HM 1L surplus circuit protective moulded case breaker series(hereinafter called circuit breaker)are applied for the power distribution network of AC 50Hz, rated voltage 400V and rated current 630A . The circuit breaker can protect against direct contact with dangerous electric current and prevent fire disaster caused by insulation fault and single-phase ground fault. It can be used to distribute electric power and protect power equipment against overload and short current. The circuit breaker can change the circuit and start motor infrequently. The circuit breaker can work normally in the case that one of three phases is lacked.