PURPOSE: To make it possible to trip a control object upon detection of an overvoltage by a constitution wherein an electromagnetic coil is de-energized to disable closing of the control object when the control voltage exceeds a maximum allowable level whereas the electromagnetic coil is energized to enable reclosing of the control object when the control voltage drops down to a resetting level.

CONSTITUTION: When a control voltage Vo increases over a maximum allowable level, an overvoltage appears at a voltage dividing point C and a first voltage detector 11 produces an OFF signal T1 of logic 1. The OFF signal T1 is fed to an OR gate 13 to produce a trip signal T for tripping a drive circuit 3 thus de-energizing an electromagnetic coil 4 and disabling closing of a circuit breaker. When the control voltage drops under light load, a potential having resetting level appears at the voltage dividing point D and a second voltage detector 12 produces an output signal of logic 0 thus stopping an OFF signal T2. Consequently, the trip signal T goes L to turn the driving circuit 3 ON thus energizing the electromagnetic coil 4 and enabling reclosing.