John Roberts {JR} on May 23, 2012, 11:38:52 am
Yes, to all of the above. And you can still buy Neon lamps with a panel-mount from Radio Shack for around $3 a pair. I have one sitting on my desk in front of me.
Fine for one off... surely cheaper in production quantities.
Quote
And I don't think this is the fix-everything solution. But it will indicate if you have one rogue outlet on stage that's been wired incorrectly as a RPBG. The real problem is that even if the DI's ground path wire is heavy enough to withstand the full short circuit current sufficient to trip a CB, it will likely never come to that. I'll have to calculate the resistance of 100 ft of BelFoil drain wire and do the calculation, but I'm pretty sure there's enough series resistance there to reduce the short circuit current into the 30 to 50 amp peak range, possibly even below 20 amps. In that case the CB won't trip, but your snake will simply heat up red hot and melt. Something's going to become the fuse, and Murphy's Law dictates it will be the most expensive piece of gear in the current path.... 
That is the classic question I wrestled with a decade or two ago... Do you go open circuit or hard short... One could argue for wimply fusible link grounds inside products, but this would probably still pass enough current to kill meat puppets, and not clear the fault condition by killing the mains power.

External cables are far easier to replace than fried gear.
Quote
And remember that even if the "good" outlet is wired properly with a GFCI, it won't know there's a current imbalance (because all the shorting current is in the ground wires) and won't trip at 5 mA or whatever. This fault will need to trip the 20 amp panel circuit breaker in the panel feeding the rouge outlet. This fault won't even trip the circuit breaker in your power strip, since it's not in the ground fault path.

This is not a simple problem to address with product design (and I know you are not suggesting that)... it really is a failure by whomever does the outlet wiring, which unfortunately all to often is DIY and/or uninformed professionals.

keep up the good fight.

JR
Title: Re: freak accident? help!
Post by: Jonathan Johnson on May 23, 2012, 03:39:24 pm
...I'm modifying an IMP-2 to have a neon bulb in parallel across the ground-lift switch. See diagram...

The potential problem I see is a false sense of security.

If the two devices connected using the modified IMP-2 both have hot grounds (that is, they are both plugged into RPBG outlets), the neon bulb may not light. This could lead a poor fool to believe that it's A-OK, then they fry themselves when they grab a hot mic.

While it's a valuable indicator, it needs to be made clear that the absence of an indication is not an indication of an absence of a problem. (Hah! I'll bet your head is spinning on that last sentence!)

It is but one tool that needs to be used in conjunction with other tools by trained personnel to ensure a safe stage.

Title: Re: freak accident? help!
Post by: Mike Sokol on May 23, 2012, 04:06:57 pm
While it's a valuable indicator, it needs to be made clear that the absence of an indication is not an indication of an absence of a problem. (Hah! I'll bet your head is spinning on that last sentence!)

Gotta love the English language. And yes, you are correct. That's why I'm an advocate for walking around with a non-contact tester both before you plug into outlets, and also as a final test on mics and guitars after everything is plugged in. I saw an article a year or so ago where someone was suggesting a tongue test, of holding onto the guitar and licking the mic to prove to the guitar players they wouldn't get shocked. I think a non-contact tester such as a Fluke Volt-Alert is a little better way to go.

And my Neon bulb mod is exactly as you describe, just one more testing tool, not the final test. This will become part of my test regimen I always do on stage. I'm proud of the fact that after I started testing outlets back in the early 80's I never got shocked on stage as a musican again, and none of the musicians I ran sound for have been shocked either. Knock on wood, but I think it's because I have a very healthy fear of getting shocked. I used to play with BIG POWER as an IE where they would maybe have found my ashes blowing in the wind if I screwed up the power (like the picture of the poor fools in the previous posting). But dead is dead, so I never take 120 volt shocks for granted.

BTW: Everybody... thanks for all the feedback and ideas so far. If we all work together on this we'll make stages safer for everyone!
Title: Re: freak accident? help!
Post by: Mike Sokol on June 11, 2012, 02:17:16 pm
I've just received a care package from Amprobe for more of my testing, and really like their little VP-600SB VoltProbe. Not only is it on all the time (they tell me a year or more battery life) it runs on two standard AAA batteries. Plus it not only beeps (loudly) and blinks (brightly), it also shakes in your hand when it triggers. Plus it's sorta flat with a pen grip to stay in your pocket. I found it at Sears for less than $14  http://www.sears.com/shc/s/p_10153_12605_SP101A11576S6083631101P?sid=IDx20070921x00003a&ci_src=14110944&ci_sku=SPM6045437007

Also, I edited my original RPBG proof-of-concept video and launched it on a new www.youtube.com/noshockzone channel. It's now available for general public viewing, so pass it on to anyone you know who plugs into an outlet.
Title: Re: freak accident? help!
Post by: Mike Sokol on June 16, 2012, 09:15:35 pm
Also, I edited my original RPBG proof-of-concept video and launched it on a new www.youtube.com/noshockzone channel. It's now available for general public viewing, so pass it on to anyone you know who plugs into an outlet.

I now have Amprobe INSP-3, Ideal SureTest, and Extech CT-70 Circuit Testers in my hot little hands, and NONE of them will find a RPBG outlet on their own. See my latest video on the SureTest:  http://youtu.be/_04HmpFBxdQ

What's needed is for the manufacturers to add a non-contact AC test function into the ground contact of their GLIT's (Ground Loop Impedance Testers). In the meantime, add a non-contact tester such as a Fluke VoltAlert of Amprobe VoltProbe to your tool kit along with a standard 3-light outlet tester. That combination should identify 99.99% of problems.

As a quick extra, note that if you use a non-contact tester on a 120-volt outlet that's been rewired with 240 volts for the floor buffer in the gym, the VoltAlert will BEEP on both the Hot and Neutral slots, but the Ground will show as OK. That's the hint not to plug ANYTHING into the outlet since equipment damage will surely result from the over-voltage.
Title: Re: freak accident? help!
Post by: George Friedman-Jimenez on June 16, 2012, 11:06:29 pm
Any recommendation among the Fluke Voltalert 1AC-A1-II, Fluke 2AC, or Amprobe VP-600SB Voltprobe?
Title: Re: freak accident? help!
Post by: Mike Sokol on June 16, 2012, 11:23:29 pm
Any recommendation among the Fluke Voltalert 1AC-A1-II, Fluke 2AC, or Amprobe VP-600SB Voltprobe?

Well, I've been using Fluke gear for more than a quarter century, and it's NEVER let me down. I've been using a VoltAlert for the last year and can always count on it to work. It's ruggedly built and works every time.

But I've got to admit that I'm a bit smitten with the operation of the Amprobe VP-600SB VoltProbe. I like that it's always on (no on/off switch to remember) and has a really bright light with a really loud beeper and really powerful buzzer. So you can see, hear, and feel it alarm. The Amprobe VoltProbe feels like a sold piece of gear, and I think it will hold up quite well. Plus it uses AAA batteries available anywhere.

I've been carrying the VoltProbe around for a week and really like it. But if you're a Fluke guy, then by all means go with the Fluke.

FYI: Amprobe is owned by Fluke and they're both located in Seattle, so for all I know they're all built on the same factory floor.
Title: Re: freak accident? help!
Post by: George Friedman-Jimenez on June 17, 2012, 01:06:23 am
Thanks Mike. Might be useful insurance in some of the substandard buildings we play in. By the way, how often would one expect to encounter a RPBG in real life playing clubs in NYC or NJ? I have never heard of anyone around here getting electrocuted or seriously zapped just playing a gig, although the OP's experience is sobering.
Title: Re: freak accident? help!
Post by: Jonathan Johnson on June 17, 2012, 02:10:54 am
By the way, how often would one expect to encounter a RPBG in real life playing clubs in NYC or NJ?

I don't know anything about construction practices in NYC, but I would guess you're most likely to find RPBG in anything built before the safety ground conductor was required, which I think was about 1974 (based on limited research).

Probably more likely in an older church (volunteer "electricians") or a converted home (DIY) than in a building that's always been a commercial structure. Of course, electricians can make mistakes just like anybody.

This Wikipedia article on cheater plugs (http://en.wikipedia.org/wiki/Cheater_plug) includes a short discussion about audio ground loops.
Title: Re: freak accident? help!
Post by: Mike Sokol on June 17, 2012, 07:17:33 am
I don't know anything about construction practices in NYC, but I would guess you're most likely to find RPBG in anything built before the safety ground conductor was required, which I think was about 1974 (based on limited research).

Probably more likely in an older church (volunteer "electricians") or a converted home (DIY) than in a building that's always been a commercial structure. Of course, electricians can make mistakes just like anybody.

This Wikipedia article on cheater plugs (http://en.wikipedia.org/wiki/Cheater_plug) includes a short discussion about audio ground loops.

You're absolutely correct. Modern wiring should be nearly impossible to hook up incorrectly (note I say "nearly") since you just hook the colors together. Mostly I see swapped H-N connections in modern buildings because some electricians can't tell the diff between a brass screw and a silver screw. Swapped H-N connections are what most of us think of as "reversed polarity". But it's not really dangerous to consumers since modern appliances and gear really don't care if the Hot and Neutral are reversed. That wasn't true in the old days because AC/DC radios and a few early guitar amps (I have one) didn't have power transformers, just a bunch of tubes who's filament voltages added up to 120 volts. That meant the "neutral" wire on the ungrounded power cord was directly connected to the chassis via a "polarized" plug. Hook it up backwards, and the chassis was energized by the now "hot" neutral wire.

However, any building from the early 70's on back never had grounded wiring to begin with. And inspectors in the 80's and 90's required that outlets be updated to NEMA 5-15 outlets with a ground connection. Many millions of bootleg grounds were likely done to get buildings to pass inspection. Of course, if the Hot and Neutral lines are swapped somewhere in the walls, or knob and tube wiring was originally used (everything is black), then at least some of these bootleg outlets were wired with reverse polarity. Just how many RPBG's were created nobody knows, but RPBG outlets are so dangerous because they test as safe using any conventional testing methods such as a 3-light tester or even using $300 Ground Loop Impedance Testers like an INSP-3 or SureTest. You either have to run a test wire to a known earth source such as a copper water pipe or use a non-contact tester which finds earth reference by capacitively coupling through your hand and body. You become the earth reference ground plane.

What's really disturbing is that I contacted the engineering departments at Amprobe, Extech, and SureTest (and a few others) to ask them if they thought their fancy $300 testers could find an RPBG outlet and all them said their products would easily find it. I then showed them my initial demo video and sent them a schematic asking them to duplicate the demonstration. Every one of them acknowledged that they were wrong and that their testers will report an RPBG outlet as having "correct" polarity. Pretty scary, isn't it? 

That's why I think we have to test outlets in all older building (pre 1970's) ourselves. Churches are notorious for not having enough extra money for proper wiring upgrades, and don't get me started on old bars. I'm guessing Tootsies in Nashville could have some RPBG outlets, so perhaps I'll have to spend some time there testing beer and outlets next time I'm in Music City.  ;)

But I am making headway with the information and now writing a feature article for EC&M magazine (Electrical Construction & Maintenance) which is a printed monthly publication going to 120,000 electricians, inspectors and engineers. That will be a great start. I'll let you know when it's available online.

