PURPOSE: To continuously (detect fault within a target fault detection range and to protect a power system by comparing an input current with an operating current of a relay corresponding to the operating state of a transformer.

CONSTITUTION: Operating currents of a relay, corresponding to fault currents, are stored in an operating current memory means 17. Operating state of a transformer is them detected through an operating state detecting means 18, and an operating current value corresponding thereto is selected through an operating current value memory means 17 and set in a current value selecting/ setting means 19. A protective operation means 20 compares an input current with the operating current value set in the operating current selecting/setting means 17. If the input current is higher than the set value, a judgment is made that a short circuit fault has occurred within fault detectable range of the relay and a protective output is provided in order to protect a power system. By such arrangement, a fault within a target detection range can be detected reliably at all times.