Originally posted by TedS
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The GFCI doesn’t use the ground wire to detect a ground fault or to test. The test function works by routing a little bit of current via a resistor from one wire on a parallel path outside the sensing coil to create a current imbalance within the coil. The test button is a very good way to verify GFCI operation.
Grounding or the lack thereof, bonding or the lack thereof, or even a ground fault prior to the GFCI won’t trip the GFCI except in the case of both an upstream and downstream bond which can result in part of the neutral current running in the EGC.
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