An interlock mechanism for preventing a first circuit breaker having a first actuating mechanism and a second circuit breaker having a second actuating mechanism from simultaneously being in a closed condition. The interlock mechanism includes a frame, a sliding bar slideably mounted within the frame, and one or more retaining protrusions extending outwardly from one or more inner edges of the frame. The sliding bar is movable between a first position and a second position. In the first position the sliding bar blocks a path of movement of the first actuating mechanism and in the second position the sliding bar blocks a path of movement of the second actuating mechanism. The one or more retaining protrusions frictionally engage the sliding bar when the sliding bar is in the first position and holds it in place, preferably with a frictional force sufficient to resist the force of gravity and/or vibrations.