PURPOSE: To provide possibility of checking the open phase protecting motion of a neutral line even under normal operation by connecting an impedance means having different impedance between two voltage lines and the neutral line, and thereby generating an unbalanced voltage state in the same as at the time the neutral line is in open phase.

CONSTITUTION: When a neutral line 1c goes into open phase, the loads L1 and L2 become off equilibrium to cause either voltage to boost. This balanced voltage is divided and impressed on a terminal 8 of an integrated circuit IC, and is another terminal 2 generates an operational output. After a capacitor C3 as a timer is charged, a thyristor Th is turned on, and the output resistance value of a rectifier circuit 3 is lessened. A current is supplied to a trip coil Ry of a breaker, and its main contact RyS is opened, and the power to the loads L1, L2 is shut off. In testing, unequilibrium is generated on the voltages of resistances Z1, Z2 by turning on a test switch SW, and the main contact RyS is operated in the same manner as at the time the neutral line goes in open phase.