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Author Topic: Dangerous AC situation in reception hall - PLEASE READ  (Read 184401 times)

Tim McCulloch

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Re: freak accident? help!
« Reply #110 on: May 15, 2012, 10:58:19 AM »

An Edison NEMA 5-15 or 5-20 outlet can NEVER be legally wired to 240 volts, no matter how big the signage.

There's a really quick way to test for 240 volt wiring using a non-contact tester such as a Volt-Alert. If there's interest here I'll make a video of it and post.

Mike, the key word here is "legally."  Up thread I posted about an Edison outlet I found in a hotel ballroom that was wired for either 208v or 240v.  It was a 20 amp, 120v outlet.  After the squints blew 3 follow spot lamps (on a Saturday in a non-major metropolitan city I metered it for them.

The hotel insisted there was nothing wrong with the outlet.  If I could have gotten a city inspector there on Saturday, I'd have tried to prove them wrong... oh well.
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Mike Sokol

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Re: freak accident? help!
« Reply #111 on: May 15, 2012, 12:07:41 PM »


The hotel insisted there was nothing wrong with the outlet.  If I could have gotten a city inspector there on Saturday, I'd have tried to prove them wrong... oh well.

The scary part is I'll bet that outlet is STILL wired for 208/240 volts, and it's waiting for the next piece of 120 volt A-V gear to destroy.

Greg_Cameron

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Re: freak accident? help!
« Reply #112 on: May 15, 2012, 12:39:50 PM »

Dryers and ranges do not ship with cords. The cordset is attached by the installer. Because 3-prong 120/240V receptacles (hot-hot-neutral) are common in older homes while newer homes have 4-prong 120/140V receptacles (hot-hot-neutral-ground), the manufacturer leaves it up to the installer to choose and install the cordset that matches the existing receptacle.

FWIW, this is a gas dryer with a standard 120V 15A Edison plug, not an "electric" dryer with a 240V plug. So the cord in this case was installed by the manufacturer.

Greg
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Mike Sokol

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Re: freak accident? help!
« Reply #113 on: May 15, 2012, 01:45:27 PM »

FWIW, this is a gas dryer with a standard 120V 15A Edison plug, not an "electric" dryer with a 240V plug. So the cord in this case was installed by the manufacturer.

Greg
So is this fixed now?

Another interesting thing is that a non-contact tester (Fluke VoltAlert) will alert if pointed at a chassis that's energized to more than 40 volts. See my video of an energized VW micro/micro bus (yes, that's two micro's). http://www.youtube.com/watch?v=rtT3te_XNBM

If you touch a VoltAlert to any appliance that has a grounded cord, it should NOT beep at you. If it beeps, then you have a failed ground, and the mic or mixing console (or dryer, in this case) is electrically energized.

If you touch a VoltAlert to a double-insulated appliance (one without a ground pin on the factory cord), then the appliance chassis will float up to approximately 1/2 line voltage (around 60 volts) which will make a non-contact tester beep. But there's not enough current (way under 1 mA) to cause a shock. That's how manufacturers qualify to leave off a grounded cord on double-insulated appliances, by certifying there's minimal internal AC leakage to the chassis.

The reasons that a "grounded" appliance shows voltage on its chassis can vary from an improperly grounded outlet, to a broken ground pin on the power cord plug, to a disconnected green/ground wire inside the appliance (or guitar amp). I've seen all of them too many times to even count. 
« Last Edit: May 15, 2012, 01:51:53 PM by Mike Sokol »
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Mike Sokol

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Re: freak accident? help!
« Reply #114 on: May 15, 2012, 02:02:50 PM »

If any of you tie directly into power panels in older industrial buildings (like I used to do for my band's 50KW lighting system in my mis-spent youth) you should be aware of something called a High-Leg or Red-Leg Delta panel. See http://en.wikipedia.org/wiki/High-leg_delta

Too much to go into within this thread, but quickly note that from Neutral to two of the phases it measures 120 volts, but from Neutral to the High-Leg phase it measures 208 volts. This was common practice in industrial buildings in the 50's and 60's since it saved the power company from installing a separate single-phase transformer for office power.

I first ran into this at the Old Mill Inn back in the late 70's. It was actually a mill converted into a club, and when an incoming group tapped into the main panel to run their lights, they fed 208 volts into their lighting system by mistake. Blew out dozens of bulbs in one shot. I had a meter with me and worked as an IE for Corning, so I knew to stay away from the High-Leg 208-volt buss with my own lighting distro.

Meters are cheap... replacement bulbs are not...!

John Livings

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Re: freak accident? help!
« Reply #115 on: May 15, 2012, 03:46:51 PM »

A source of good information is;

"Electricity for the Entertainment Electrician & Technician"

By  Richard Cadena.

http://www.amazon.com/Electricity-Entertainment-Electrician-Technician-Richard/dp/0240809955/ref=la_B001IO9WAG_1_1?ie=UTF8&qid=1337110400&sr=1-1

It really covers a lot different scenarios,  most helpful information.

Regards,  John
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Mike Sokol

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Re: freak accident? help!
« Reply #116 on: May 16, 2012, 10:00:56 PM »

A source of good information is;

"Electricity for the Entertainment Electrician & Technician"

By  Richard Cadena.

http://www.amazon.com/Electricity-Entertainment-Electrician-Technician-Richard/dp/0240809955/ref=la_B001IO9WAG_1_1?ie=UTF8&qid=1337110400&sr=1-1

It really covers a lot different scenarios,  most helpful information.

Regards,  John

Looks like a good book. I'll order a copy for my library...

Mike Sokol

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Re: freak accident? help!
« Reply #117 on: May 18, 2012, 06:53:53 PM »

For those interested in grounding and ground-loop issues, the saga continues on this thread  8)

http://forums.prosoundweb.com/index.php/topic,138041.0.html
« Last Edit: May 18, 2012, 06:56:01 PM by Mike Sokol »
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Mike Sokol

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Re: freak accident? help!
« Reply #118 on: May 20, 2012, 04:25:02 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?   
« Last Edit: May 21, 2012, 12:23:46 PM by Mike Sokol »
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John Roberts {JR}

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Re: freak accident? help!
« Reply #119 on: May 20, 2012, 06:15:32 PM »

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


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Re: freak accident? help!
« Reply #119 on: May 20, 2012, 06:15:32 PM »


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