PURPOSE: To detect overvoltage with considerable precision by providing a plurality of circuits set at specified values of rated voltage and overvoltage on the primary side of a circuit breaker.

CONSTITUTION: On the primary side of a circuit breaker 1 the following two circuits are connected: (a) a circuit 11 in which a resistance R11, a constant voltage diode D11 and a light emitting diode D12 (a protective diode D13 for inverse voltage prevention inclusive) are connected in series. (b) a circuit 12 in which a resistance R12; a constant voltage diode D12 and a light emitting diode D12 (a diode D13 inclusive) are connected in series. The constant voltage diode D11 in the circuit 11 will turn in the light emitting diode by inputting operating voltage at rated voltage, while the constant voltage diode D12 in the circuit 12 will turn on the light emitting diode by applying operating voltage at a specified value of overvoltage.