PURPOSE: To prevent the burning of a load by inserting an impedance with a proper value between an optional voltage line and an abnormal voltage detection terminal.

CONSTITUTION: In case an impedance Z3 is provided between a voltage line L2 and an abnormal voltage detection terminal U, the current is supplied to another voltage line L1 through an impedance Z2, Z1 and a rectifier section. When the impedance Z1 is so set that voltage VZ1 to be generated in the impedance Z1 is VZ1VS against reference voltage VS of a comparison section by the current, a signal is sent to a switching element (SCR)2 through the comparison section to operate the switching element 2, and a specific voltage is applied to a trip coil 3 of a breaker to open the breaker.