Answer Posted / surajit mishra

A Circuit Breaker is often required to operate when there is no power. If the auxiliary power in a station is taken for the Main power supply, the CB Trip coil and closing coil will not have power to operate. The CBs have Closing Coil for switching On and a Shunt Trip coil for switching off. In substations, DC Control supply, usually 110V DC, from Lead Acid Battery sets are maintained to provide DC supply for operating the Closing Coil and Trip Coil. Not only MV Breakers, if the system requires even an LV Breaker to operate when mains power is not available, DC Control supply is used.

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