PURPOSE: To permit detection of principally intentional breaker throwing command from an operating unit upon throwing a breaker without adding any new circuit or component.

CONSTITUTION: Resistors R1-R3 and photocouplers PC1-PC3 are provided so far whereby it is permitted to detect whether or not any disconnection is generated in circuits after a point B. However, a command receiving relay becomes impossible to be operated by the provision of such a disconnection detecting circuit even when a breaker throwing command is given whereby it becomes impossible to detect principally intentional breaker throwing command from an operating unit 1 upon throwing the breaker. Then, a disconnection detecting circuit is used both for the detecting circuit of principally intentional breaker throwing command from the operating unit 1 upon throwing a breaker whereby detection of this kind can be effected simply and inexpensively.