To facilitate the protection on a customer receiving equipment side eve if an intermittent ground fault with a long failure rest time by a method wherein, if a period during which a system failure is not detected continuous longer than a predetermined time, the integral element of the system is reset.

If a ground fault current flows through the customer power receiving equipment, an A/D converter 11 converts the inputted failure current value into a digital value. A failure detection element 12 judges the system failure from the digital value and a detection signal which is delayed by an operation time and a restoration time from an intermittent ground fault time is outputted. The system failure time is integrated by an integral element 13. If a failure rest time occurs, a rest detection element 14 detects a period during which a system failure is not continuously detected and, if the continuous period is longer than a predetermined time, the integral element 13 is reset. As the detection time of the rest detection element 14 is short and resets the integral element 13 periodically, the integral value does not exceed the predetermined value and a circuit breaker is not tripped. Therefore, the grounding failure does not give influence upon the substation side.