Abstract: A circuit breaker comprises a housing containing a trip mechanism, a handle coupled to the trip mechanism and movable between on and off positions, and a lock-off mechanism capable of holding the handle in its off position. The lock-off mechanism includes a base member adapted to be mounted on the circuit breaker housing adjacent the movable handle. A slide is mounted on the base member for movement to an advanced position in which a portion of the slide projects into the path of movement of the breaker handle for engaging the handle and blocks movement of the handle from the “off” position to the “on” position. In the retracted position of the slide, the path of movement of the handle is unobstructed by the slide.