A molded case circuit breaker is taught with a housing base and a primary cover disposed on the housing base. The primary cover has a recess therein for an auxiliary module which is disposed in the recess. A secondary cover is disposed on the primary cover for covering the recess when the auxiliary module is disposed therein. A combination manual trip and secondary cover interlock is provided which is accessible from outside of the secondary cover for manually opening separable main contacts or for automatically opening separable main contacts when the secondary cover is removed. There is also provided on the back plane of the housing a single piece DIN rail attachment for securing the back plane to a DIN rail. There is provided an under voltage release mechanism for actuating the circuit breaker to trip on the occurrence of an under voltage condition of pre-determined magnitude across the lines served by the circuit interrupter. The under voltage release mechanism has an adjustment spring which is controlled by a nut or thumb screw on a shaft to vary the spring force for calibrating the under voltage release mechanism. There is also provided a self-retaining collar member for the load and line terminals of the circuit interrupter. The collar member comprises a locating protrusion in the bottom thereof which aligns with a hole in the load or line conductors. Side entrapment members which entrap the line or load conductor therebetween and between the bottom of the collar member are also provided.