PURPOSE: To detect at a high speed a grounding accident in a power receiving/ distributing system by constituting a device of a positive current value detecting means, negative current value detecting means, current ratio deciding means and an operating output deciding means.

CONSTITUTION: In positive and negative current value detecting means 14, 15, a secondary current of an instrument current transformer, connected to a power system, is fetched as an input data, to obtain positive and negative waveform components of this input current respectively separated. In a current ratio deciding mans 18, ratio of a difference between positive and negative current values is obtained from the respective data of the positive and negative current value detecting means 14, 15. In an operating output deciding means 20, operation is output on condition that both the positive and negative current value detecting means 14, 15 are a preset value or more and further ratio of the positive current value to the negative current value is a preset value or less in the current ratio deciding means 18. In this way, wrong operation is prevented due to an exciting rush current in a transformer, and an earth accident in a power receiving/distributing system can be detected at a high speed.