PURPOSE: To prevent the extension over the permanent failure by disposing a pressure detector, and performing a stored-energy trip at the time of a grounding accident when gas pressure goes down to not more than the predetermined value.

CONSTITUTION: When the grounding accident occurs, a grounding current is detected in a zero-phase-sequence current path and stored in a memory circuit 22 after a predetermined period of time through an amplifier 17, a sensitivity controller 18 and a comparator 19. At that time, whenever a contact 5 is actuating, no signal is inputted to an OR circuit 16 by means of an inhibit circuit 21. And the voltage of a distribution line 3 is detected by an operating transformer 11 and interrupted by a substation breaker. And then only when no- voltage is detected by a novolt detector, a trip coil 8 is operated through a timer circuit 25, an AND circuit 23 and the OR circuit 16, and a gas-blast load- break switch is opened in the novolt state, whereby as reconnection cycle of the substation is successful, the service to only a non-utility electric installation is interrupted and the service to an electric power company and general users is not interrupted, a secondary failure may be prevented.