Testing Class A GFCI

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MD84

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Stow, Ohio, USA
I would usually consider pressing the test button on a GFCI the best way to test a GFCI. I happened to test a few GFCI receptacles in my house with some resistors of different sizes. I used an equivalent 18k ohm resistor and one of my receptacles did not trip. All the others did trip.

The test button on this GFCI did trip it and it also reset. A 5k ohm resistor did trip it. I put an ammeter in series with the 18k ohm resistor and verified 6.6mA with no trip.

Shouldn't the GFCI have tripped? Do these have a tolerance beyond 4-6mA?
 
The standard says the GFCI must not trip below 4 mA, is permitted to trip between 4 and 6 mA and must trip above 6 mA.
 
Thank you. I think I have a bad GFCI. I wonder what will cause it to have an increased trip level. I will likely do some further testing to get an accurate lower and upper trip threshold and then dissect the device to see what is going on.
 
So I am starting some further troubleshooting of this questionable GFCI. One bit of additional information; a 2 lamp T8 fixture was plugged in and operating during these tests. I unplugged the light and re-tested and it tripped. I plugged it back in and it did not trip.

I am thinking that the T8 fixture is offsetting the imbalance in the opposite way as the resistor. Any thoughts?
 
To support the theory that leakage current caused by the T8 lamps is canceling with the ground current caused by the resistor I removed the EGC from the GFCI and tested with the fixture operating. With the EGC lifted and placing the resistor from the ungrounded conductor to the EGC the GFCI tripped. Terminating the EGC back on the GFCI and placing the resistor from the ungrounded conductor to EGC resulted in no trip.

Placing an ammeter between the EGC and the GFCI equipment grounding terminal with the light operating resulted in 1.35mA.
 
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