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Author Topic: Dangerous AC situation in reception hall - PLEASE READ  (Read 184444 times)

Mike Sokol

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Re: freak accident? help!
« Reply #60 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.   

Rob Spence

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freak accident? help!
« Reply #61 on: May 07, 2012, 08:34:18 PM »

To the OP
I was referring to the unity processor ( not digital ? ).


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Vic Snyder

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Re: freak accident? help!
« Reply #62 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
« Last Edit: May 07, 2012, 08:45:20 PM by AP »
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Mac Kerr

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Re: freak accident? help!
« Reply #63 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 in the Forum Announcements section, and on the registration page when you registered.

Mac
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Mike Sokol

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Re: freak accident? help!
« Reply #64 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
« Last Edit: May 19, 2012, 01:56:27 PM by Mike Sokol »
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Mike Sokol

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Re: freak accident? help!
« Reply #65 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

John Roberts {JR}

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Re: freak accident? help!
« Reply #66 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
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J. Taylor Webb

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Re: freak accident? help!
« Reply #67 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
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Taylor Webb

Mike Sokol

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Re: freak accident? help!
« Reply #68 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
« Last Edit: May 08, 2012, 06:48:35 PM by Mike Sokol »
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Jonathan Johnson

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Re: freak accident? help!
« Reply #69 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.
« Last Edit: May 08, 2012, 07:17:37 PM by Jonathan Johnson »
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Re: freak accident? help!
« Reply #69 on: May 08, 2012, 06:55:09 PM »


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