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Author Topic: GFCI leakage testing  (Read 12294 times)

Stephen Swaffer

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Re: GFCI leakage testing
« Reply #30 on: June 08, 2014, 03:42:36 pm »


Sorry, I should have typed using my twangy 230v Australian accent.  :-) 


I wondered if that were the case-but rest of the thread was talking about 120 V systems.  I run into to many people that work enough with electricity that they should know better, but they don't understand basic relationships.  Ohm's Law and power calculations are very simple, basic physics - we might as well encourage that understanding with a discussion about a real world test.

Also, I know no one else would do this, but it is very easy to overlook a side effect of a solution like clamping diodes.  Although it gives plenty of range for this test-next month I might try to measure something entirely different and grab my handy tester-only to get confusing results!

I agree that a 1 ohm resistor seems right for this test. 
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Steve Swaffer

Lyle Williams

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Re: GFCI leakage testing
« Reply #31 on: June 08, 2014, 06:41:03 pm »

For my no-shunt jig with the diodes, there is a side effect of the diodes worth mentioning. They can clip the ac waveform peaks.  The measurement limit with two diodes is less than 10ma.  Current measurements are only linear until the diodes start to clip the peaks.

A one ohm resistor is good as long as your voltmeter reads millivolts (or even microvolts) well.  My multimeter jumps around quite a bit at low levels.
« Last Edit: June 08, 2014, 07:01:07 pm by Lyle Williams »
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Mike Sokol

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Re: GFCI leakage testing
« Reply #32 on: June 09, 2014, 07:18:43 pm »

A one ohm resistor is good as long as your voltmeter reads millivolts (or even microvolts) well.  My multimeter jumps around quite a bit at low levels.

I was messing with GFCI leakage measurment ideas today and revisited my idea of looping a wire multiple times through a clamp meters jaws to increase the resolution. Looking for a plastic spool for the loops I found this, a spool of 12-gauge magnet wire already on a plastic spool. I could simply drill a big hole in the center for the clamp jaws to fit through, and rewind it for a 10X or 100X multiplier depending on how many wraps you want to use. Solder on some green stranded THHN (I think) wire to run between the ground screws in a pair of male and female Edison plugs, and now you have a fractional milli-amp tester from any cheap clamp meter with 10 mA resolution you have laying around. What do you all think? 
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Mike Sokol
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Stephen Swaffer

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Re: GFCI leakage testing
« Reply #33 on: June 09, 2014, 11:31:55 pm »

How well the wire/loops is centered in the jaws affects accuracy.  I would suggest drilling a hole in a way that centers the coils as much as possible, and also limits movement to reduce variability. Then you could test for accuracy with a low voltage AC supply, a potentiometer, and an accurate mA wired in series with the loop.

Of course, the more coils you have the more inductive impedance you introduce to the circuit. Without some serious calculations, I have no idea if this even approaches the 1 ohm resistive impedance we had been discussing-that may very well more of a theoritical consideration than a practical one.
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Steve Swaffer

Mike Sokol

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Re: GFCI leakage testing
« Reply #34 on: June 09, 2014, 11:48:27 pm »

How well the wire/loops is centered in the jaws affects accuracy.  I would suggest drilling a hole in a way that centers the coils as much as possible, and also limits movement to reduce variability. Then you could test for accuracy with a low voltage AC supply, a potentiometer, and an accurate mA wired in series with the loop.

Of course, the more coils you have the more inductive impedance you introduce to the circuit. Without some serious calculations, I have no idea if this even approaches the 1 ohm resistive impedance we had been discussing-that may very well more of a theoritical consideration than a practical one.

My plan is to build both the resistive tester as well as the coil multiplier and compare field accuracy. I'm fairly certain that centering the clamp in the coil isn't required for accuracy, but perhaps I'm wrong. That's easy enough to test once this is all set up on the bench. And yes, I'll check overall calibration against my fancy Fluke meter and external clamp that's goes to 4 decimal place accuracy and has a factory calibration sticker on the side. It reads down to 1/10 of an mA in the jaw clamp, but unfortunately costs around $1,000.

I just want a simple and affordable system that anyone can use to read ground leakage currents to 1 mA sensitivity. I think that's the best way to troubleshoot GFCI tripping, not only for ProSound users, but for RV owners as well who are always complaining about GFCI random tripping.
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Mike Sokol
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Jeff Carter

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Re: GFCI leakage testing
« Reply #35 on: June 10, 2014, 12:20:01 am »

Of course, the more coils you have the more inductive impedance you introduce to the circuit. Without some serious calculations, I have no idea if this even approaches the 1 ohm resistive impedance we had been discussing-that may very well more of a theoritical consideration than a practical one.

