PURPOSE: To detect the whole area of a generator output with the same detecting sensitivity by comparing the output voltage of a converter for generating a third harmonic component responsive to the output of the generator with a zero-phase voltage.

CONSTITUTION: A voltage at both ends of a neutral point ground resistor 4 is detected through a transformer 5 for an instrument by a voltage converter 12, and the output of a generator 3 is detected through a transformer 8 for an instrument and a current transformer 9 for an instrument by a power converter 10. A third harmonic voltage responsive to the output of the generator 3 is generated from a converter 11 which inputs the output of the converter 10. The output voltage of the converter 11 is compared by a ground fault detecting relay 13 with that of the converter 12, and if the output of the converter 12 is larger than that of the converter 11, the relay 13 is operated to detect a ground fault.