Abstract: A circuit breaker mechanical interlock apparatus is provided for preventing an electrical system from being energized by more than one of a plurality of power sources at any given time. The apparatus includes a housing that defines a receptacle for receiving two or more circuit breakers having actuating switches for energizing the electrical system. The apparatus further includes one or more interlock slides movably mounted on the housing and a faceplate for restricting unauthorized access to the slides. The number of interlock slides is one fewer than the number of circuit breakers so that the slides can be positioned to provide access to only one of the switches of the circuit breakers, while the remaining switches of the circuit breakers cannot be accessed. Each of the switches of the circuit breakers can be individually accessed without the slides extending beyond the periphery of the housing.