GFCI Operation:
FYI most GFCI receptacles and circuit breakers use an Op-Amp to determine that the currents in the hot and the neutral are equal. In short what goes in must come out, however if they are not equal it must mean that another path to ground has been found, that being someone's tender nervous system and it trips. I have one that is designed to be used at the end of an extension cord and it sure can be handy when you're under a house or in a damp location. Dale