A final note on the Wikipedia link you reference, there's an error when they describe a correct polarity bootleg outlet as being dangerous. It really isn't dangerous unless the neutral wire becomes loose or corroded. Then the appliance body can be energized via it's own load leakage. You'll likely know something is wrong because an open neutral will shut down the appliance. I'm more worried about REVERSE Polarity Bootleg Grounds since there's a 100% certainty that they'll energize the chassis of anything with a grounded power plug to 120 volts AC. And any "grounded" appliance plugged into an RPBG outlet will operate normally, albeit with its chassis energized to 120 volts. 
Title: Re: freak accident? help!
Post by: Mike Sokol on June 17, 2012, 06:31:27 pm
Thanks Mike. Might be useful insurance in some of the substandard buildings we play in. By the way, how often would one expect to encounter a RPBG in real life playing clubs in NYC or NJ? I have never heard of anyone around here getting electrocuted or seriously zapped just playing a gig, although the OP's experience is sobering.

Nobody knows just how many outlets are miswired because up to this point there has been no simple way to test outlets for all reversal conditions. And because RPBG outlets operate completely normal (except for the electrocution and burned up gear possibilities) they can exist for years and never produce any problems. My 18 year old boy just found an RPBG outlet in his bedroom that's been there at least 30 years, way before I bought my house. Since it's in his bedroom on a hardwood floor with only the air-conditioner plugged into it, this was unknown until last week when he tried a VoltAlert on it for grins. Unless he cross connects his computer on a correct outlet to a grounded printer on that RPBG outlet, it could go on for another 30 years without causing problems.

But since this is a 30 second test (at most) that takes around $20 worth of gear, I think it's a good thing to check for in any unknown old building. 
Title: Re: freak accident? help!
Post by: Mike Sokol on June 18, 2012, 08:01:07 pm
Per your PM'ed requests (plus a few of my readers in the EE community) I've replaced the steel screw in the center of the RPBG outlet with a nylon one. That's right, it's electrified to 120 volts AC and a serious shock hazard. You're all correct that I shouldn't take unnecessary chances with electricity. That' s just not setting a good example for the readership. And certainly I'm way too distracted when trying to capture the demonstration on video camera while talking about it. I normally focus completely on the task at hand while working on live circuitry since it only takes a second of inattention to end up dead.

Thanks for the heads up...  8)
Title: Re: freak accident? help!
Post by: George Dougherty on June 19, 2012, 12:06:22 am
I found it at Sears for less than $14  http://www.sears.com/shc/s/p_10153_12605_SP101A11576S6083631101P?sid=IDx20070921x00003a&ci_src=14110944&ci_sku=SPM6045437007
Plus about $7 shipping. 

$15, free shipping.
http://www.ktool.net/servlet/the-6985/VP-dsh-600SB/Detail

Added one of these guys and I'm at $19
http://www.ktool.net/servlet/the-3994/AMPROBE-ST101A-Socket-Tester/Detail

Money very well spent.  Thanks for converting a young fool into a careful tester.  My Multi-meter's going into the kit as well.
Title: Re: freak accident? help!
Post by: Mike Sokol on June 19, 2012, 07:55:00 am
Plus about $7 shipping. 

$15, free shipping.
http://www.ktool.net/servlet/the-6985/VP-dsh-600SB/Detail

Added one of these guys and I'm at $19
http://www.ktool.net/servlet/the-3994/AMPROBE-ST101A-Socket-Tester/Detail

Money very well spent.  Thanks for converting a young fool into a careful tester.  My Multi-meter's going into the kit as well.

Good find on the price(s). Yup, with $20 worth of testers you can slip in your pocket, you should be able to diagnose nearly every outlet mis-wiring condition I can think of. You can even discover when a 120-volt outlet has been jury-rigged for 240 volts. If your non-contact tester beeps on both the hot and neutral slots, but not on the ground slot, then there are two hot wires connected into it. That's how 240 wiring is done on the correct style outlet, but must never be done on a NEMA 5-15 or  5 -20 (Edison) outlets.

That's why I think the best test sequence is to use a non-contact tester on the outlets first, followed by the 3-light tester. Just sweep your non-contact tester from the Hot (small slot) to Neutral (tall slot) to Ground (u-shaped slot), then back to Hot. You always start with the "energized" slot first to verify that your batteries aren't dead in your non-contact tester. If you get the correct pattern of Beep-Silence-Silence-Beep, then plug in your 3-light tester and confirm you get 2 amber lights and no red light. If the outlet passes both tests, then you're good to go. Now this test wont find abnormally high (130+ volts) or low (100-) volts, but those are extremely rare circumstances on installed wiring. However, generator power is its own can of worms.

If you do a lot of festival work outside with generators, I would follow up your basic outlet test with a digital meter and check that H-N measures between 115 and 125 volts unloaded. Of course, if your amplifiers are pulling a lot of watts you might see their AC supply dropping below 110 volts on loud passages (I've seen 105 volts a few times on my own racks) which is not dangerous, but you really want your starting (unloaded) voltage to be as close to 120 volts as possible. At some point I plan to write a piece on testing generator power, including frequency and proper grounding. 

Lots to do... ;D
Title: Re: freak accident? help!
Post by: Jordan Wolf on June 19, 2012, 10:35:21 am
At some point I plan to write a piece on testing generator power, including frequency and proper grounding. 
That would be a welcome article for this industry to have, Mike.  I think the biggest problem most people have is not knowing why, in a particular situation, to do something one way and not another (i.e. how to wire the gennie when you plug straight in to it vs. using a spider box/distro that's distant from the gennie). 

I think it would be a good idea to start a new poll thread to get some ideas for specific topics that apply all-around (following the NEC, of course), but may not be necessarily be mandated in all areas of the country due to more-lax interpretations and utilizations of The Code, etc.
Title: Re: freak accident? help!
Post by: Mike Sokol on September 03, 2013, 08:17:54 am
Just an update on Reverse Polarity Bootleg Ground (RPBG) information. My article in EC&M Magazine "Failures In Outlet Testing Exposed" was published in the July issue and is now available online. Here it is: http://ecmweb.com/contractor/failures-outlet-testing-exposed

Just in the last month I've answered dozens of inquiries from electricians and inspectors around the country about outlet testing, which should eventually translate into safer power for our sound systems. But don't take electrical power for granted. Be sure to test any unknown outlets BEFORE plugging in your expensive sound gear.

Mike Sokol
mike@noshockzone.org
www.NoShockZone.org 

   
Title: Re: freak accident? help!
Post by: Marjan Milosevic on September 03, 2013, 09:32:16 am
Must admit i didnt read all of the comments but one thing got my attention.
Swapping hot and neutral.
How can this make any problem?
I can only conclude that your system is far different fro the Euro standard where you can plug any way you like. Hot and neutral connectors are same. You can put the plug in both ways.
Title: Re: freak accident? help!
Post by: John Roberts {JR} on September 03, 2013, 10:21:30 am
Must admit i didnt read all of the comments but one thing got my attention.
Swapping hot and neutral.
How can this make any problem?
I can only conclude that your system is far different fro the Euro standard where you can plug any way you like. Hot and neutral connectors are same. You can put the plug in both ways.

I bet you can find the answer earlier in this thread...

JR
Title: Re: freak accident? help!
Post by: Steve M Smith on September 03, 2013, 10:30:23 am
Swapping hot and neutral.
How can this make any problem?
I can only conclude that your system is far different fro the Euro standard where you can plug any way you like. Hot and neutral connectors are same. You can put the plug in both ways.


It's not a problem for us in Europe where all outlets have a ground connection.  It also isn't a problem in the US when using three way outlets with a correctly wired ground connection.  In all of thse cases, live and neutral being swapped makes no difference.

There appear to be cases in the US where outlets have been changed from two pin to three pin grounded where rather than run a separate ground wire, it has been done on the cheap and the earth pin has been connected directly to the neutral pin at the outlet.

If the live and neutral are correct, the ground pin is connected to ground via the neutral wire.  However, if live and neutral were reversed at the outlet terminals, the ground pin would now be connected to live - not ideal or safe.


Steve.
Title: Re: freak accident? help!
Post by: Jonathan Johnson on September 03, 2013, 01:30:00 pm
It's not a problem for us in Europe where all outlets have a ground connection.  It also isn't a problem in the US when using three way outlets with a correctly wired ground connection.  In all of thse cases, live and neutral being swapped makes no difference.

In North America there are still old appliances out there that tie the neutral to the chassis, or use the neutral as a zero voltage reference. This is no longer acceptable practice and will not pass UL* testing. For this reason alone, swapping the hot and neutral is a very, very bad idea. (I'm sure Europeans, being smarter than us Americans, have never created anything with this level of stupidity. ;) )

Note that "neutral" is a misnomer. It is indeed NOT "neutral" as it does carry current in a single-pole, single-phase circuit. In the United States, the "neutral" wire is properly termed a "grounded" current-carrying conductor, while the "ground" is properly termed a "grounding" safety conductor (and called "earth" in Europe). The grounded and grounding conductors should be bonded (tied together) only in the service entrance panel. But everybody calls it "neutral" and understands what that means, so I'll continue to use that term.

So you can see that while most devices don't care whether the hot/live and neutral are swapped, people do. If you swap the hot/live and neutral, you increase the potential for electrical shock in the event of a failure.

One significant difference between NA and EU: in EU, single-phase circuits are "single-pole" 240V with a "live" and "neutral" while in NA, single-phase circuits are available in two versions: single-pole 120V with "hot" and "neutral" or double-pole 240V with two "hots." Each leg of the 240V circuit is measured at 120V referenced to neutral. (The terms "live" and "hot" mean the same thing, just differences in terminology between regions of the globe.)

*"UL" is Underwriters' Laboratories, a product safety testing organization in the United States that is supported by the insurance industry. It is considered the premier testing organization in the United States and has great respect here. Any device which passes UL testing gets "listed" which means that UL has determined that it is safe when used as intended.
Title: Re: freak accident? help!
Post by: Jonathan Johnson on September 03, 2013, 01:35:15 pm
A fairly well-written document about the Neutral wire:

http://www.eetimes.com/document.asp?doc_id=1272972
Title: Re: freak accident? help!
Post by: Steve M Smith on September 03, 2013, 03:13:16 pm
That document seems to make sense.

One comment though
Quote
In Europe, the normal 3-wire receptacle is symmetrical so that the neutral and hot wire connections can be swapped by simply rotating the plug.

In the UK we use a different plug which cannot be rotated.  Not that it actually matters because if the live and neutral are reversed in the plug or the socket it just means that the neutral line will have the fuse rather than the live.  The rest of the input circuit will be symmetrical and will work fine.

Unlike US or European plugs, our plug is fused and a 3, 5 or 13 amp fuse is fitted as appropriate to the load.

http://en.wikipedia.org/wiki/BS_1363

Our neutral is connected to earth at the stepdown transformer.  Is this the case in the US or is it grounded where it enters the building?


Steve.
Title: Re: freak accident? help!
Post by: Jonathan Johnson on September 03, 2013, 04:50:10 pm
Our neutral is connected to earth at the stepdown transformer.  Is this the case in the US or is it grounded where it enters the building?

Thank you for the clarification. My experience with European wiring ends at knowing what the typical receptacle looks like. Any other knowledge I purport to claim is gained from Internet research... and I haven't found much that seems authoritative on European wiring.

