PURPOSE: To improve the time delay due to smoothening and eliminate the use of a large time-measuring integrating csapacitor because the conventional overcurrent protective relay adopts an analog circuit and the overcurrent is judged only after the continuation of the state of exceeding DC reference level for a predetermined time period after rectifying and smoothening a detected current from a objective power system.

CONSTITUTION: A waveform (fig. a) of detected current of a system or a full- wave rectified waveform (fig. b) by a circuit 12 is compared with a predetermined value other than zero in a comparator 11 and is turned into square wave. Time of H or L level of this square wave is determined by a circut 71 or 71 consisting of a microcomputer and compared with a predetermined value for deciding the presence of overcurrent.