... coal mine contains explosive gas (methane) mixture, as main switch or branch switch in the power supply system, with the frequency of 50Hz ... .3 The vacuum circuit breaker is mainly composed of vacuum switch tubes, operating mechanism, protection plug-ins and other components, ... leakage resistance pass through 1KΩ; as a branch switch≤30ms;as a main switch≤100ms(no delayed), 200~400ms(delayed) Inlet ...