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Author Topic: Dangerous AC situation in reception hall - PLEASE READ  (Read 35334 times)

Bill Whitlock

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Re: Dangerous AC situation in reception hall - PLEASE READ
« Reply #200 on: June 13, 2014, 01:07:28 am »

Providing sound for a banquet at a reception hall this past weekend and I ran into some problems.

Rig
2 Yorkville U15 run off rmx2450 each channel
2 Yorkville UCS1 run off 2 rmx2450 in bridged mono
Mixer (Mackie SR24)
Effects rack (dbx 2231s, Yorkville unity processors, dbx compressors)
Snake (horizon 16x4)

Setup
-Please see attached file

Power
-There was an outlet in the front of the stage and I plugged the two amps for the subs there.
-I plugged the mixer, effects rack, and poweramp for the mains into an outlet at the back of the stage

Signal
-So my tech was working on the tops while I worked on the subs.  He routed it incorrectly, so Iíll explain the chain. 

Tops : Main outs from mixer > Unity Processor (crossover) > Unity Bi-amp/high out > Eq in > Snake A Channel tail end > From snake box, ran a cable back to the poweramp > U15s

Subs: Sub Out from unity processor > Snake C channel > from head of snake, channel C into poweramp > UCS1s via parallel inputs

-I had the tech correct the chain for the tops because he didnít have to go through the snake because the poweramp was right there by the mixer/effects rack.  Also had him bypass the EQ.

New Signal Chain
Tops: Main outs from mixer  > Unity Processor > Unity Biamp out > Poweramp > U15s


Now, hereís where it gets funky.  Had him power up everything.  Mixer turned on, effects turned on, and as we are about to turn on the poweramps, we smell smoke and I have him shut everything off.  At this point I had no idea what was going on because none of the poweramps were on.  My first thought was that I was drawing too much power somewhere.  I ran to unplug everything, and the first piece I went to get was the cable in channel C of the snake (which is for the subs).  It was HOT, so I left it.  I took off the plugs from the outlet.

It was total chaos.  Channel A of the snake on the tail end lost its coating (which probably was what the smoke was) and the wire was glowing red (about a foot of cable).  Now, if you were following along, Channel A was plugged into the EQ output.  A cable on the box end was originally patched to the poweramp (the one in the back for the mains) but it was pulled out.  So, Channel A on the box end had nothing connected to it. 

I still cant explain what happened.
What caused Channel A to short and heat up like that?  It was only connected to the output of the EQ.  And it only melted from the jack to about a foot in.

Why was channel C hot?  Matter of fact, I touched the entire snake cable and it was really warm.  My guess is that the heat from channel A was heating all the cables?  Or was it channel C that was causing the problem?

None of the poweramps were on.  Mixer and effects were on for about 20-30 seconds.

So after everything cooled down, I used another outlet for the mixer, and used separate outlets on opposite walls for the poweramps.  Not knowing how the circuit is split up, I took my best guess.  Powered everything back on and nothing smoked.  Channel C on the snake passed signal for the subs.  Other channels on the snakes worked as well.  Had a flawless night with speech and presentations, and two hours of pumping music at levels barely clipping.

What happened?  Iím glad nothing serious, like fire, started.  But something DID happen, and itís serious and I donít want it to happen again.  Please input, and Iíll try to answer questions if I left things out.

Thanks.

I think you may well have a NEUTRAL/GROUND swap at AC outlets powering gear at one or the other end of the snake. If it were a HOT/GROUND swap, I believe the results would have been much more spectacular!  The bad news is that these are hard to find ... the $10 tester shown in this thread won't find it because it only looks for 120 VAC on the proper prong and, since both N and G are bonded back at the main panel, it shows OK as long as they're both connected. However, it makes the full load current on the mis-wired outlet flow in the ground conductor (in this case the shield of the snake) which will likely just cause some serious overheating. If you're lucky, you can open up the outlets and check the colors of the wires connected:  green (or bare) is safety ground, black is "hot" 120 V (to the narrower slot), and white is "neutral" (to the wider slot) are the correct connections. I've seen this several times but usually it just causes very serious hum issues. "Bootleg" grounds, where there's a connection between neutral and ground somewhere besides the main panel can cause serious hum problems but I doubt this much current in the snake because it puts a section of ground and neutral wiring effectively in parallel, sharing the load current.
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Bill Whitlock
President & Chief Engineer
Jensen Transformers, Inc.
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"Sadly, marketing has become the art of deception by omission."

