Which of the following conditions will cause a ground fault circuit interrupter (GFCI) to disconnect AC power?
Why A GFCI continuously compares the current leaving on the hot conductor with the current returning on the neutral. Those should be equal; if they differ by roughly 5 milliamps, some current is escaping to ground, possibly through a person, and the device trips in a fraction of a second. So it is current leaking from hot directly to ground that causes the disconnect, not the size of the load current.
Watch out Current from hot to neutral is simply normal load current, which a GFCI ignores no matter how large; overcurrent is the job of the fuse or breaker, and overvoltage is handled by surge protection, not a GFCI.
GFCI = balance check. Hot out must equal neutral back; about 5 mA going missing to ground trips it.