CYD series miniature circuit breakers are composed of the moulded case, operating mechanism, contact system, arc-extinguishing system, tripping mechanism, etc. The case is made of the special plastic high in fire-resistance and strength, so it is very capable of resisting impact and light in weight. The parts of the operating mechanism employ high strength plastic products in order to assure the flexible, reliable operation and obtain the minimum moment of inertia as well so that the time from the moment the short-circuit fault occurs to the operation of the tripping mechanism is very little. The tripping mechanism is composed of the two part of bimetal sheet overload counter time limit tripping mechanism and short-circuit instantaneous electromagnetic mechanism. The contact arcextinguishing system employs the special arc-conducting angle and gangway arc-extinguishing chamber so it has distinct current limiting characteristics.