PURPOSE: To prevent unnecessary operation or non-operation by detecting its badness as soon as the filter chracteristics were changed due to the falut of band pass filter circuit.

CONSTITUTION: The both poles A and B of power system are provided with devices of the same constitution. Of these, the pole A is provided with the input circuit 1A for the phase-comparing relay of the terminal A, the band pass filter 2A, the differential circuit 7A to calculate the output difference between the input circuit 1A and the filter circuit 2A, the level circuit 8A to generate output when the output of the circuit 7A exceeds a predetermined level, and the supplementary relay 9A to display an alarm in response to the output of the level circuit 8A. With the circuit, the output voltages of the circuits 1A and 2A are compared by constant monitoring, and in case where each circuit is normal, no difference exists between the output voltages of both, whereas if a badness was discovered, a difference is detected and a badness of relay is therefore detected.