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. The contacts themselves are manufactured from carefully chosen materials, selected specifically for their low electrical resistance and low propensity to weld under fault conditions. A mechanism of the Sentry MCB has been carefully designed and engineered using thermal and magnetic elements to detect overcurrent due to both overload and fault currents. It will operate and interrupt the supply to prevent damage to the installation. The thermal component is a carefully calibrated, thermally operated bi-metal element. Larger overloads and fault current situations are dealt with using the magnetic tripping mode of the MCB. This acts very quickly, overriding the thermal operation.