PURPOSE: To reduce the working time with the simple working by arranging a connection point of both terminals of a series circuit and one phase of a main circuit in the main circuit at a position, which is positioned in the power source side and the load side of a zero-phase current transformer.

CONSTITUTION: When a push button switch 8 of a series circuit is turned ON, the main circuit current flowed from a power source inside of a printed board 1, which is obtained from a main circuit, is flowed in order of a connecting plate 4 - a test resistor 2 - the switch 8, and returned to the power source inside of the printed board 1, and the main circuit current passing through a ZCT 9 is the only one phase, and an earth leakage breaker is turned OFF. The condition similar with the one-line ground is thereby generated in the real circuit, and OFF operation of the circuit-breaker can be tested.