Because protective equipment must be ready to clear faults all times. The reliability will not achieve without an absolutely reliable source of supply to operate the trip coils of circuit breakers and all auxiliary relays participation in the tripping process.
To reach this reliability, Battery with nominal voltage of (110 or 220) V is installed. This battery is charged by one of two charges. One of the two charges is connected to the battery and the other is stand by to be connected if the first charger is disconnected.
The load is connected to battery and the charger by dc buses. In this situation, the charger acts as a source of dc to feed the load and charge the battery. If the AC supply of the charger is switched, then the charger is out of service and the battery acts as a source of DC to feed the DC load. Connection of charger, battery, and loads as shown in figure (7).