In North America (Canada, Mexico, United States), the neutral is connected to the ground (earth) at the stepdown transformer and at the service entrance panel. There is typically no grounding (earth) conductor between the stepdown transformer and the service entrance panel -- just a neutral. There is also a grounding rod (earthing rod) just outside the building that connects to the neutral/grounding busbar in the service entrance panel. In a typical installation, the ground and neutral are connected at exactly one point in the building: the service entrance. No other panel can be considered the service entrance; other panels that are fed from the service entrance are called subpanels and must have separate ground and neutral back to the service entrance.

(Sort-of exception in the U.S.: manufactured homes are connected with a grounding conductor and the neutral and ground are NOT tied together in the breaker panel, nor is there a ground rod connected to the panel in the home. However, in the case of manufactured homes, it is required to have a separate disconnect outside the home, mounted on a pole or pedestal. This disconnect is considered the service entrance, and therefore the ground and neutral are bonded at this point, as well as the ground rods. The reason for this separate disconnect as a service entrance is because manufactured homes are still considered mobile structures in the eyes of the National Electrical Code and therefore are treated similarly to recreational vehicles.)
Title: Re: freak accident? help!
Post by: Geoff Doane on September 03, 2013, 08:02:20 pm
No other panel can be considered the service entrance; other panels that are fed from the service entrance are called subpanels and must have separate ground and neutral back to the service entrance.

And there's always one exception to prove the rule.   ;)

You're not likely to run into it in a home, but many commercial buildings will have local step-down transformers feeding specific groups of loads, and in this case, they are considered another service entrance, and the ground will be connected to the newly created neutral at that point.  You're most likely to run into this in arenas, where ice making equipment and discharge lighting runs off higher voltage (277/480V in the US or 347/600V in Canada), or high rise buildings where each floor has its own 120/208 service from a high voltage riser.  An isolation transformer might also be used for a studio or theatre technical power panel, so even if the voltage was the same on primary and secondary, the new neutral would be bonded to ground on the secondary side of the transformer.

I once made the mistake of tying my distro into a 347/600V panel, in a church hall, of all places.  The setup looked a little odd to me, but the thought never crossed my mind that such a configuration could exist.  Very fortunately, I discovered the error before anything blew up.  Ah, the arrogance of youth.  :-[

GTD
Title: Re: freak accident? help!
Post by: Paul Dershem on September 03, 2013, 09:34:59 pm
Just an update on Reverse Polarity Bootleg Ground (RPBG) information. My article in EC&M Magazine "Failures In Outlet Testing Exposed" was published in the July issue and is now available online. Here it is: http://ecmweb.com/contractor/failures-outlet-testing-exposed

Just in the last month I've answered dozens of inquiries from electricians and inspectors around the country about outlet testing, which should eventually translate into safer power for our sound systems. But don't take electrical power for granted. Be sure to test any unknown outlets BEFORE plugging in your expensive sound gear.

Mike Sokol
mike@noshockzone.org
www.NoShockZone.org 

   

Great news! Thanks for sharing the link; I'll be forwarding it to several of my friends and co-workers.
Title: Re: freak accident? help!
Post by: George Friedman-Jimenez on September 03, 2013, 10:41:08 pm
I have seen multiple reviews of different brands of non contact voltage testers that warn of frequent  "false negative" results. Some have gone so far as to call them "death sticks". The claim is that sometimes the unit will indicate no hot wire when there actually is, potentially leading the electrician to trust that the wire is not hot when it actually is. Mike Sokol, you have had a lot of experience with this, have obviously thought a lot about it, and have probably tested many many units against known gold standard situations. My questions are two.

1) In your experience, or in published studies, how common are these "false negative" results when there is enough voltage on the line to cause injury? 1 in 10 tests of known hot wires? 1 in 100 tests of known hot wires? 1 in 1,000 tests of known hot wires? Even less common?

2) Are any of the specific brands of testers less likely than others to give "false negative" results without also giving a lot more "false postive" results?
Title: Re: freak accident? help!
Post by: Per Sovik on September 04, 2013, 03:15:36 am
One significant difference between NA and EU: in EU, single-phase circuits are "single-pole" 240V with a "live" and "neutral" while in NA, single-phase circuits are available in two versions: single-pole 120V with "hot" and "neutral" or double-pole 240V with two "hots." Each leg of the 240V circuit is measured at 120V referenced to neutral. (The terms "live" and "hot" mean the same thing, just differences in terminology between regions of the globe.)

No, in most European countries the phases are "floating", so any single phase can be anything from 0 to 240v relative to ground depending on possible fault conditions that may exist. In practice the supply is 120-0-120, but this cannot be assumed (and certainly not relied upon to supply 120v).
Title: Re: freak accident? help!
Post by: Mike Sokol on September 04, 2013, 07:43:20 am
I have seen multiple reviews of different brands of non contact voltage testers that warn of frequent  "false negative" results. Some have gone so far as to call them "death sticks". The claim is that sometimes the unit will indicate no hot wire when there actually is, potentially leading the electrician to trust that the wire is not hot when it actually is. Mike Sokol, you have had a lot of experience with this, have obviously thought a lot about it, and have probably tested many many units against known gold standard situations. My questions are two.

1) In your experience, or in published studies, how common are these "false negative" results when there is enough voltage on the line to cause injury? 1 in 10 tests of known hot wires? 1 in 100 tests of known hot wires? 1 in 1,000 tests of known hot wires? Even less common?

2) Are any of the specific brands of testers less likely than others to give "false negative" results without also giving a lot more "false positive" results?

I had a table-top full of different brand NCVT's, and have indeed built a calibration standard using a Variac feeding different size charged surfaces and wire types. While I've not done a formal survey, I've noted some trends.

#1)  I don't trust Always On testers. These are the types that don't have an ON/OFF switch, but which start buzzing/beeping when finding a charged surface. Since there's no blinking light to tell you the battery is OK and the unit is in "on" mode, then it's possible to have a dead battery which means the unit won't light up near a wire that's actually on. This is a modern trend towards destupification (that should be a word) since the manufacturer assumes we're too stupid to push the ON switch.  :P

#2)  I don't like variable sensitivity testers with sliding/rotary voltage threshold. Since you won't have a know voltage to calibrate it in the field, the tendency is to keep moving the threshold up until it doesn't beep at all or down until it beeps on everything. Warning gear should NEVER have a threshold/sensitivity adjustment. You don't want to calibrate your radiation counter to whatever you feel like, do you?  ;D

#3)  The best testers for basic field operation as I describe are the ones rated for 90 to 1,000 volts sensitivity. Those will find a romex wire as low as 90 volts, or something the size of a microphone as low as 40 volts, or a microphone at 120-volts from 4 inches away. As you can see in my video at http://www.youtube.com/watch?v=Y8h64X33aKg  it will find an RV with 120-volts on it from nearly 2 feet away.  :D

#4)  Don't put yourself in a Faraday cage or isolate yourself from the earth. I think this is what gives these NCVT "death sticks" such a bad rap. The manufacturers say that standing on a fiberglass ladder can isolate you enough from the earth so that a NCVT won't beep on an active wire, but I've not found that to be the case. All of my testers beep just fine while I'm on a ladder. However, I've also not tried this in a attic to be sure, so that would be an interesting experiment. Also, if you're inside a charge box (like an RV) then pointing a NCVT at a water faucet inside the RV won't cause it to beep, because your body and the test surface are at exactly the same voltage. Duh!! But standing on the RV steps and pointing it out at the ground WILL make it beep. Which makes perfect sense when you think about it.  ::)

5)  It's supposed to be possible to hold a NCVT exactly in the middle of a 3-phase circuit and have it cancel out so it won't beep, but that seems like a crazy stretch to me. I have access to an industrial building with 3-phase power, so perhaps I'll give it a try, just for grins. But that's a pretty far-out scenario.  ???

6)  I look for a NCVT that's rugged enough to stand up to being thrown in a toolbox, so the Fluke VoltAlert (not their always-on model) and the Klein NCVT-1 (not their dual-sensitivity model) are my favorites right now. Amprobe is sending me some of their newest products to test soon, and if it meets my above criteria, that could be another favorite.  :)

7)  Don't go cheap... Your life depends on it. The most expensive Non Contact Voltage Testers only cost about $25, so go ahead and splurge. I don't think I would trust my life to a $5 tester from Harbor Freight or those foreign $7 with "shipping included" eBay deals. A good piece of test gear should last you for decades, just like a guitar or microphone. Buy the best you can afford.  8)

Mike Sokol
mike@noshockzone.org
www.noshockzone.org
   
Title: Re: freak accident? help!
Post by: Jerome Malsack on September 04, 2013, 07:59:45 am
Another question on the subject would be what is the effect of a  Ground Fault Interrupt GFI on the resulting problems.  I feel the  GFI would trip and shut down and potentially save the snake or DI box.  This would not fix the problem however.  It just screams trouble. 
Title: Re: freak accident? help!
Post by: Mike Sokol on September 04, 2013, 08:30:19 am
Another question on the subject would be what is the effect of a  Ground Fault Interrupt GFI on the resulting problems.  I feel the  GFI would trip and shut down and potentially save the snake or DI box.  This would not fix the problem however.  It just screams trouble.

Here's the really interesting thing. I'm pretty sure that a GFCI in the receptacle itself MAY NOT trip during a ground fault event if it was mis-wired as an RPBG. And it certainly won't disconnect the ground pin from the incoming "hot" line even if it DOES trip. I see this in my head and have drawn it out and traced it on paper, but have never mis-wired one like this intentionally. But I'm in the 90% sure confidence range on this statement.

Wouldn't that be a real kicker if it's indeed true? I think this may have been the mis-wiring problem which killed the 12-year old girl in Orlando last year from a water feature pump on a miniature golf course. The news report said the GFCI was mis-wired so it didn't stop the current from killing her. I never saw the police report or pictures of the mis-wired GFCI, but it sure sound suspicious to me. Now THAT would be an interesting experiment to design and run.

Wonder if PSW would sponsor such an experiment. Is the forum moderator listening?

Mike Sokol   
Title: Re: freak accident? help!
Post by: Bob Leonard on September 04, 2013, 08:41:29 am
Mike,
I've been following this thread and would like to first say thank you for the time and effort you've spent on the subject.

I'm in the middle of a remodel project at home and am installing 12 6" remotely dimmed recessed lights, 14 outlets, media center wiring, etc. Basically I've gutted and rebuilt the living room, moved some walls, removed all of the 65 year old cable, etc.. Yesterday my Jr. electrician black lab ran away with my Fluke tester which has now gone to lab toy heaven. I needed to pick up some stock, and while doing that saw the Klein NCVT-2 and bought one.

Why don't you like the -2 dual range tester?
Title: Re: freak accident? help!
Post by: Mike Sokol on September 04, 2013, 08:51:38 am
Mike,
I've been following this thread and would like to first say thank you for the time and effort you've spent on the subject.

I'm in the middle of a remodel project at home and am installing 12 6" remotely dimmed recessed lights, 14 outlets, media center wiring, etc. Basically I've gutted and rebuilt the living room, moved some walls, removed all of the 65 year old cable, etc.. Yesterday my Jr. electrician black lab ran away with my Fluke tester which has now gone to lab toy heaven. I needed to pick up some stock, and while doing that saw the Klein NCVT-2 and bought one.

Why don't you like the -2 dual range tester?

I do like the Klein NCVT-2 and works just fine in my tests, but for the casual user it's way easy to get confused when setting the high-low/beep-quite modes. For a professional like yourself I think it's a good choice.