One example of a coil inductance calculator here. At some level this is an approximation but gives a bit of a feel for magnitude and scaling:
http://www.66pacific.com/calculators/coil_calc.aspx

Inductance goes up as the square of the number of turns so it's most worrisome for a large number of turns. Depending on the geometry inductance scales somewhere between linearly (short coil) and quadratically (long solenoid) with radius, so a small coil will have lower inductance than a large one for the same number of turns.

In terms of actual numbers, I wound a few magnetic field coils in my PhD work. Inductances for the largest ones I did (square, about 12" a side, 64 turns) were on the order of 1 mH give or take, or a reactance of about 0.3 ohms at 60 Hz. As such I think a 10x multiplier with the dimensions of the spool should definitely be OK, and even 100x is probably OK given that the reactance of the coil adds in quadrature with the 1 ohm sense resistance to give the total impedance of the circuit (I assume you're worried about the coil impedance and sense resistor forming a voltage divider?)
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Mike Sokol

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Re: GFCI leakage testing
« Reply #36 on: June 10, 2014, 12:28:45 am »

In terms of actual numbers, I wound a few magnetic field coils in my PhD work. Inductances for the largest ones I did (square, about 12" a side, 64 turns) were on the order of 1 mH give or take, or a reactance of about 0.3 ohms at 60 Hz. As such I think a 10x multiplier with the dimensions of the spool should definitely be OK, and even 100x is probably OK given that the reactance of the coil adds in quadrature with the 1 ohm sense resistance to give the total impedance of the circuit (I assume you're worried about the coil impedance and sense resistor forming a voltage divider?)

I love it when the eggheads crawl out of the woodwork.  ;D

I would only do one current measuring method at a time, so too much series impedance at 60 Hz would reduce the actual current leakage and mess with the measurment. But my gut feel from messing with coils this size for speaker crossover systems is that any inductive effect would be quite small and not significantly change the measurement. I'll run the numbers and gather some empirical data to compare them against once I get the experiment on the bench.

Too much fun...  8)
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Mike Sokol
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Mike Sokol

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Re: GFCI leakage testing
« Reply #37 on: June 10, 2014, 06:35:21 am »

For my no-shunt jig with the diodes, there is a side effect of the diodes worth mentioning. They can clip the ac waveform peaks.  The measurement limit with two diodes is less than 10ma.  Current measurements are only linear until the diodes start to clip the peaks.

A one ohm resistor is good as long as your voltmeter reads millivolts (or even microvolts) well.  My multimeter jumps around quite a bit at low levels.

Yes, diodes are a double-edged sword... they offer protection but also the possibility of non-linear readings. My plan was going to be to mark the unit with a max number it will measure before being suspect. Of course, clipping diodes plus a 100-watt dissipation resistor is kind of a belt and suspenders approach. But I've learned a long time ago that as soon as you think something can't happen, worlds collide. Besides, as mentioned earlier, this is a great mind experiment for all of us to understand the design and calibration of test gear. This will help when we want to develop actual procedures to troubleshoot GFCI tripping scenarios.
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Mike Sokol
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Mike Sokol

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Re: GFCI leakage testing
« Reply #38 on: June 10, 2014, 06:54:35 am »

As such I think a 10x multiplier with the dimensions of the spool should definitely be OK, and even 100x is probably OK...

Case in point... you can buy a commercial product called an Amprobe ELS2A that splits out the hot wire of an Edison outlet for clamp meter measurements. It includes one section marked X1 and a second section marked X10 which obviously has 10 loops internally. I just want something similar that measures the EGC (Ground Wire) current, and maybe has a 100X multiplier loop since the ground leakage currents should be really small for properly functioning gear.

No, this isn't going to be laboratory grade gear with traceable calibration and certification. I want something simple that any ProSound or RV technician can build or purchase in order to better troubleshoot GFCI issues. I just hate flying blind when troubleshooting, and think that GFCI tripping problems will only get more common. 
« Last Edit: June 10, 2014, 06:59:24 am by Mike Sokol »
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Mike Sokol
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Stephen Swaffer

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Re: GFCI leakage testing
« Reply #39 on: June 10, 2014, 08:17:34 am »

The only reason I mention centering the coils on the clamp is that Klein mentions centering the wire in the clamps for max accuracy.  I know you aren't supposed to read instruction manuals, but its amazing how many times people asked me, "How did you know that?" when I fixed something in the factory!   In any case, I was reading to understand the variation I commonly notice when trying to check the amp draw on the motor.
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Steve Swaffer

ProSoundWeb Community

Re: GFCI leakage testing
« Reply #39 on: June 10, 2014, 08:17:34 am »


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