There are two arrangement of operation of miniature circuit breaker. One due to thermal effect of over current and other due to electromagnetic effect  of over current. The thermal operation of miniature circuit breaker is achieved with a bimetallic strip whenever continuous over current flows through  MCB, the bimetallic strip is heated and deflects by bending. This deflection of bimetallic strip releases mechanical latch. As this mechanical latch is  attached with operating mechanism, it causes to open the miniature circuit breaker contacts. But during short circuit condition, sudden rising of electric  current, causes electromechanical displacement of plunger associated with tripping coil or solenoid of MCB. The plunger strikes the trip lever causing  immediate release of latch mechanism consequently open the circuit breaker contacts. This was a simple explanation of miniature circuit breaker