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Author Topic: Dangerous AC situation in reception hall - PLEASE READ  (Read 113226 times)

John Halliburton

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Re: freak accident? help!
« Reply #40 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
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John Hyun

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Re: freak accident? help!
« Reply #41 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?
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John Hyun

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Re: freak accident? help!
« Reply #42 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.
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Mike Sokol

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Re: freak accident? help!
« Reply #43 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? 
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Mike Sokol
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Mike Sokol

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Re: freak accident? help!
« Reply #44 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
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Mike Sokol
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John Roberts {JR}

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Re: freak accident? help!
« Reply #45 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
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On the internet people tell you everything "they" know, not the answer to "your" question.....  http://circularscience.com/

Bob Charest

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Re: freak accident? help!
« Reply #46 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!
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Mike Sokol

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Re: freak accident? help!
« Reply #47 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
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Mike Sokol
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Guy Luckert

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Re: freak accident? help!
« Reply #48 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 !!
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Mike Sokol

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Re: freak accident? help!
« Reply #49 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....
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Mike Sokol
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