You do read directions, don't you? We all know that 99% of the time the directions are still unread in the shrink wrap years later. Keeps them nice and safe, I guess... For the average consumer, just getting them to remember to turn on the switch is asking too much. 

Mike Sokol
Title: Re: freak accident? help!
Post by: Mike Sokol on September 04, 2013, 09:08:36 am
Duh, double post....
Title: Re: freak accident? help!
Post by: Mike Sokol on September 04, 2013, 09:10:02 am
Yesterday my Jr. electrician black lab ran away with my Fluke tester which has now gone to lab toy heaven.

PLEASE post a picture of the Fluke chew-toy.

Mike
Title: Re: freak accident? help!
Post by: Geoff Doane on September 04, 2013, 09:37:28 am
Here's the really interesting thing. I'm pretty sure that a GFCI in the receptacle itself MAY NOT trip during a ground fault event if it was mis-wired as an RPBG. And it certainly won't disconnect the ground pin from the incoming "hot" line even if it DOES trip. I see this in my head and have drawn it out and traced it on paper, but have never mis-wired one like this intentionally. But I'm in the 90% sure confidence range on this statement.

Wouldn't that be a real kicker if it's indeed true? I think this may have been the mis-wiring problem which killed the 12-year old girl in Orlando last year from a water feature pump on a miniature golf course. The news report said the GFCI was mis-wired so it didn't stop the current from killing her. I never saw the police report or pictures of the mis-wired GFCI, but it sure sound suspicious to me. Now THAT would be an interesting experiment to design and run.


If the GFCI was wired as RPBG, then it is a shock hazard by definition.  The hot wire is connected to ground, and nothing inside the GFCI will disconnect the ground, no matter what happens.  It should still trip if a 2-wire device was plugged in and had a ground fault.  This would be a rare case of a 2-wire device being safer than a properly "grounded" one.

Apparently a common mistake with GFCIs is that the installer mixes up the LINE and LOAD sides, which even with a proper ground, makes them ineffective.  To make matters worse, I suspect the TEST button on the GFCI will still indicate that it is working properly, even if it's mis-wired. 

Older GFCIs simply compared the current on the hot and neutral, and tripped if the difference exceeded a certain threshold.  This doesn't protect against an open neutral, because the circuit needed to be powered for it to work.  Newer units add the requirement of voltage potential on the LINE side before they will make the connection to the outlet and LOAD terminals.

GTD
Title: Re: freak accident? help!
Post by: TJ (Tom) Cornish on September 04, 2013, 09:37:52 am
PLEASE post a picture of the Fluke chew-toy.

Mike
The Flukes are CAT rated.  Maybe they need to work on a DOG rating.

Ba dum...
Title: Re: freak accident? help!
Post by: Bob Leonard on September 04, 2013, 12:17:45 pm
I wish I could post a picture. we let her get to be a fatty (88lbs), and the vet had us put her on a diet. She's down in weight now, but always hungry. So I'm working on the wiring and I hear a beeping sound. This lab goes shithouse over chew toys that beep so at first I thought she had one of her toys until I hear my wife yell "God damn it Lucy, give that back.", the universal lab signal to play keep away.

Well the door was open so the dog ran out to the fenced yard, all the time going beep, beep, beep. I got down off of the ladder, went outside, and by the time I could get to her and chase her down it was gone to doggy toy heaven without a trace.
Title: Re: freak accident? help!
Post by: Mike Sokol on September 04, 2013, 12:47:57 pm
I got down off of the ladder, went outside, and by the time I could get to her and chase her down it was gone to doggy toy heaven without a trace.

My wife had Dobermans when she was growing up. One would eat ANYTHING, such as a hamburger right out the hot frying pan on the stove, or a beret from her school band uniform, and a Timex watch right off the dresser. The beret and watch made it through the doggie recycling process, but the beret was never the same, and the Timex didn't "keep on ticking" since the crystal broke on a tooth while the dog was chomping it down. They were planning to send the watch into the Timex "Keeps On Ticking" television ad campaign at the time, but it was not to be...

Perhaps someday you'll find your Fluke tester in a hole somewhere. If so, you should see if it still works and offer to send it to Fluke for a replacement. I'll bet they'll send you a new one in trade just for fun.

Mike Sokol
Title: Re: freak accident? help!
Post by: Mark McFarlane on September 04, 2013, 02:09:29 pm
A few 'curious' observations from my year-old Fluke Volt Alert 2AC (90-1000V) (always on, but with a battery test button)

1) Anywhere within two inches of my Macbook Pro wallwart and it lights up
2) Anywhere within about 4 inches of my MacBook Pro laptop it lights up
3) Anywhere within an inch of my Apple Airport Express and it lights up
4) On a cheap outlet strip (9 outlets spread over 30 inches) (that is plugged into a GFCI outlet) it doesn't light up anywhere, including inserted into the hot legs, but the outlet strip powers devices.  The GFCI 'source' tests fine.  I haven't opened up the outlet strip (yet) to see how it is wired.

On a dozen other outlets around the house it seems to work correctly.
Title: Re: Defecation Occurs
Post by: Russ Davis on September 04, 2013, 02:10:18 pm
...by the time I could get to her and chase her down it was gone to doggy toy heaven without a trace.

PLEASE post a picture of the Fluke chew-toy.

If "without a trace" means what I think it does, it'll need a thorough cleaning once it reappears.
Title: Re: freak accident? help!
Post by: Mike Sokol on September 04, 2013, 02:59:05 pm
A few 'curious' observations from my year-old Fluke Volt Alert 2AC (90-1000V) (always on, but with a battery test button)

1) Anywhere within two inches of my Macbook Pro wallwart and it lights up
2) Anywhere within about 4 inches of my MacBook Pro laptop it lights up
3) Anywhere within an inch of my Apple Airport Express and it lights up
4) On a cheap outlet strip (9 outlets spread over 30 inches) (that is plugged into a GFCI outlet) it doesn't light up anywhere, including inserted into the hot legs, but the outlet strip powers devices.  The GFCI 'source' tests fine.  I haven't opened up the outlet strip (yet) to see how it is wired.

On a dozen other outlets around the house it seems to work correctly.

I've recently done some tests on iPhones plugged into Apple chargers, and found much the same thing. Basically, anything without a ground plug (double insulated) plugged into a wall outlet will bias up to around 50% of the line voltage. So depending on which way you plug in your wall wart, you'll likely measure 40 to 80 volts on chassis of the appliance or gear. Now, this isn't normally dangerous because the double insulation keeps the leakage current through you to less than 1 mA (I think UL limits are 0.5 mA, if memory serves).

However, if your wall wart becomes damaged (dropped too many times) or wet (dunked in the shower), then I do believe it's possible for this double-insulation to fail, which will pass the line current into you. And if you're on something nicely grounded (or standing in the shower like that woman in China last month) and reach out to pick up your iPhone while plugged into a wall charger, then you can die from electrocution.

Again, 2 or 3 mA is a small shock. 10 mA is a big shock. 20 mA you can't let go of the conductors. 30 mA your heart goes into fibrillation in a few seconds. 100 mA nerve damage starts and death is almost certain. Since your body has about 1,000 ohm hand to hand or foot, then it's easy to do the math and see that 30 volts AC can cause 30 mA of current to flow through your heart. Without immediate CPR and AED intervention, you're now dead.

I just won't touch anything directly plugged into an ungrounded wall wart while standing in water. Way too dangerous for me, and I've done some pretty crazy technical things.

Much more on this later, but I'm so glad you brought up the point. Yes, you're iPhone is indeed at 60 volts AC while holding it to your ear while plugged into a wall charger. The current limiting of the insulation is the only thing keeping you alive. Pretty scary when you think about it.

Title: Re: freak accident? help!
Post by: Jonathan Johnson on September 04, 2013, 04:27:48 pm
#1)  I don't trust Always On testers. These are the types that don't have an ON/OFF switch, but which start buzzing/beeping when finding a charged surface. Since there's no blinking light to tell you the battery is OK and the unit is in "on" mode, then it's possible to have a dead battery which means the unit won't light up near a wire that's actually on. This is a modern trend towards destupification (that should be a word) since the manufacturer assumes we're too stupid to push the ON switch.  :P

...

#4)  Don't put yourself in a Faraday cage or isolate yourself from the earth. I think this is what gives these NCVT "death sticks" such a bad rap. The manufacturers say that standing on a fiberglass ladder can isolate you enough from the earth so that a NCVT won't beep on an active wire, but I've not found that to be the case. All of my testers beep just fine while I'm on a ladder. However, I've also not tried this in a attic to be sure, so that would be an interesting experiment. Also, if you're inside a charge box (like an RV) then pointing a NCVT at a water faucet inside the RV won't cause it to beep, because your body and the test surface are at exactly the same voltage. Duh!! But standing on the RV steps and pointing it out at the ground WILL make it beep. Which makes perfect sense when you think about it.  ::)

...

6)  I look for a NCVT that's rugged enough to stand up to being thrown in a toolbox, so the Fluke VoltAlert (not their always-on model) and the Klein NCVT-1 (not their dual-sensitivity model) are my favorites right now. Amprobe is sending me some of their newest products to test soon, and if it meets my above criteria, that could be another favorite.  :)

I don't remember the model, but I have what must be one of the first non-contact voltage detectors on the market. If memory serves me correcly it was marketed by Gardner-Bender, but all markings have long since worn off. I think it's only good for AC. I've had it for at least 15 years, maybe 20. Alas, this model isn't made anymore.

It has no power switch. It has no battery test. It's always on. It flickers when you bump it against something. It rattles around in my toolbox. I last changed the batteries 10+ years ago and it still works.

I can stick it in the hot side of an outlet and it lights up, even when I let go of it. It doesn't depend on the capacitance of my body. That's useful when I need to identify a circuit. I've had others, but this one has always been the most trustworthy and my favorite.

But before I use a tester, I always test it against known conditions. Nobody hears the watchdog that doesn't bark.
Title: Re: freak accident? help!
Post by: Dan Mortensen on September 04, 2013, 06:35:17 pm
I had a table-top full of different brand NCVT's, and have indeed built a calibration standard using a Variac feeding different size charged surfaces and wire types. While I've not done a formal survey, I've noted some trends.

#1)  I don't trust Always On testers. These are the types that don't have an ON/OFF switch, but which start buzzing/beeping when finding a charged surface. Since there's no blinking light to tell you the battery is OK and the unit is in "on" mode, then it's possible to have a dead battery which means the unit won't light up near a wire that's actually on. This is a modern trend towards destupification (that should be a word) since the manufacturer assumes we're too stupid to push the ON switch.  :P


Hi Mike,

First, thanks for this great thread! I missed it the first time, and am glad you revived it.

Thanks to you, I'm in the process of buying a bunch of non-contact testers and socket testers for my crew.

You said the above today, but in June you found this:

I've just received a care package from Amprobe for more of my testing, and really like their little VP-600SB VoltProbe. Not only is it on all the time (they tell me a year or more battery life) it runs on two standard AAA batteries. Plus it not only beeps (loudly) and blinks (brightly), it also shakes in your hand when it triggers. Plus it's sorta flat with a pen grip to stay in your pocket. I found it at Sears for less than $14  http://www.sears.com/shc/s/p_10153_12605_SP101A11576S6083631101P?sid=IDx20070921x00003a&ci_src=14110944&ci_sku=SPM6045437007


What changed?

