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Author Topic: Dangerous AC situation in reception hall - PLEASE READ  (Read 80794 times)

John Roberts {JR}

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Re: freak accident? help!
« Reply #10 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
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Samuel Rees

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freak accident? help!
« Reply #11 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?
« Last Edit: May 03, 2012, 10:34:38 am by Samuel Rees »
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Jay Barracato

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Re: freak accident? help!
« Reply #12 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.
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Jay Barracato

John Roberts {JR}

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Re: freak accident? help!
« Reply #13 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
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Tim McCulloch

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Re: freak accident? help!
« Reply #14 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.
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Nick Enright

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Re: freak accident? help!
« Reply #15 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)
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Geoff Doane

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Re: freak accident? help!
« Reply #16 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
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John Roberts {JR}

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Re: freak accident? help!
« Reply #17 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
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Mike Sokol

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Re: freak accident? help!
« Reply #18 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
« Last Edit: May 19, 2012, 01:36:07 pm by Mike Sokol »
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Al Keltz

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Re: freak accident? help!
« Reply #19 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
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