Mike Sokol

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Re: Dangerous AC situation in reception hall - PLEASE READ
« Reply #201 on: June 13, 2014, 07:00:21 am »

I think you may well have a NEUTRAL/GROUND swap at AC outlets powering gear at one or the other end of the snake. If it were a HOT/GROUND swap, I believe the results would have been much more spectacular!  The bad news is that these are hard to find ... the $10 tester shown in this thread won't find it because it only looks for 120 VAC on the proper prong and, since both N and G are bonded back at the main panel, it shows OK as long as they're both connected. However, it makes the full load current on the mis-wired outlet flow in the ground conductor (in this case the shield of the snake) which will likely just cause some serious overheating. If you're lucky, you can open up the outlets and check the colors of the wires connected:  green (or bare) is safety ground, black is "hot" 120 V (to the narrower slot), and white is "neutral" (to the wider slot) are the correct connections. I've seen this several times but usually it just causes very serious hum issues. "Bootleg" grounds, where there's a connection between neutral and ground somewhere besides the main panel can cause serious hum problems but I doubt this much current in the snake because it puts a section of ground and neutral wiring effectively in parallel, sharing the load current.

Bill, welcome to the forum. For a synopsis of this rather long thread including a graphic showing the fault current path cause by an RPBG outlet condition, please look at http://ecmweb.com/contractor/failures-outlet-testing-exposed There's also a sidebar about a home studio that was damaged by an electrician who was paid to upgrade a room to grounded outlets, but instead created an RPBG.

Bill Whitlock

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Re: Dangerous AC situation in reception hall - PLEASE READ
« Reply #202 on: June 15, 2014, 01:58:50 am »

Bill, welcome to the forum. For a synopsis of this rather long thread including a graphic showing the fault current path cause by an RPBG outlet condition, please look at http://ecmweb.com/contractor/failures-outlet-testing-exposed There's also a sidebar about a home studio that was damaged by an electrician who was paid to upgrade a room to grounded outlets, but instead created an RPBG.
I'm familiar with the error, but not the acronym. Just seems to me that if the snake were grounded via a properly wired outlet at one end and an "RPBG" outlet at the other, putting 120 between the two ends of the cable's shield conductor, the result would have been much more serious than just a partly burned cable. Did a breaker open, thus reducing the energy delivered?  If not, I'd guess that the cable was a drain wire and foil shielded type ... which has a much higher end-to-end resistance than copper braid (in addition to having very, very poor SCIN performance).
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Bill Whitlock
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Jensen Transformers, Inc.
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"Sadly, marketing has become the art of deception by omission."

Johnlackner

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Re: freak accident? help!
« Reply #203 on: July 23, 2014, 04:56:07 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

I went on Amazon and noticed there are a variety of Voltalerts available, with prices ranging from about $26 to $130. which is the right one to get?
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Mike Sokol

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Re: freak accident? help!
« Reply #204 on: July 23, 2014, 05:21:25 pm »

I went on Amazon and noticed there are a variety of Voltalerts available, with prices ranging from about $26 to $130. which is the right one to get?

Here's what I use http://www.amazon.com/Fluke-1AC-A1-II-VoltAlert-Non-Contact-Voltage/dp/B000EJ332O/ref=sr_1_1?ie=UTF8&qid=1406150368&sr=8-1&keywords=Fluke+VoltAlert

Get the 90 to 1,000 volt version for standard outlet testing.

Scott Holtzman

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Re: freak accident? help!
« Reply #205 on: July 27, 2014, 03:39:55 am »

One of the other posters mentioned he had a neutral hot swap on one floor of his house.

I had this happen in my home office, didn't notice for years until one day I unscrewed the outlet from the cable modem and got hit.  First started cursing the cable company and went and tested the entrance.

After a while I finally checked the outlet.  The wiring isn't 10 years old in this house, was redone from the original 2 conductor without ground from '62 just before I bought it.

I even test stuff I just built and am staring at.  Anybody can make a mistake.

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Passion Fuel Productions, Cleveland OH
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