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Author Topic: Help understand cause of Shock  (Read 6153 times)

Nitin Sidhu

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Help understand cause of Shock
« on: November 21, 2014, 05:55:41 am »

Hello!

Please educate me on possible theories for cause of the following accident.

Bar gig. Stage has fixed power receptacles which have been measured and found good.

Chain is power from receptacle to a Voice live, voice live outputs feeding Allen/Heath AR24 stage box, SM58 into the Voice live. Vocalist also playing bassguitar feeding a Hartke amp, on a different power drop but same circuit. Towards end of soundcheck he gets a solid hit from the mic. The hartke is also grounded, DI link feed to the AR24, ground is enabled.

Now the ground on the power drop feeding the helicon is live, and continues to remain so when everything is disconnected, all other power drops on stage check out fine. This was measured using a basic voltage detector.

TC Live outputs are fried, bank of 8 mic pres on the AR24 is fried. Same bank where the TC was connected.

Fortunately the vocalist was ok, and the gear was not mine.

Regards,
Sidhu
« Last Edit: November 21, 2014, 01:35:07 pm by Nitin Sidhu »
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Mike Sokol

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Re: Help understand cause of Shock
« Reply #1 on: November 21, 2014, 07:48:21 am »

Now the ground on the power drop feeding the helicon is live, and continues to remain so when everything is disconnected, all other power drops on stage check out fine. This was measured using a basic voltage detector.

Sounds like something I call an RPBG (Reverse Polarity Bootleg Ground). That occurs when an electrical outlet has its ground terminated to the "neutral" wire rather than running a proper EGC (Equipment Grounding Conductor) back to the service panel. If the Hot and Neutral are accidentally swapped (easy to do) then the Neutral and Ground connections will be "hot" and the Hot connection will be at "earth ground". An outlet with RPBG miswiring will measure correctly using any standard testing method. See my article describing this condition to the electrical construction industry. http://ecmweb.com/contractor/failures-outlet-testing-exposed and here's a thread right on this forum where I introduced the concept of an RPBG to the pro-sound industry. http://forums.prosoundweb.com/index.php/topic,137760.0.html

Anything with a ground plug connected to an RPBG outlet will have its chassis energized to full line voltage and current (in the USA that would be 120 volts and 20 amperes). However, it will operate normally without any visible problems. It's a very dangerous shock condition because touching that chassis/mic/guitar and anything grounded can be deadly. But cross connecting gear that's plugged into different outlets (one correctly wired and one RPBG outlet) will result in full outlet fault current traveling through any XLR, USB or signal cables, blowing up input and output circuits in a fraction of a second, and many times causing the signal cables to overheat and melt down. See graphic below.
« Last Edit: November 21, 2014, 07:50:57 am by Mike Sokol »
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Mike Sokol
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g'bye, Dick Rees

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Re: Help understand cause of Shock
« Reply #2 on: November 21, 2014, 08:00:34 am »


Bar gig. Stage has fixed power receptacles which have been measured and found good.

Regards,
Sidhu

Unfortunately, "measured and found good" is an assumption of safety when in fact it is only a test of raw voltage levels, not circuit safety.  Now you can expand your testing procedures for safety as well as mere current levels.

A NCVD...non-contact voltage detector...is a necessary part of an everyday kit.

http://www.fluke.com/Fluke/caen/Electrical-Test-Tools/Electrical-Testers/2AC.htm?PID=70668
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Mike Sokol

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Re: Help understand cause of Shock
« Reply #3 on: November 21, 2014, 08:51:47 am »

Unfortunately, "measured and found good" is an assumption of safety when in fact it is only a test of raw voltage levels, not circuit safety.  Now you can expand your testing procedures for safety as well as mere current levels.

A NCVD...non-contact voltage detector...is a necessary part of an everyday kit.

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

+1

Below is a pic of how a standard 3-light tester will fail to find an RPBG outlet. This is from one of my table-top demonstrations about electrical grounding safety.  And here's a video I made about how an RPBG can even fool a $300 Ground Impedance Tester, but a $20 NCVT will find it in a few seconds. https://www.youtube.com/watch?v=XfAPkJVYUpY
« Last Edit: November 21, 2014, 09:03:07 am by Mike Sokol »
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Jerome Malsack

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Re: Help understand cause of Shock
« Reply #4 on: November 21, 2014, 09:10:14 am »

Hello!

Please educate me on possible theories for cause of the following accident.

Bar gig. Stage has fixed power receptacles which have been measured and found good.

