This is a Miniature Circuit Breaker, thermo -magnetic, current limiting type and are available with Type B operating characteristics. MCB operating dolly may be locked in either the ON or OFF position without affecting the ability of the trip mechanism to operate. . A mechanism of the MCB has been carefully designed and engineered using thermal and magnetic elements to detect overcurrent due to both overload and fault currents. The high speed current limiting action ensures that the MCB operates before the full prospective fault current is allowed to develop. Back-up protection is required only if the prospective short circuit current at the point of installation exceeds the breaking capacity of the MCB. When providing back-up protection consideration must be given to discrimination between the MCB and fuse.