PURPOSE: To prevent unnecessary function upon occurrence of ground fault due to an excessive surge such as a lightning surge by a method wherein a synthesizing means synthesizes an output from a ground fault detector and an output lagged behind by a predetermined time and then the level is judged based on thus synthesized output.

CONSTITUTION: Ground fault current flowing through an AC path 1 is detected through a ZCT 6 and amplified through an amplifier 20 and then fed to a level judging unit 21 and a delay circuit 25. The level judging unit 21 produces a pulse signal having time width corresponding to the interval of negative level. The delay circuit 25 produces an output lagged by half period behind the output from the amplifier 20. An adder 27 adds the outputs from the amplifier 20 and the delay circuit 25. A retriggerable one-shot circuit 28 generates a pulse signal having time width corresponding to the interval of positive level. An AND gate 31 performs logical operation on both pulse signals and provides the output to the gate electrode of a switching element 8 thus operating a circuit breaker 5 to interrupt the AC path 1.