Chain is power from receptacle to a Voice live, voice live outputs feeding Allen/Heath AR24 stage box, SM58 into the Voice live. Vocalist also playing bassguitar feeding a Hartke amp, on a different power drop but same circuit. Towards end of soundcheck he gets a solid hit from the mic. The hartke is also grounded, DI link feed to the AR24, ground is enabled.

Now the ground on the power drop feeding the helicon is live, and continues to remain so when everything is disconnected, all other power drops on stage check out fine. This was measured using a basic voltage detector.

TC Live outputs are fried, bank of 8 mic pres on the AR24 is fried. Same bank where the TC was connected.

Fortunately the vocalist was ok, and the gear was not mine.

Regards,
Sidhu

So buy tested and found good, what was the test that confirmed ??   

3 prong tester with lights,  or Metered. 

was the No Contact Voltage tester used ?  (NCV)
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Nitin Sidhu

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Re: Help understand cause of Shock
« Reply #5 on: November 21, 2014, 01:34:14 pm »

Hello all!

Thank you for your time.

@Mike. Why would this happen only towards the end of SC ? I would presume he was singing into the mic all through, I know the band and thats how he sings. I will confirm. I will also spend time on your article soon. thank you.

The test was metered. Live to Ground good at 220v. Neutral to ground at 0v. Live to neutral at 220v.

More so, I believe that the establishment has RCD's (GFCI) installed, should be part of code, and they would follow. But that did not trip. I have asked for a follow up on it.

Sidhu
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John Roberts {JR}

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Re: Help understand cause of Shock
« Reply #6 on: November 21, 2014, 01:45:53 pm »


The non-contact tester will help identify unusual voltages on ground.

If products were plugged into different power drops (outlets)  maybe measure voltage between grounds on the different power drops.

JR
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frank kayser

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Re: Help understand cause of Shock
« Reply #7 on: November 21, 2014, 01:56:05 pm »

Hello all!

Thank you for your time.

@Mike. Why would this happen only towards the end of SC ? I would presume he was singing into the mic all through, I know the band and thats how he sings. I will confirm. I will also spend time on your article soon. thank you.

The test was metered. Live to Ground good at 220v. Neutral to ground at 0v. Live to neutral at 220v.

More so, I believe that the establishment has RCD's (GFCI) installed, should be part of code, and they would follow. But that did not trip. I have asked for a follow up on it.

Sidhu
That's the nasty part of this RPBG wiring mistake that is so insidious - the metering you went through (congrats on that, BTW) will produce perfect results -just as you saw.  It seems the ONLY way to see if this condition exists (save opening up the outlet and tracing wires) is to use a NCVT to see if there is voltage on the ground before anything is plugged in.  As stated above something plugged into a socket wired like this will operate just about everything - but with a hot chassis ready to strike someone dead.


Any type of bootleg ground and GFCI is a false safety.


Why that long into the SC?  I'll let better minds than mine address this.
frank
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Mike Sokol

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Re: Help understand cause of Shock
« Reply #8 on: November 21, 2014, 03:05:47 pm »

That's the nasty part of this RPBG wiring mistake that is so insidious - the metering you went through (congrats on that, BTW) will produce perfect results -just as you saw.  It seems the ONLY way to see if this condition exists (save opening up the outlet and tracing wires) is to use a NCVT to see if there is voltage on the ground before anything is plugged in.  As stated above something plugged into a socket wired like this will operate just about everything - but with a hot chassis ready to strike someone dead.


Any type of bootleg ground and GFCI is a false safety.


Why that long into the SC?  I'll let better minds than mine address this.
frank
A receptacle based RCD or GFCI generally won't trip from a RPBG fault. That's because the current is flowing from the ground wire which is upstream of the RCD/GFCI's current transformer. Even if it does trip it won't disconnect its own ground wire, which is unfortunately now hot. And the OCP breaker may not trip due to all the series resistance of the mic cables and snake. So your signal wires can heat up like a toaster and burn out all your electronics. A quick test with a NCVT will identify any hot grounds before plugging in. And it will find hot mics and guitar strings as well.
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John Roberts {JR}

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Re: Help understand cause of Shock
« Reply #9 on: November 21, 2014, 05:21:06 pm »

The other vector for shock is the bass amp... The mic could be providing a valid safety ground but if the bass amp energizes the guitar and player the current will flow to ground through the mic.

Does the bass amp have 3 wire plug with chassis bonded to safety ground.

JR
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Re: Help understand cause of Shock
« Reply #9 on: November 21, 2014, 05:21:06 pm »


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