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The GFCI's built-in test button and your handheld test button do two different things.

The one that's built into the GFCI passes some power through a resistor to create an imbalance in the circuit. The GFCI sees this imbalance and trips. It'll test the GFCI even if the device itself isn't grounded.

The one on your handheld tester shunts some power to the ground pin to create a ground fault. If that ground pin isn't actually connected to the equipment grounding system, then there'll be no fault, the GFCI won't see an imbalance, and it won't trip.

Perhaps the GFCI you were testing wasn't grounded or was "grounded" with a bootleg ground.

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The circuitry on the two are different so they work differently. Even if both were designed to trip at the same level of ground-fault current, an error might still result because different designs might yield different tolerances. I'm not sure all GFCI device manufacturers use the same design and I doubt they do. Engineers quibble.

Lastly, specifying a standard trip value (for all manufacturers to abide by) when the current in question fits no mathematical description (faults don't necessarily follow rules) can be tricky.

A few years ago, I reviewed a common design used on those 'big box' GFCI testers. It had an obvious fault that would cause an incorrect display under certain test conditions. They're handy and cheap but they do err sometimes, just keep their limitations in mind.

What Jim K said.

Marc

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