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Author Topic: Stacked GFI's? Had an issue at a show last night.  (Read 3539 times)

Stephen Swaffer

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Re: Stacked GFI's? Had an issue at a show last night.
« Reply #10 on: July 17, 2017, 12:55:28 pm »

YThat's why it's a bad idea to have a single 50-amp hot tub GFCI protecting your entire stage. Far better to have half a dozen power drops, with each 20-amp drop protected by a GFCI. Then when something trips a GFCI, it only takes down one instrument on stage. Much easier to troubleshoot during/after the show. and much less impact during the show itself. It's a win/win situation (is that me repeating myself?)

I agree-however the 2017 NEC requires all outdoor 50 amp single phase receptacles to be GFCI protected.  Indoors you will OK-but outdoors it's gong to be a problem sooner or later.
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Frank Koenig

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Re: Stacked GFI's? Had an issue at a show last night.
« Reply #11 on: July 17, 2017, 01:26:53 pm »

It is pretty simple to unplug the suspect gear and measure resistance from one pin of the plug to chassis ground with a VOM.

JR,

True this. The Ohmmeter is the first test. (That's what I suggested for sorting the tingling tubs.) But if the device passes that and still trips the GFCI we need to see what the GFCI is seeing.

I built that fixture when trying to sort out nuisance trips with some Powersoft K-10s. They tripped the GFCIs in a certain venue, but would not trip any of the GFCIs in my home, or anywhere else I have used them. My fixture revealed a good bit of differential HF hash, which, for lack of a better explanation, I believe is responsible for the problem.

Anyway, I just like building little test fixtures :)

Best,

--Frank
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Mike Sokol

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Re: Stacked GFI's? Had an issue at a show last night.
« Reply #12 on: July 17, 2017, 04:01:40 pm »

I built myself a little ground leakage probe by opening up a hardware store GFCI receptacle and bringing the secondary leads of its differential transformer out to some banana jacks. The rest of the GFCI circuitry is abandoned. An AC voltmeter (or scope) plugged into the banana jacks measures leakage. Calibration is performed using a known resistor from leg to ground.

I don't know about its frequency response, which would be useful for investigating SMPS induced nuisance trips, but for simple 60 Hz leakage it works fine.

--Frank

Frank, what's the ratio of leakage current to measured voltage across the test resistor? You knew I was going to ask that, didn't you?
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Mike Sokol
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Frank Koenig

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Re: Stacked GFI's? Had an issue at a show last night.
« Reply #13 on: July 17, 2017, 05:07:06 pm »

Frank, what's the ratio of leakage current to measured voltage across the test resistor? You knew I was going to ask that, didn't you?

Hi Mike,

With a line voltage of 122 V and a test resister of 17.5 kOhm I should have a leakage current of 7 mA. I, somewhat arbitrarily, selected a burden resister of 1 kOhm, across which I get 5.4 mV under these conditions. The calibration appears to be pretty linear for a factor of 2 up and down.

The short-circuit current (as close to a short as the AC-uA range of the Fluke meter gets) is  6.1 uA. The DC resistance of the secondary is about 50 Ohms. We're dealing with pretty small signals in any case, and need a good AC meter.

--Frank
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Jonathan Johnson

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Re: Stacked GFI's? Had an issue at a show last night.
« Reply #14 on: July 18, 2017, 12:55:11 pm »

Let's put our heads together and come up with a cheap/simple way to read line-to-ground leakage currents on individual pieces of gear. I've got a $1,000 Fluke meter for that will go below 1mA, but cheap ($50) clamp-on ammeters usually bottom out at 10 mA resolution. I have a few ideas that should work but I'll need your help to try them. Anyone else have experience in sub-mA clamp ammeters? Maybe this should be a simple inline ammeter?

Make up a short cord with male and female plugs. In between, loop the ground wire 10 times.

Then use your cheap clamp-on ammeter with the 10 mA resolution to measure the current on the ground wire, and divide by 10 to get the current.

Of course, this assumes that ALL the leakage current returns on the ground wire.

You could also loop both the hot and neutral wires together 10 times and measure that instead (a differential current transformer), but I'm not certain of the safety of that idea in terms of the ability of the loop to dissipate heat. Of course, you'd need to leave the ground out of THAT loop.

However you do it, the wiring and loop of the cord would need to have an outer jacket to protect from physical damage.
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Mike Sokol

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Re: Stacked GFI's? Had an issue at a show last night.
« Reply #15 on: July 18, 2017, 01:48:57 pm »

Make up a short cord with male and female plugs. In between, loop the ground wire 10 times.

Then use your cheap clamp-on ammeter with the 10 mA resolution to measure the current on the ground wire, and divide by 10 to get the current.

Of course, this assumes that ALL the leakage current returns on the ground wire.


Yep, I done that and it works. If you only have one piece of gear plugged into test, then the ground current should be the total leakage current. If the gear is connected to other gear via XLR connectors and such, then you could have secondary current paths. But for testing a single piece of suspect gear on the bench that's probably the most cost effective way since you can use a $50 clamp-on ammeter.
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Mike Sokol
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Re: Stacked GFI's? Had an issue at a show last night.
« Reply #15 on: July 18, 2017, 01:48:57 pm »


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