Thanks,
Dan
Title: Re: freak accident? help!
Post by: Mike Sokol on September 04, 2013, 07:29:19 pm

You said the above today, but in June you found this:

What changed?

Thanks,
Dan

I've had one tester with a dead battery scare me a bit when it didn't indicate on a live circuit. Since I always double-check on a known live circuit everything was safe for me. However, I kept thinking about the average user who won't check to make sure it beeps on something. They'll just go diving in with a tester that has a dead battery and possibly get hurt or killed.

Of course, if you're NOT average and understand the risks and how to double-check your gear, then this is a great tester. But I'm getting more cautious as I teach consumers how to use a NCVT to detect hot-wire and hot-skin conditions on their RV.
Title: Re: freak accident? help!
Post by: Tim Perry on September 04, 2013, 11:06:23 pm
I've had one tester with a dead battery scare me a bit when it didn't indicate on a live circuit. Since I always double-check on a known live circuit everything was safe for me. However, I kept thinking about the average user who won't check to make sure it beeps on something. They'll just go diving in with a tester that has a dead battery and possibly get hurt or killed.

Of course, if you're NOT average and understand the risks and how to double-check your gear, then this is a great tester. But I'm getting more cautious as I teach consumers how to use a NCVT to detect hot-wire and hot-skin conditions on their RV.

If you are above average you may buy the more expensive lithium batteries that last 12 times longer for this application.
Title: Re: freak accident? help!
Post by: Nathan Vanderslice on September 05, 2013, 01:58:52 am
My wife had Dobermans when she was growing up. One would eat ANYTHING, such as a hamburger right out the hot frying pan on the stove, or a beret from her school band uniform, and a Timex watch right off the dresser. The beret and watch made it through the doggie recycling process, but the beret was never the same, and the Timex didn't "keep on ticking" since the crystal broke on a tooth while the dog was chomping it down. They were planning to send the watch into the Timex "Keeps On Ticking" television ad campaign at the time, but it was not to be...

Perhaps someday you'll find your Fluke tester in a hole somewhere. If so, you should see if it still works and offer to send it to Fluke for a replacement. I'll bet they'll send you a new one in trade just for fun.

Mike Sokol

This reminds me of a story I heard years ago when an event was happening at an estate. Long story short, the day of load out, they backed the truck up that was picking up the piano. Bear in mind that this was a stage built on a slope. Well they had gotten the truck in position so that the truck bed and stage lined up. By this time there was a torrential downpour. Somehow the crew moving the piano completely missed the truck and the piano went off the stage to the low side of the hill and into the mud. It was taken back to the company, and they were so impressed at how well it held together, all they charged them for was the broken ivory on one of the keys.
Title: Re: freak accident? help!
Post by: Mike Sokol on September 05, 2013, 06:34:25 am
If you are above average you may buy the more expensive lithium batteries that last 12 times longer for this application.

Hey, it had the cheap OEM batteries in it from the factory. But yes, that's why you buy the best batteries possible.  ;)

More than 30 years ago I had a voltmeter with cheap test leads, and one of the plugs fell out while I testing a power supply. Of course, the supply appeared to be "off" when it was indeed "hot".  But as I was reaching my hand into the supply I noticed the test lead plug laying beside the meter. I stopped quickly enough, since reaching a bare hand into a live high-voltage supply is a bad idea. After that I bought the best meter leads and meters I could get. I still remember this incident like it was yesterday. This electrical stuff can kill you if you're not careful.
Title: Re: freak accident? help!
Post by: Dan Mortensen on September 08, 2013, 02:42:56 am
Thanks to this thread, I got one of the Fluke probes and also the Amprobe one.

The buzzing of the Amprobe is nice, but it reacts the same to 120v and to whatever power is on a USB cable. It even reacts the same to the face of an iPad 1. It also doesn't necssarily react the same way to the same stimulus each time.

The Fluke, OTOH, has but a single beep at the lower voltage rather than a continuous tone, and behaves consistently.

Yes, I read Mike's response to Mark about odd behavior, but the two respond differently to the same stimulus and I like the Fluke's action better.

Also, I got one of the Amprobe INSP-3's, and it pops all the GFCI's in my kitchen when plugging in. (They are all the same make/model/vintage.) That's without activating the GFCI mode. Will test on three other GFCI make/model/vintages before saying anything more. Interesting data on non-GFCI's, though.

Thanks again for publicizing this, Mike.

Dan

Title: Re: freak accident? help!
Post by: Mark McFarlane on September 08, 2013, 09:54:05 am
Thanks to this thread, I got one of the Fluke probes and also the Amprobe one....

Fluke alone sells 5 or more models. Which one did you get?
Title: Re: freak accident? help!
Post by: Mike Sokol on September 08, 2013, 10:35:53 am
The buzzing of the Amprobe is nice, but it reacts the same to 120v and to whatever power is on a USB cable. It even reacts the same to the face of an iPad 1. It also doesn't necessarily react the same way to the same stimulus each time.

FYI: I'm going to be presenting a 90-minute No~Shock~Zone seminar for the student AES chapter at Shenandoah University in Winchester, VA on Wednesday, Sept 18 starting at 6 PM. It will include a bunch of different NCVT demonstrations on various electrified objects such as guitars and microphones, as well as my Ground Loop hum diagnostics clamp procedure, and an introduction of my latest theory on GLID (Ground Loop Inter-modulation Distortion) in sound systems and how it affects bass clarity. We'll take some pictures and post them here after the seminar. It's free and open to the pro-sound public, but you should contact me directly in advance if you want to attend so I can put your name on "the list". 
Title: Re: freak accident? help!
Post by: Dan Mortensen on September 08, 2013, 02:32:27 pm
Fluke alone sells 5 or more models. Which one did you get?

Sorry, didn't realize that.

This one:

http://www.amazon.com/gp/product/B000EJ332O/ref=oh_details_o01_s00_i00?ie=UTF8&psc=1

It was a little cheaper, and came from a different Amazon source.
Title: Re: freak accident? help!
Post by: Mike Sokol on September 08, 2013, 06:23:15 pm
This one:

http://www.amazon.com/gp/product/B000EJ332O/ref=oh_details_o01_s00_i00?ie=UTF8&psc=1


That's exactly the one I've used in hundreds of NO~Shock~Zone hot-chassis experiments and seminars. Get the standard 90-1000 volt version, which will find a hot microphone or guitar down as low as 40 volts.

I think that spending $25 to potentially save your life is a pretty good payback.  ;)
Title: Re: freak accident? help!
Post by: Dan Mortensen on September 08, 2013, 07:28:04 pm
That's exactly the one I've used in hundreds of NO~Shock~Zone hot-chassis experiments and seminars. Get the standard 90-1000 volt version, which will find a hot microphone or guitar down as low as 40 volts.

I think that spending $25 to potentially save your life is a pretty good payback.  ;)

Good, thanks, and I agree on the return on investment, which is why I'm giving them to my crew as a thanks for doing such great work this summer. There's a 5 pack offer on Amazon for $22.80 each, which along with the neon light checkers will give them tools to be electrically safe at work and at home.

Thanks again for your work. I wish I was closer to VA to see your talk. Do you ever come West? I'm confident my AES Section would be happy to host you sometime.
Title: Re: freak accident? help!
Post by: Mike Sokol on September 08, 2013, 10:18:26 pm

I wish I was closer to VA to see your talk. Do you ever come West? I'm confident my AES Section would be happy to host you sometime.

I'm not coming out to the west coast until spring 2014, but I will be in eastern Texas Oct 12 thru 17 next month. I've taught seminars in the past at Austin Community College, and since I'll have downtime on Monday-Wednesday that week, I've already contacted the staff there to see if I could hookup with the student AES chapter to do a No~Shock~Zone presentation. Is anyone on this forum with the Austin student AES chapter, or any other pro-audio organizations in the area? If so, contact me directly at mike@noshockzone.org. 
Title: Re: freak accident? help!
Post by: Jonathan Johnson on September 09, 2013, 02:09:43 am
To summarize this thread:

If you've ever been shocked by touching an appliance or audio device or know someone who has, suspect an RPBG issue. It's not a minor issue; someone could get killed. Take it seriously and don't let anyone poo-poo the problem.

Further reading:

(P.S. -- I posted a similar summary in another thread that refers to this thread. I apologize in advance for making you read it twice.)
Title: Re: freak accident? help!
Post by: Mike Sokol on September 09, 2013, 06:57:27 am
To summarize this thread:

That's an excellent summary. Thanks for putting it together.

The only addition I might note is that ALL bootleg grounds (reverse or correct polarity) are a violation of National electrical code and should be corrected. Also, while a CPBG (Correct Polarity Bootleg Ground) receptacle doesn't appear to be immediately dangerous since its ground isn't hot, if the neutral wire is lost somehow (loose screw or corrosion) then anything with a ground plug connected to it will become hot-chassis energized to 120 volts.

I'm also working on a theory describing how double-bonded G-N bus bars in electrical sub panels as well as CPBG mis-wired outlets can cause an audio distortion I've termed "GLID" for Ground Loop Inter-modulation Distortion. Could explain why some sound systems have undefined or "fuzzy" bass. Stay tuned.... 

Title: Re: freak accident? help!
Post by: Dan Mortensen on September 09, 2013, 01:35:40 pm
Thanks for this summary. I thought the part about indicating voltage presence on double insulated and computer parts was important, too, so the user can know what's a problem and what's likely not a problem.

To summarize this thread:
  • 2-wire ungrounded 120V circuits (hot and neutral only) may be present in older construction. The insulation on very old wiring can be discolored making it difficult to distinguish hot and neutral -- if the wires even were color-coded in the first place.
  • When a receptacle is replaced or added to an ungrounded 2-wire circuit, the hot and neutral may be inadvertently reversed.
  • If a grounding-type receptacle is used on this circuit, the installer may "bootleg" the ground by jumpering between the ground and neutral connections.
  • When both of the above conditions are present, the "ground" wire will be hot: 120V relative to true earth ground. This condition has been termed by Mike Sokol (who has performed extensive research into the issue) to be a "Reverse Polarity Bootleg Ground" (RPBG).
  • RPBG cannot be detected by most traditional testing methods. Using a neon test lamp, a three-light receptacle tester, or a voltmeter will not reveal a problem with the wiring unless referenced to a second known, tested & trusted grounding reference.
  • The use of a non-contact voltage tester (NCVT) can be used fairly reliably to identify RPBG conditions, when used in conjunction with the above tools. However, certain conditions may still fool the NCVT.
  • A person who comes in contact with a metal device connected to an RPBG receptacle and another grounded object will be subjected to a serious electrical shock.
  • GFI breakers and circuit breakers may not protect against RPBG faults.
  • Churches may be the most likely structures to experience wiring problems due to the prevalence of old construction and potentially unqualified volunteer labor. Wiring problems can occur in new construction as well.
  • Surveys and research indicate RPBG may be a widespread problem.
  • If this condition (or any abnormal electrical condition) is observed, the faulty circuit must immediately be locked and tagged out-of-service until a qualified electrician can rectify the problem.

If you've ever been shocked by touching an appliance or audio device or know someone who has, suspect an RPBG issue. It's not a minor issue; someone could get killed. Take it seriously and don't let anyone poo-poo the problem.

