PURPOSE: To enable a grounding fault circuit to be cut without interrupting normal circuits, by calculating the phase angle of the zero-phase current of each circuit to zero-phase voltage, or the phase angle of the displacement component of the zero-phase current to the change component of the zero-phase voltage before or after a grounding fault.

CONSTITUTION: The zero-phase voltage of a high tension distribution line and the zero-phase current of each circuit are sample-held by sampling hold circuits 1A-1N. The input of the output of the respective sample hold circuits 1A-1N to a fault circuit detecting means 5 via a multiplexer 3 and an A/D converter 4 is provided. By the fault circuit detecting means 5, calculation is performed on the phase angles of the zero-phase voltage and the zero-phase current on every circuit, and by the phase angles, arbitrary phase whose lag ranges 45 degrees to zero degree is set to be of a range limit on one side, and a circuit in an area for setting the phase of 180 degrees lead to be of a range limit on the other side is set to be a grounding fault circuit. Via an interface circuit 6, interrupting command is applied to the circuit.