Abstract: This concerns a molded case circuit breaker having separable main contacts and an operating mechanism structured to cause the separable main contacts to open and close. A trip unit is provided to actuate the operating mechanism in desirable circumstances. The circuit breaker housing has an auxiliary device compartment therein into which may be inserted a key lock arrangement. The trip unit has a plunger member which protrudes into the auxiliary device compartment and which, when actuated, causes the trip unit to actuate the operating mechanism to open the main contacts. When the key lock is actuated, a slideable member interacts with the plunger member in the trip unit and maintains the trip unit in a configuration that prevents the main contacts from engaging. Thus, as long as the lock member is maintained in the locked state, the circuit breaker may not be reset.