Further reading:
  • Mike Sokol's article summarizing his research: http://ecmweb.com/contractor/failures-outlet-testing-exposed
  • Tom (TJ) Cornish's discussion of basic receptacle testing: http://soundforums.net/hub/1856-basic-receptacle-testing.html

(P.S. -- I posted a similar summary in another thread that refers to this thread. I apologize in advance for making you read it twice.)
Title: Re: freak accident? help!
Post by: Mike Sokol on September 09, 2013, 01:49:49 pm
Thanks for this summary. I thought the part about indicating voltage presence on double insulated and computer parts was important, too, so the user can know what's a problem and what's likely not a problem.

Yes, that's a good point. I'll note here that a NCVT is an accurate test on anything with a GROUND PLUG. There are many appliances that use double-insulation to protect you from shock (such as iPhone chargers and computer power supplies), and these will cause a NCVT to beep even though the appliance is safe to touch. That's because the computer or iPhone or even your wife's slow cooker in the kitchen actually DOES have 60 volts or so on the chassis. But the current is limited to below 1 mA, so you're not electrocuted. Makes me feel a bit uncomfortable when I measure it though....

However, I believe that ALL stage and studio gear should be grounded somehow since there's way too many grounding paths for you to get shocked by if something goes wrong in the double insulation of the power supply. And no matter how well you think you have everything grounded, it's still foolhardy to wade into a baptismal pool playing an electric guitar plugged into a stage amp (I've personally seen this) or holding a hard-wired microphone which provides the ground for water that's been electrified by a broken electric heater element (I've read the obits and written articles about it).

I'm so glad this forum has embraced the idea of electrical safety and spread it around the world. Thank you all for your time and attention to this serous issue. Getting shocked is no laughing matter.   
Title: Re: freak accident? help!
Post by: Jerome Malsack on September 09, 2013, 02:39:43 pm
Besides the audio sound system this problem could present itself with a TV/video network, Or Computer network. 

Not as likely with the phones but more phones are starting to have a power wall plug. 
Title: Re: freak accident? help!
Post by: Jonathan Johnson on September 09, 2013, 05:02:45 pm
Thanks for this summary. I thought the part about indicating voltage presence on double insulated and computer parts was important, too, so the user can know what's a problem and what's likely not a problem.

Thanks for reminding me of that. At the risk of making the summary as long as the whole thread, I modified it with a couple of points:

Besides the audio sound system this problem could present itself with a TV/video network, Or Computer network. 

Not as likely with the phones but more phones are starting to have a power wall plug. 

Any device with grounded components, whether an audio system, refrigerator, or power tool can be hazardous when connected to an RPBG receptacle, or a CPBG receptacle with a failed neutral wire upstream of the bootleg ground.

Any system with multiple components interconnected by signal lines with a grounded shield can be damaged by these conditions.
Title: Re: freak accident? help!
Post by: Jonathan Johnson on September 09, 2013, 05:19:13 pm
And no matter how well you think you have everything grounded, it's still foolhardy to wade into a baptismal pool playing an electric guitar plugged into a stage amp (I've personally seen this) or holding a hard-wired microphone which provides the ground for water that's been electrified by a broken electric heater element (I've read the obits and written articles about it).

The underlying lesson is that people using a baptismal pool should ONLY use wireless microphones and interconnects. Wired microphones should not be within reach of the baptismal pool, and the pump and heater for the baptismal pool should be connected to a GFI breaker (required for hot tubs by the National Electrical Code).

It's interesting to note that the National Electrical Code specifies an exclusion zone above bathtubs: any lighting fixture within three feet horizontally or eight feet vertically above the rim of the bathtub must be listed as suitable for damp or wet locations. Any receptacle within six feet of a sink, tub, or other water source must also be GFI protected. I think we can apply these same principles to audio devices and baptismal pools.
Title: freak accident? help!
Post by: Matt Edmonds on September 11, 2013, 01:02:52 pm
Glad I found this thread. I will be buying a voltalert ASAP. I found some 277v ac the hard way trying to repair some ceiling tiles in a church. However this was due to my own negligence. Let's just say I'm lucky to only have a scar on my left hand. It could have been way worse. Anyways thanks for this super informative thread. :)(http://img.tapatalk.com/d/13/09/12/4eqyheny.jpg)
Title: Re: freak accident? help!
Post by: Tommy Peel on September 11, 2013, 04:17:03 pm
I've recently done some tests on iPhones plugged into Apple chargers, and found much the same thing. Basically, anything without a ground plug (double insulated) plugged into a wall outlet will bias up to around 50% of the line voltage. So depending on which way you plug in your wall wart, you'll likely measure 40 to 80 volts on chassis of the appliance or gear. Now, this isn't normally dangerous because the double insulation keeps the leakage current through you to less than 1 mA (I think UL limits are 0.5 mA, if memory serves).

However, if your wall wart becomes damaged (dropped too many times) or wet (dunked in the shower), then I do believe it's possible for this double-insulation to fail, which will pass the line current into you. And if you're on something nicely grounded (or standing in the shower like that woman in China last month) and reach out to pick up your iPhone while plugged into a wall charger, then you can die from electrocution.

Again, 2 or 3 mA is a small shock. 10 mA is a big shock. 20 mA you can't let go of the conductors. 30 mA your heart goes into fibrillation in a few seconds. 100 mA nerve damage starts and death is almost certain. Since your body has about 1,000 ohm hand to hand or foot, then it's easy to do the math and see that 30 volts AC can cause 30 mA of current to flow through your heart. Without immediate CPR and AED intervention, you're now dead.

I just won't touch anything directly plugged into an ungrounded wall wart while standing in water. Way too dangerous for me, and I've done some pretty crazy technical things.

Much more on this later, but I'm so glad you brought up the point. Yes, you're iPhone is indeed at 60 volts AC while holding it to your ear while plugged into a wall charger. The current limiting of the insulation is the only thing keeping you alive. Pretty scary when you think about it.

That is pretty scary... Just curious do laptop(including MacBook) chargers that have a ground exhibit this effect? On my MacBook Pro's charger you can use the longer cord which has a ground(and metal lug in on the charger end to make it seem like it does something) or the one that lets you use it as a wall wart style two prong charger.
Title: Re: freak accident? help!
Post by: Mike Sokol on September 11, 2013, 06:36:29 pm
That is pretty scary... Just curious do laptop(including MacBook) chargers that have a ground exhibit this effect? On my MacBook Pro's charger you can use the longer cord which has a ground(and metal lug in on the charger end to make it seem like it does something) or the one that lets you use it as a wall wart style two prong charger.

The longer cord on the MacBook chargers does indeed have a ground pin on the plug which will ground the chassis of your laptop. So the short "ungrounded" plug will allow your laptop chassis to float around 60 volt AC or so, while the long "grounded" wire will tie your laptop chassis to the receptacle's safety ground. I never worry about this in the house, but plugging your laptop into an ungrounded "wall wart" while relaxing by the pool would be a bad idea.

Again, ANYTHING with an ungrounded plug is "supposed" to be double-insulated and not allow more than a fraction of a milliamp of shocking current. HOWEVER that double-insulated shock barrier can be defeated by damage to the "wall wart" such as dunking in water or perhaps dropping it too many times and breaking something internally. I've not done any serious tests on this, but I do know that if you use a high-impedance meter on an "ungrounded" appliance, stage amp, mixing console, or whatever, it will generally have around half of line voltage on it. So while 60 volts at 0.5 mA isn't a shock hazard, if the current leakage goes up, then 60 volts could cause up to 60 mA of current through your body. And only 30 mA of shock current is deadly.   

Please keep me posted of any shock condition you find. I'm building a database.
Title: Re: freak accident? help!
Post by: Tom Bourke on September 19, 2013, 07:31:57 pm
Please keep me posted of any shock condition you find. I'm building a database.
I have a couple for you.

I have run into MANY VCRs or DVD/VCR combo units, polarized cords, that will put about 30 to 35 volts on the chassis and shell of any coax connectors on the back if plugged into a revers polarity outlet.  I have gotten shocked by this and seen it damage other equipment.

I have a line lump PSU with a grounded IEC plug that has the output power floating about 35 V.  It has a molex connector like an old computer hard drive with 12V and 5V outputs.  Common to each rail is right, just that the common is 35V above ground.  This is on a known good outlet.

Another non shock situation are the laptops that put out hum on the audio connector when running from a grounded outlet. I think it was Dell that was the main offender.  Those would quiet down with a non grounded PSU, normal PSU with a ground lift on the plug, or from battery.  An audio ISO transformer would NOT make them quiet.  They were the only situation I would condone a widow maker ground lift adaptor.
Title: Re: freak accident? help!
Post by: John Roberts {JR} on September 19, 2013, 09:27:50 pm
I have a couple for you.

I have run into MANY VCRs or DVD/VCR combo units, polarized cords, that will put about 30 to 35 volts on the chassis and shell of any coax connectors on the back if plugged into a revers polarity outlet.  I have gotten shocked by this and seen it damage other equipment.
I recall seeing something similar to the stinger cap inside some consumer VCRs trying to cop a ground from the neutral. These were fairly small caps but I suspect the voltage you see depends on a divider formed with whatever you measured it with. Still not cool, but if UL blessed it they are probably small (safe) currents.
Quote
I have a line lump PSU with a grounded IEC plug that has the output power floating about 35 V.  It has a molex connector like an old computer hard drive with 12V and 5V outputs.  Common to each rail is right, just that the common is 35V above ground.  This is on a known good outlet.
Again this voltage may depend on how you measure it. Have you tried to ground it to see how much current? A VOM on it's current scale will tell you, If you want to know.
Quote
Another non shock situation are the laptops that put out hum on the audio connector when running from a grounded outlet. I think it was Dell that was the main offender.  Those would quiet down with a non grounded PSU, normal PSU with a ground lift on the plug, or from battery.  An audio ISO transformer would NOT make them quiet.  They were the only situation I would condone a widow maker ground lift adaptor.

Computers suck, but they are better than no computers.

JR
Title: Re: freak accident? help!
Post by: Tom Bourke on September 21, 2013, 01:44:14 pm
I recall seeing something similar to the stinger cap inside some consumer VCRs trying to cop a ground from the neutral. These were fairly small caps but I suspect the voltage you see depends on a divider formed with whatever you measured it with. Still not cool, but if UL blessed it they are probably small (safe) currents. Again this voltage may depend on how you measure it. Have you tried to ground it to see how much current? A VOM on it's current scale will tell you, If you want to know.
Computers suck, but they are better than no computers.

JR

I just moved cross country so I am still unpacking all my stuff.  When I can get to the power supplies again I will measure them.  It was enough current to get a decent shock from.
Title: Re: freak accident? help!
Post by: Brian Jones on October 01, 2013, 08:24:53 pm
Guys... this is not a freak accident. It was caused by something I call a Reverse Polarity Bootleg Ground (or RPBG). This occurs when an old building has new grounded outlets added by bonding the ground screw to the neutral screw because there was no separate ground wire to begin with. That by itself is electrically safe (however, it's illegal per the NEC), but many older buildings had black power wires for both the hot and neutral, and some were simply wired backwards with the white/neutral wire being hot and the black/power line being actual neutral. See the attached diagram. In that case any piece of gear plugged into a Reverse Polarity Bootleg Grounded outlet will have its chassis energized to 120 volts. If you then connect that piece of audio gear to something else that's plugged into a correctly wired outlet, you can have 20 amps or more of current flow down the shield, which melts wires and destroys gear. The really scary thing is that a 3-light tester will tell you that this reversed outlet is wired correctly, when in fact both the neutral and ground contacts are at 120 volts and the hot side is at earth potential. Please see http://www.youtube.com/watch?v=_pwCY4_LwJo&feature=youtu.be&noredirect=1  for a video I did a few weeks ago that describes how you can use a $20 non-contact AC tester in conjunction with a cheap 3-light tester to qualify grounds in power plugs. That's the only easy way to determine if an outlet will blow up your gear.

