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Author Topic: Dangerous AC situation in reception hall - PLEASE READ  (Read 87149 times)

Mike Sokol

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Re: freak accident? help!
« Reply #20 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

Jordan Wolf

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Re: freak accident? help!
« Reply #21 on: May 03, 2012, 07:19:32 pm »

Al, et, uh...al  ::)

I had no idea that such a thing existedů"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.
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Jordan Wolf
<><

"We want our sound to go into the soul of the audience, and see if it can awaken some little thing in their minds... Cause there are so many sleeping people." - Jimi Hendrix

John Hyun

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Re: freak accident? help!
« Reply #22 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.
« Last Edit: May 03, 2012, 07:25:36 pm by John Hyun »
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Mike Sokol

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Re: freak accident? help!
« Reply #23 on: May 03, 2012, 07:39:04 pm »

Al, et, uh...al  ::)

I had no idea that such a thing existedů"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

Mike Sokol

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Re: freak accident? help!
« Reply #24 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

John Hyun

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Re: freak accident? help!
« Reply #25 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...
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Ivan Beaver

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Re: freak accident? help!
« Reply #26 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.
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Guy Luckert

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Re: freak accident? help!
« Reply #27 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
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Mike Sokol

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Re: freak accident? help!
« Reply #28 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"....

Mike Sokol

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Re: freak accident? help!
« Reply #29 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
« Last Edit: May 03, 2012, 08:40:36 pm by Mike Sokol »
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