PURPOSE: To absorb a surge only by a voltage absorbing element, and prevent failure of an internal circuit by connecting resistors to divide voltage between electric wires and a trip coil in series to each other, and monitoring voltages on both ends of the resistors by an overvoltage detecting circuit.

CONSTITUTION: Overvoltage between voltage lines is detected in a passage of a voltage line L1 - a trip coil TC1 - a resistor R1 - a resistor R2 - a diode D1 - a voltage line L2. When large surge voltage is impressed between the voltage lines L1 and L2, first of all, it is absorbed by a voltage absorbing element ZNR1, and unabsorbed surge voltage is divided by the resistors R1 and R2 and the coil TC1. At this time, the trip coil TC1 acts as an inductance to voltage short in the time like the surge voltage, and the surge voltage impressed on the resistors R1 and R2 is reduced, and this voltage is monitored between an overvoltage detecting circuit A-B, and when detecting voltage becomes a prescribed value or higher, a thyristor SCR1 is turned on, and breaks a main circuit contact point 4.