The short-circuit withstand capacity of the semiconductor in US-N/K/Htype solid state contactor is specified and the flowing current at short-circuit must not exceed the withstand capacity for protecting againstshort-circuit. In order to meet the condition, it is neccessary to combinewith specified protection fuse. Therefore, MCCB can not protect againstshot-circuit since the passing energy exceeds the withstand capacity of thesemiconductor element.