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Author Topic: Dangerous AC situation in reception hall - PLEASE READ  (Read 184435 times)

Mike Sokol

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Re: freak accident? help!
« Reply #120 on: May 20, 2012, 06:20:27 PM »

While a minor quibble it's AC current so shouldn't arrow heads point both ways?
JR

Yeah, you're right.... It's a tough room, isn't it???

Done
« Last Edit: May 20, 2012, 06:28:23 PM by Mike Sokol »
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Tim McCulloch

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Re: freak accident? help!
« Reply #121 on: May 20, 2012, 07:38:44 PM »

At the request of one of my meter manufacturer contacts, I drew up a diagram of what happens when you interconnect two pieces of audio gear that are powered from a correctly grounded outlet and an RPBG outlet. Note from the attached diagram that you don't even have to turn on the power switch for the mixer or speakers, short circuit currents will flow from the grounds. However, with 100' of XLR interconnecting the gear (typical) you'll probably have a max peak shorting current of 100 amps before the Circuit Breaker tips.

Also note that this type of connection will NOT trip a GFCI on the properly wired outlet because the fault current is from ground-to-ground, and GFCI outlets don't care about ground current, only a balance of the hot and neutral currents.

Of course, any sort of DI box in the XLR line will have its internal ground wires vaporized by this fault current, and Belfoil shielded wire will be subject to turning red hot since it doesn't have the wider current path of a braided shield. This means that the shield in your snake will melt down, while the XLR cable connecting it to the offending piece of gear won't be damaged. No AC currents will flow in the 22 gauge twisted-pair signal wires, only in the shield. Of course, a pin-1 XLR lift in this connection will dump 120 volts AC right into your speakers' balanced input circuitry.

Getting even more interesting, isn't it?

Nice diagram, Mike.  The picture does indeed replace 1000 words.

Now you need to add the heartache and trauma with another frame that shows the effects of heating on the cable or the electricity being delivered to a human.  Just enough blood and gore to get the point across: this can kill you and/or burn down a building.
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Mike Sokol

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Re: freak accident? help!
« Reply #122 on: May 20, 2012, 07:56:37 PM »

Nice diagram, Mike.  The picture does indeed replace 1000 words.

Now you need to add the heartache and trauma with another frame that shows the effects of heating on the cable or the electricity being delivered to a human.  Just enough blood and gore to get the point across: this can kill you and/or burn down a building.

Something like this? 8)

« Last Edit: May 20, 2012, 08:01:33 PM by Mike Sokol »
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Tim McCulloch

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Re: freak accident? help!
« Reply #123 on: May 20, 2012, 08:15:10 PM »

Something like this? 8)

Needs some zombie food or heart exploding from the chest, lightning bolts radiating from the guitar or microphone... I was serious about the blood and guts.
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Mike Sokol

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Re: freak accident? help!
« Reply #124 on: May 20, 2012, 08:29:52 PM »

Needs some zombie food or heart exploding from the chest, lightning bolts radiating from the guitar or microphone... I was serious about the blood and guts.

When I went through my OSHA safety training back in the late 70's, they made us look at a bunch of autopsy photos of arc flash burns. I had nightmares for weeks after that.

One of things we're pitching to the PSW editors is a 12-part series on all aspects of sound system electrical hookups and safety, how to find and stop hum from ground loops, reasons for buzz, shock avoidance, grounding issues, etc... I have the ability to create all these situations at will on a bench using real instruments, amplifiers and speakers. If you think a picture is good, a video of each of these situations is great.

But you're correct, we should probably animate some of these shock situations to make a point. 

Marty McCann

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Re: freak accident? help!
« Reply #125 on: May 22, 2012, 11:36:38 AM »


When I went through my OSHA safety training back in the late 70's, they made us look at a bunch of autopsy photos of arc flash burns. I had nightmares for weeks after that.

One of things we're pitching to the PSW editors is a 12-part series on all aspects of sound system electrical hookups and safety, how to find and stop hum from ground loops, reasons for buzz, shock avoidance, grounding issues, etc... I have the ability to create all these situations at will on a bench using real instruments, amplifiers and speakers. If you think a picture is good, a video of each of these situations is great.

But you're correct, we should probably animate some of these shock situations to make a point.

This attachment should make a point . . . .
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Mike Sokol

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Re: freak accident? help!
« Reply #126 on: May 22, 2012, 11:50:06 AM »

This attachment should make a point . . . .
Yes it does. I studied that particular fault last year, which was a pair of copper thieves who tried to cut through a live high-voltage cable in a sub-station. They were using "insulated" fiberglass handle bolt cutters, but the load current in the cable created a huge arc-flash, which is basically an explosion of super-heated copper plasma on fire. Nice, huh?

