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Dangerous AC situation in reception hall - PLEASE READ

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John Hyun:
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.

Jordan Wolf:
John,

It sounds like there was a hot/ground swap in one of the outlets you used.  Anything tied to the chassis ground of the equipment that was plugged into that faulty outlet would have current flowing through it and, likely, through any snake channels connected to it.  My guess is that the smaller gauge wire in the snake heated up faster than the larger gauge metals that made up the shells of the rackmounted gear, so it smoked and became a heating coil.

Unfortunately, a tester cannot detect this fault - a visual inspection is the easiest way to find out and the best way to verify that everything is "ok".  If you were able to use a multimeter to measure the voltage between the outlet's "ground" and the rack rails or some piece of gear that had a metal chassis, you might have seen the voltage.  Anytime you measure an outlet's ground against anything other than a hot leg, and it reads 1V or more, I'd be concerned and investigate further or refuse to use that outlet.

Any other takers?

brian maddox:

--- Quote from: Jordan Wolf on May 03, 2012, 12:36:40 am ---John,

It sounds like there was a hot/ground swap in one of the outlets you used.  Anything tied to the chassis ground of the equipment that was plugged into that faulty outlet would have current flowing through it and, likely, through any snake channels connected to it.  My guess is that the smaller gauge wire in the snake heated up faster than the larger gauge metals that made up the shells of the rackmounted gear, so it smoked and became a heating coil.

Unfortunately, a tester cannot detect this fault - a visual inspection is the easiest way to find out and the best way to verify that everything is "ok".  If you were able to use a multimeter to measure the voltage between the outlet's "ground" and the rack rails or some piece of gear that had a metal chassis, you might have seen the voltage.  Anytime you measure an outlet's ground against anything other than a hot leg, and it reads 1V or more, I'd be concerned.

Any other takers?

--- End quote ---

i concur that this is a likely cause.  it's always a good idea to test for voltage between neutral and ground, even straight from a wall outlet.  that would have shown this issue immediately.

i suspect it was the outlet that the mixer was plugged into...

Jordan Wolf:

--- Quote from: brian maddox on May 03, 2012, 12:42:39 am ---it's always a good idea to test for voltage between neutral and ground, even straight from a wall outlet.  that would have shown this issue immediately.

--- End quote ---

One time is one time too many for me. 

I make absolutely certain that I meter my power when :
 - I go to a new venue
 - I think something may have changed since I was there last
 - any type of tie-in is made (cams, twistlock)
 - a generator is used (along with  >:(or without in-wall power)

I don't care as much about smoked gear as I do smoked people; smoked salmon on the other hand...

Tim Perry:

--- Quote from: Jordan Wolf on May 03, 2012, 12:55:51 am ---One time is one time too many for me. 

I make absolutely certain that I meter my power when :
 - I go to a new venue
 - I think something may have changed since I was there last
 - any type of tie-in is made (cams, twistlock)
 - a generator is used (along with  >:(or without in-wall power)

I don't care as much about smoked gear as I do smoked people; smoked salmon on the other hand...

--- End quote ---

A $3 tester from most any hardware store may save your life... if you use it every time.  The one outlet you fail to check may get you.

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