After discussing this testing issue with a few meter manufacturers, it seems that the entire industry has missed this problem. In fact, electrical inspectors routinely use a 3-light tester to qualify outlets in renovated buildings, but that's where the hot and neutral wires in the wall are most likely to be reversed.

I'm covering a lot of this on www.noshockzone.org and trying to get Lowes and Home Depot to offer training to consumers and electricians on how to check for this condition. Please contact me with any questions or comments.

Mike Sokol - mike@fitsandstarts.com

Thanks for this. I picked up a non-contact voltage tester today and it will be in my toolkit and used at every gig from here forward. I, like most, already had the three-light tester.
Title: Re: Dangerous AC situation in reception hall - PLEASE READ
Post by: Gus Housen on January 30, 2014, 01:00:36 pm
I would get rid of that snake, I dont think I could trust it
Title: Re: Dangerous AC situation in reception hall - PLEASE READ
Post by: Bill Whitlock on June 13, 2014, 01:07:28 am
Providing sound for a banquet at a reception hall this past weekend and I ran into some problems.

Rig
2 Yorkville U15 run off rmx2450 each channel
2 Yorkville UCS1 run off 2 rmx2450 in bridged mono
Mixer (Mackie SR24)
Effects rack (dbx 2231s, Yorkville unity processors, dbx compressors)
Snake (horizon 16x4)

Setup
-Please see attached file

Power
-There was an outlet in the front of the stage and I plugged the two amps for the subs there.
-I plugged the mixer, effects rack, and poweramp for the mains into an outlet at the back of the stage

Signal
-So my tech was working on the tops while I worked on the subs.  He routed it incorrectly, so Iíll explain the chain. 

Tops : Main outs from mixer > Unity Processor (crossover) > Unity Bi-amp/high out > Eq in > Snake A Channel tail end > From snake box, ran a cable back to the poweramp > U15s

Subs: Sub Out from unity processor > Snake C channel > from head of snake, channel C into poweramp > UCS1s via parallel inputs

-I had the tech correct the chain for the tops because he didnít have to go through the snake because the poweramp was right there by the mixer/effects rack.  Also had him bypass the EQ.

New Signal Chain
Tops: Main outs from mixer  > Unity Processor > Unity Biamp out > Poweramp > U15s


Now, hereís where it gets funky.  Had him power up everything.  Mixer turned on, effects turned on, and as we are about to turn on the poweramps, we smell smoke and I have him shut everything off.  At this point I had no idea what was going on because none of the poweramps were on.  My first thought was that I was drawing too much power somewhere.  I ran to unplug everything, and the first piece I went to get was the cable in channel C of the snake (which is for the subs).  It was HOT, so I left it.  I took off the plugs from the outlet.

It was total chaos.  Channel A of the snake on the tail end lost its coating (which probably was what the smoke was) and the wire was glowing red (about a foot of cable).  Now, if you were following along, Channel A was plugged into the EQ output.  A cable on the box end was originally patched to the poweramp (the one in the back for the mains) but it was pulled out.  So, Channel A on the box end had nothing connected to it. 

I still cant explain what happened.
What caused Channel A to short and heat up like that?  It was only connected to the output of the EQ.  And it only melted from the jack to about a foot in.

Why was channel C hot?  Matter of fact, I touched the entire snake cable and it was really warm.  My guess is that the heat from channel A was heating all the cables?  Or was it channel C that was causing the problem?

None of the poweramps were on.  Mixer and effects were on for about 20-30 seconds.

So after everything cooled down, I used another outlet for the mixer, and used separate outlets on opposite walls for the poweramps.  Not knowing how the circuit is split up, I took my best guess.  Powered everything back on and nothing smoked.  Channel C on the snake passed signal for the subs.  Other channels on the snakes worked as well.  Had a flawless night with speech and presentations, and two hours of pumping music at levels barely clipping.

What happened?  Iím glad nothing serious, like fire, started.  But something DID happen, and itís serious and I donít want it to happen again.  Please input, and Iíll try to answer questions if I left things out.

Thanks.

I think you may well have a NEUTRAL/GROUND swap at AC outlets powering gear at one or the other end of the snake. If it were a HOT/GROUND swap, I believe the results would have been much more spectacular!  The bad news is that these are hard to find ... the $10 tester shown in this thread won't find it because it only looks for 120 VAC on the proper prong and, since both N and G are bonded back at the main panel, it shows OK as long as they're both connected. However, it makes the full load current on the mis-wired outlet flow in the ground conductor (in this case the shield of the snake) which will likely just cause some serious overheating. If you're lucky, you can open up the outlets and check the colors of the wires connected:  green (or bare) is safety ground, black is "hot" 120 V (to the narrower slot), and white is "neutral" (to the wider slot) are the correct connections. I've seen this several times but usually it just causes very serious hum issues. "Bootleg" grounds, where there's a connection between neutral and ground somewhere besides the main panel can cause serious hum problems but I doubt this much current in the snake because it puts a section of ground and neutral wiring effectively in parallel, sharing the load current.
Title: Re: Dangerous AC situation in reception hall - PLEASE READ
Post by: Mike Sokol on June 13, 2014, 07:00:21 am
I think you may well have a NEUTRAL/GROUND swap at AC outlets powering gear at one or the other end of the snake. If it were a HOT/GROUND swap, I believe the results would have been much more spectacular!  The bad news is that these are hard to find ... the $10 tester shown in this thread won't find it because it only looks for 120 VAC on the proper prong and, since both N and G are bonded back at the main panel, it shows OK as long as they're both connected. However, it makes the full load current on the mis-wired outlet flow in the ground conductor (in this case the shield of the snake) which will likely just cause some serious overheating. If you're lucky, you can open up the outlets and check the colors of the wires connected:  green (or bare) is safety ground, black is "hot" 120 V (to the narrower slot), and white is "neutral" (to the wider slot) are the correct connections. I've seen this several times but usually it just causes very serious hum issues. "Bootleg" grounds, where there's a connection between neutral and ground somewhere besides the main panel can cause serious hum problems but I doubt this much current in the snake because it puts a section of ground and neutral wiring effectively in parallel, sharing the load current.

Bill, welcome to the forum. For a synopsis of this rather long thread including a graphic showing the fault current path cause by an RPBG outlet condition, please look at http://ecmweb.com/contractor/failures-outlet-testing-exposed (http://ecmweb.com/contractor/failures-outlet-testing-exposed) There's also a sidebar about a home studio that was damaged by an electrician who was paid to upgrade a room to grounded outlets, but instead created an RPBG.
Title: Re: Dangerous AC situation in reception hall - PLEASE READ
Post by: Bill Whitlock on June 15, 2014, 01:58:50 am
Bill, welcome to the forum. For a synopsis of this rather long thread including a graphic showing the fault current path cause by an RPBG outlet condition, please look at http://ecmweb.com/contractor/failures-outlet-testing-exposed (http://ecmweb.com/contractor/failures-outlet-testing-exposed) There's also a sidebar about a home studio that was damaged by an electrician who was paid to upgrade a room to grounded outlets, but instead created an RPBG.
I'm familiar with the error, but not the acronym. Just seems to me that if the snake were grounded via a properly wired outlet at one end and an "RPBG" outlet at the other, putting 120 between the two ends of the cable's shield conductor, the result would have been much more serious than just a partly burned cable. Did a breaker open, thus reducing the energy delivered?  If not, I'd guess that the cable was a drain wire and foil shielded type ... which has a much higher end-to-end resistance than copper braid (in addition to having very, very poor SCIN performance).
Title: Re: freak accident? help!
Post by: John Lackner on July 23, 2014, 04:56:07 pm
Guys... this is not a freak accident. It was caused by something I call a Reverse Polarity Bootleg Ground (or RPBG). This occurs when an old building has new grounded outlets added by bonding the ground screw to the neutral screw because there was no separate ground wire to begin with. That by itself is electrically safe (however, it's illegal per the NEC), but many older buildings had black power wires for both the hot and neutral, and some were simply wired backwards with the white/neutral wire being hot and the black/power line being actual neutral. See the attached diagram. In that case any piece of gear plugged into a Reverse Polarity Bootleg Grounded outlet will have its chassis energized to 120 volts. If you then connect that piece of audio gear to something else that's plugged into a correctly wired outlet, you can have 20 amps or more of current flow down the shield, which melts wires and destroys gear. The really scary thing is that a 3-light tester will tell you that this reversed outlet is wired correctly, when in fact both the neutral and ground contacts are at 120 volts and the hot side is at earth potential. Please see http://www.youtube.com/watch?v=_pwCY4_LwJo&feature=youtu.be&noredirect=1  for a video I did a few weeks ago that describes how you can use a $20 non-contact AC tester in conjunction with a cheap 3-light tester to qualify grounds in power plugs. That's the only easy way to determine if an outlet will blow up your gear.

After discussing this testing issue with a few meter manufacturers, it seems that the entire industry has missed this problem. In fact, electrical inspectors routinely use a 3-light tester to qualify outlets in renovated buildings, but that's where the hot and neutral wires in the wall are most likely to be reversed.

I'm covering a lot of this on www.noshockzone.org and trying to get Lowes and Home Depot to offer training to consumers and electricians on how to check for this condition. Please contact me with any questions or comments.

Mike Sokol - mike@fitsandstarts.com

I went on Amazon and noticed there are a variety of Voltalerts available, with prices ranging from about $26 to $130. which is the right one to get?
Title: Re: freak accident? help!
Post by: Mike Sokol on July 23, 2014, 05:21:25 pm
I went on Amazon and noticed there are a variety of Voltalerts available, with prices ranging from about $26 to $130. which is the right one to get?

Here's what I use http://www.amazon.com/Fluke-1AC-A1-II-VoltAlert-Non-Contact-Voltage/dp/B000EJ332O/ref=sr_1_1?ie=UTF8&qid=1406150368&sr=8-1&keywords=Fluke+VoltAlert

Get the 90 to 1,000 volt version for standard outlet testing.
Title: Re: freak accident? help!
Post by: Scott Holtzman on July 27, 2014, 03:39:55 am
One of the other posters mentioned he had a neutral hot swap on one floor of his house.

I had this happen in my home office, didn't notice for years until one day I unscrewed the outlet from the cable modem and got hit.  First started cursing the cable company and went and tested the entrance.

After a while I finally checked the outlet.  The wiring isn't 10 years old in this house, was redone from the original 2 conductor without ground from '62 just before I bought it.

I even test stuff I just built and am staring at.  Anybody can make a mistake.

Title: Re: freak accident? help!
Post by: Bill Koonce on November 18, 2016, 07:07:56 pm
I even test stuff I just built and am staring at.  Anybody can make a mistake.
I second that!  I used to build my own "4-boxes" to provide stage power, secure in the belief that because I knew how it should be wired, that my hands would never get it wrong...until they did.