Circuit breaker panels and wiring impedance limit the peak currents possible inside a residence or on a stage with Edison plugs, but you can get a pretty serious fireball with cam-locks on a 400 amp service. Big voltage will kill you, but but amperage will kill you and cook you.

I feel like having tacos for lunch. Hmmmm..... wonder why 8)

Mike Sokol

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Re: freak accident? help!
« Reply #127 on: May 23, 2012, 09:37:19 AM »

The fun continues....

I've been in contact with both Whirlwind and Ebtech engineering, who think their DI and Hum Eliminator box transformers will withstand a full 120-volts AC common-mode differential. With that in mind, I'm modifying an IMP-2 to have a neon bulb in parallel across the ground-lift switch. See diagram. I'll place this "Super-IMP" in a RPBG situation as previously described. Don't worry, I can do this safely... but kids, don't try this at home. I'm pretty sure the Super-IMP will work normally with the switch in the lift position, and there will be no audio indication that anything is wrong. But my Neon bulb will light up, indicating a BIG voltage differential between the two power outlets. Note that adding the Neon bulb isn't what makes the IMP tolerant of 120-VAC ground differentials, that's a natural part of the transformer design. So if I flipped the switch to Ground, it would short out and burn up just like any other DI box with a ground switch. However, I think that modifying your existing DI boxes with a Neon bulb, and ALWAYS starting with the switch in lift position for any double-ground situation (a grounded instrument amp on stage feeding a grounded console in the back of the room) you'll get a visual indication (big red light) that the stage (or console) ground is hot, and that you need to proceed with extreme caution. DON'T FLIP THE SWITCH TO GROUND IF YOU SEE THE RED LIGHT!

My buddies at Whirlwind are sending me a scratch-n-dent IMP-2 for this experiment (Thanks Al Keltz) so I should be able to mod the Imp and do the experiment next week. I'll report back then, but I'm 99% certain this will all work as I've envisioned.

Stay tuned...

John Roberts {JR}

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Re: freak accident? help!
« Reply #128 on: May 23, 2012, 10:05:17 AM »

That should give indication for 90v (peak dc) or more.

For DI between two, three-wire line cord products, that should work, but for one rouge outlet that powers up a chassis ground, without a solid ground on the other end you may only get an indication "while" being shocked.

Sounds like a nice elegant inexpensive indication, while neon lamps are probably not as common as they once were for on/off indicia.

You have probably already had this conversation, but remind your product manufacturing friends that this ground path, when not lifted or open, needs to be robust enough to take out the fuse/circuit breaker (tens of amps) with no life threatening voltage rise.   

JR
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Mike Sokol

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Re: freak accident? help!
« Reply #129 on: May 23, 2012, 10:54:06 AM »

That should give indication for 90v (peak dc) or more.

For DI between two, three-wire line cord products, that should work, but for one rouge outlet that powers up a chassis ground, without a solid ground on the other end you may only get an indication "while" being shocked.

Sounds like a nice elegant inexpensive indication, while neon lamps are probably not as common as they once were for on/off indicia.

You have probably already had this conversation, but remind your product manufacturing friends that this ground path, when not lifted or open, needs to be robust enough to take out the fuse/circuit breaker (tens of amps) with no life threatening voltage rise.   

JR

Yes, to all of the above. And you can still buy Neon lamps with a panel-mount from Radio Shack for around $3 a pair. I have one sitting on my desk in front of me.

And I don't think this is the fix-everything solution. But it will indicate if you have one rogue outlet on stage that's been wired incorrectly as a RPBG. The real problem is that even if the DI's ground path wire is heavy enough to withstand the full short circuit current sufficient to trip a CB, it will likely never come to that. I'll have to calculate the resistance of 100 ft of BelFoil drain wire and do the calculation, but I'm pretty sure there's enough series resistance there to reduce the short circuit current into the 30 to 50 amp peak range, possibly even below 20 amps. In that case the CB won't trip, but your snake will simply heat up red hot and melt. Something's going to become the fuse, and Murphy's Law dictates it will be the most expensive piece of gear in the current path.... 

And remember that even if the "good" outlet is wired properly with a GFCI, it won't know there's a current imbalance (because all the shorting current is in the ground wires) and won't trip at 5 mA or whatever. This fault will need to trip the 20 amp panel circuit breaker in the panel feeding the rouge outlet. This fault won't even trip the circuit breaker in your power strip, since it's not in the ground fault path. 
« Last Edit: May 23, 2012, 10:59:57 AM by Mike Sokol »
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Re: freak accident? help!
« Reply #129 on: May 23, 2012, 10:54:06 AM »


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