PURPOSE: To instantaneously interrupt the self-circuit when the accidents of the inside of the self-circuit while eliminating false operation generated in case of the accidents of other circuits, by mounting a circuit that judges as to whether or not the points of accidents exist in the inside or the outside of the self-circuit by the action or the non-action of a ground fault direction relay.

CONSTITUTION: The distribution lines L1, L2 of parallel two circuits are connected at a comparatively short distance between a bus bar BUS-A at the power source side and a bus bar BUS-B at the load side, and zero-phase current transformers ZCT1, ZCT2 and ground fault direction relays GD1, GD2 are each installed to the power source side and the load side. Breakers CB1, CB2 are tripped by means of a control circuit formed by combining the constantly-opened contacts and constantly-closed contacts of the ground fault direction relays GD and time limit relays T. Thus, the self-circuit can instantaneously be interrupted when the accidents of the inside of the self-circuit, and false operation generated in case of the accidents of the insides of other circuits is prevented while the self-circuit can be interrupted with the time limit to outside accidents.