Would you or anyone here know of a good tester for AC wiring faults that a non-contact or "3 light" tester don't catch?  I've had a couple of situations when the FAULT light on one of my Tripp-Lite Isobar power strips lights up after my simple tester shows no problem.  Usually this happens when I don't have the time to pull out my DMM and refocus my mind on doing that.  Is there a simple-to-use tester just for ground & neutral faults?  One that I could send a well-meaning but poorly trained assistant out with?

Come to think of it, I'd pay good money for a tester that showed G-N voltage.  That's something that I'm finding more of lately.
Title: Re: freak accident? help!
Post by: Mike Sokol on November 18, 2016, 08:16:23 pm
  Is there a simple-to-use tester just for ground & neutral faults?  One that I could send a well-meaning but poorly trained assistant out with?

Come to think of it, I'd pay good money for a tester that showed G-N voltage.  That's something that I'm finding more of lately.

Here's what I use, a Suretest Analyzer. It's $300 but tells you a lot of things including ground and leg impedance, voltage drop, and lots of other measurments. Can't detect an upstream RPBG though: http://www.idealind.com/us/en/products/test-measurement/suretest-circuit-analyzer/suretest-circuit-analyzer.aspx
Title: Re: freak accident? help!
Post by: Bill Koonce on November 18, 2016, 11:40:18 pm
Thanks Mike!  That looks...well, ideal for my needs.  Not to disparage my new home town, but so far I'm batting less than .500 when it comes to finding a good, working plugin at gigs.  (Both power and Ethernet!)  $300 is cheap insurance if it saves my ENG camera and laptop from flaky power connections.  At home I have a hodgepodge of mixed aluminum and copper wiring that I suspect has not been given due care over the years.  (I found 25VAC G-N on one circuit that was supposed to be dead when I checked before removing 3 taped-together switches.)  Either way I think I'll get my money's worth in peace of mind.
Title: Re: freak accident? help!
Post by: Jonathan Johnson on November 19, 2016, 12:25:06 am
I second that!  I used to build my own "4-boxes" to provide stage power, secure in the belief that because I knew how it should be wired, that my hands would never get it wrong...until they did.

I NEVER trust the terminal colors when wiring up plugs and receptacles, ever since I discovered a duplex receptacle that had been shipped from the factory with the brass and silver screws reversed.

How many others actually wire according to the position of the terminals rather than the color?
Title: Re: freak accident? help!
Post by: Mike Sokol on November 19, 2016, 08:13:15 am
Thanks Mike!  That looks...well, ideal for my needs.  Not to disparage my new home town, but so far I'm batting less than .500 when it comes to finding a good, working plugin at gigs.  (Both power and Ethernet!)  $300 is cheap insurance if it saves my ENG camera and laptop from flaky power connections.  At home I have a hodgepodge of mixed aluminum and copper wiring that I suspect has not been given due care over the years.  (I found 25VAC G-N on one circuit that was supposed to be dead when I checked before removing 3 taped-together switches.)  Either way I think I'll get my money's worth in peace of mind.

I'm sure that 99% (or more) of outlets in modern homes or performance venues are properly wired. But it's that possible 1% that can bite you in the butt. A lot of that is due to the fact that we interconnect a lot of sensitive electronic gear that's often powered by multiple outlets of unknown pedigree. For instance, if you plug a toaster or slow coodker nto an ungrounded outlet or even an RPBG in your kitchen, you'll never know it. And many modern power supplies are auto-switching which means they don't care if they're plugged into 90 or 250 volts. And even a grounded appliance such as an air conditioner in the bedroom can be plugged into an RPBG outlet for decades and you might not know it for decades because you won't get shocked while standing on a wood floor. But plug your computer printer into that outlet and plug your computer power supply into another outlet and the USB cable will melt down as your computer fries.

I generally put my Suretest Analyzer on the outlet and look for obvious miswiring, then do a quickie test with a NCVT to check for an RPBG. Remember that while a Suretest or Amprobe INSP-3 will find a bootleg ground created by a G-N bond in the receptacle itself, they won't detect one at least 15 feet upstream. And they won't notify you of a hot ground created by any RPBG. See https://www.youtube.com/watch?v=_04HmpFBxdQ
Title: Re: Dangerous AC situation in reception hall - PLEASE READ
Post by: Kevin Graf on November 19, 2016, 08:57:42 am
I don't think it's 99 %. This Audio Precision note is from 3 years ago:

New Facility
Audio.TST June 2013       
Notes from the Test Bench
By Bruce Hofer, Chairman & Co-Founder, Audio Precision

Recently, AP celebrated the 15th anniversary of our custom designed building. Photo albums were displayed showing the construction and the move into our facility back in 1998. (My, how some people have changed in appearance!) Among my many memories of that time, I was reminded of a particularly nasty problem we experienced as we restarted operations in our new production environment.
Almost immediately after moving we began to experience failures of certain bench tests that are performed by our technicians during the course of product assembly and adjustment. After some research, we discovered our new building had some extremely large magnetic fields in the production area, almost as if it was haunted. These fields coupled high levels of hum into our products that were causing the test failures. AP products are designed to reject reasonable levels of stray magnetic fields that would typically be encountered in a lab or production environment. However the magnitude of the fields we faced were at least 20 dB worse.
We ultimately discovered that several of our AC outlets had been wired incorrectly, having their neutral and ground connections swapped. This is a big no-no from the safety viewpoint, but it also caused all of the neutral currents in a particular circuit (outside of our production area) to return through the safety ground connection and ultimately through plumbing and drainage pipes. Some of these plumbing pipes were located in the space directly above our production area while the main drainage pipe was buried directly below; thus our production area was effectively inside of a huge coil. Our electrical contractor was embarrassed but confirmed our diagnosis, and the problem was quickly fixed.
Sometimes one has to think ďoutside of the boxĒ to correctly perceive or understand a given problem. In this case, our new building plans provided the necessary insight to recognize the inadvertent source of our unwanted magnetic fields (plumbing that formed a coil around our production area). Perhaps you might want to check the neutral and safety ground connections of the AC outlets in your own work spaceóthere could be some ghosts present!

Title: Re: Dangerous AC situation in reception hall - PLEASE READ
Post by: Stephen Swaffer on November 19, 2016, 10:17:31 am
I don't think it's 99 %. This Audio Precision note is from 3 years ago:


I would agree-I was in a code update class yesterday-one student that, from overheard conversations, is in at least his second 3 year license cyle meaning he has been a licensed electrician for over 6 years + the time to get his license asked a question about grounding signs-using a "concrete encased electrode."  The instructor was talking about equipment grounding conductors-an intentional metalic path back to the service by definition-he seemed perplexed by the concept that an EGC needed to me metallic.

Another interesting historical fact that was mentioned was that when dimmer's first came out they included a "ground" wire-but that ground was actually used as a neutral for the function of the dimmer-this allowed them to be used in the place of switches that had no neutral installed in the box.  Code was changed a few cycles ago to require neutrals at switch locations to allow for this.  Still, especially with audio and sensitivity to ground loops and objectionable current on grounds, this could be a problem in some buildings.

I suspect that in Mike's area-and in most large cities-licensing and inspection has been required for a few decades.  Most of Iowa had no licensing or inspection requirement until 2007-and there are electrician's that were grandfathered in that shouldn't be wiring buildings. 

Since most inspectors in this area do a 100% test of receptacles with at least a generic 3 lamp tester, I might allow that 99% correct in any new construction that has actually been permitted and inspected :)
Title: Re: freak accident? help!
Post by: Mike Sokol on November 19, 2016, 10:40:02 am
I'm sure that 99% (or more) of outlets in modern homes or performance venues are properly wired. But it's that possible 1% that can bite you in the butt.

Note that I said perhaps 99% of "modern homes or venues" are properly wired. If you factor in all the old wiring that's been grandfathered in, then that number has to plummet. But to your point about swapped G-N wires on receptacles, I too have found these in a number of performance venues where there was all sorts of ground-loop hum that couldn't be explained. A few of these have been big conference halls with hundreds of vendor spots, and I was only measuring where I was plugging in my own gear. So how widespread are these mis-wiring conditions? I just don't know. But I do find at least several obvious mis-wired outlets every year, and I'm just casually inspecting outlets that I'm going to plug into myself. That is, I'm not doing a formal assay of a bunch of facilities. That being said, I was recently brought in to consult on a rewiring job at a church to assure there's no ground-loop hum on their youth stage. And a big performance theater in the area has asked me to figure out why some of their outlets meter at 130 volts before they install a bunch of new LED lighting gear and controllers. According to the theater tech, they've blown up a lot of control electronics, and have decided to look at what caused the failures before they hook up their new expensive gear. 
Title: Re: freak accident? help!
Post by: Bill Koonce on November 19, 2016, 12:26:54 pm
I'm sure that 99% (or more) of outlets in modern homes or performance venues are properly wired. But it's that possible 1% that can bite you in the butt. A lot of that is due to the fact that we interconnect a lot of sensitive electronic gear that's often powered by multiple outlets of unknown pedigree. For instance, if you plug a toaster or slow coodker nto an ungrounded outlet or even an RPBG in your kitchen, you'll never know it. And many modern power supplies are auto-switching which means they don't care if they're plugged into 90 or 250 volts. And even a grounded appliance such as an air conditioner in the bedroom can be plugged into an RPBG outlet for decades and you might not know it for decades because you won't get shocked while standing on a wood floor. But plug your computer printer into that outlet and plug your computer power supply into another outlet and the USB cable will melt down as your computer fries.

I generally put my Suretest Analyzer on the outlet and look for obvious miswiring, then do a quickie test with a NCVT to check for an RPBG. Remember that while a Suretest or Amprobe INSP-3 will find a bootleg ground created by a G-N bond in the receptacle itself, they won't detect one at least 15 feet upstream. And they won't notify you of a hot ground created by any RPBG. See https://www.youtube.com/watch?v=_04HmpFBxdQ
In my case, I have two potential problems to contend with.  The first is widespread deployment of aluminum wiring here since 1960, before compatible terminal gear was available.  (In Chicago, the Sears Tower had to have all switches and outlets replaced after it was discovered that they were only Cu rated when installed c. 1970 with all Al wiring.)  In my own home, I have a couple of later additions that muddy the waters even more--the additions used copper wire since that was cost effective at that time.  And I think that a lot of the home wiring was done by the owner, who was a police officer, not an electrician.  I'm pretty sure that a few of my problems are coming from less than optimal Al-Cu splices, and tired outlets.  IME when I find a flaky outlet, just cutting off the old oxidized wire, exposing shiny wire and attaching that to a shiny new socket will make it rock solid.  That's the time to fix any wrong wiring too.

At work there's the possibility of flaky Al-Cu joins, plus wall outlets that have been treated roughly by cleaning crews who think nothing of unplugging a vacuum cleaner by tugging on its power cord until something gives.  Up north a conference room will often have a "conditioned" outlet behind a panel that is verboten for the cleaning crew to use, along other structured wiring hookups just for AV use.  Although I'm just starting to explore the various venues of Albuquerque, the ones that I've worked in to date don't have modern structured wiring, and wall outlets that have taken a beating over years.  Like the old XLR jack for the PA that has had the push-to-disconnect tab mangled.

I'm a subcontractor for this gig, and would love to be able to grow my role by being the AV/PA provider too (I already lug a lot of stuff to each gig as backup in case the hotel system fails), but so far my client's client isn't interested.  C'est la vie.