PURPOSE: To cut off an emergent power source surely, by inputting a DC output from a regular power source to a switch driving circuit as a trip current, and operating a circuit breaker by the opening or the short circuit of a UEPO switch built in the driving circuit.

CONSTITUTION: An external AC power source cutoff circuit has a main power source 1 to output an external AC power source by converting to DC, and a regular power source 2, and the DC output from the regular power source 2 is input to a switch circuit 3 as a trip current. A UEPO switch 7 is built in the circuit 3, and by the opening or the short circuit of the UEPO switch 7, a circuit breaker 5 is operated. Or, the main power source 1 to output the external AC power source by converting into DC is provided, and the DC output from an external power source is input directly to the switch driving circuit 3 as a trip current. The UEPO switch 7 is built in the circuit 3, and by the opening or the short circuit of the switch 7, the circuit breaker 5 is operated.