PURPOSE: To permit a phase comparison relay to operate and make this allowable time be inversely proportional to the size of failure current by operating a failure detection relay or obtain an accurate operation by controlling the allowable time in a contrary direction of the size of failure current in stages.

CONSTITUTION: At system failure a failure detection relay 8A operates. While an inverse time limit characteristic and overcurrent relay 7A operates, an output signal is generated in an AND circuit 9A and a trip circuit 6A is energized by operating an internal failure phase comparison relay. On the other hand, after the inverse time limit characteristic and overcurrent relay 7A operates at external failure, the operation of the AND circuit 9A is locked by an inhibit gate 91A. Even when a phase comparison relay operates due to transient phenomena after the external failure is removed, mistripping does not occur