PURPOSE: To detect instantaneous detection longer than a specified time without the influence of the value of the AC voltage to be detected just prior to the instantaneous interruption by using the value obtained by dividing the rectified and smoothed output of the AC voltage to be detected as a reference voltage.

CONSTITUTION: When instantaneous interruption arises in the AC voltage V1 to be detected, a DC voltage V2 drops at the curve determined by a capacitor C1 and spurious load 7, and the voltage V4 divided by a variable resistor RV1 drops correspondingly as well. On the other hand, the output voltage V3 of a reference voltage circuit 8 consisting of a diode D1, a capacitor C2 and a voltage dividing circuit 9 remains at approximately a constant value for several tens seconds because the electric discharge to the terminals 3 and 4 of the capacitor C2 is blocked by the diode D1. As a result, the output terminal of a comparator Z1 goes to a high level, a transistor Tr Q1 goes on, a relay RL1 operates and the output terminals 5, 6 are made. At the same time, Tr Q2 goes on as well, and the DC voltage V4 attains zero volts; therefore, the comparator Z1 is latched.