As protecting equipment should be adept to clear faults at all times. The reliability will not accomplish without completely reliable source of supply to function the trip coils of circuit breakers and all auxiliary relays engagement inside the tripping method.
To reach this reliability, electric battery with nominal voltage of (110 or 220) V is put in. This electric battery is charged by one amongst the two charges. One of the two charges is connected to the electric battery and thus the alternate is standby to be connected if the primary charger is disconnected. The load is connected to electric battery and thus the charger by DC motor buses. Throughout this state of activities, the charger acts as a source of dc to feed the load and charged the battery. If the AC supply of the charger is swapped, then the charger is out of service and thus the battery acts as a supply of DC to feed the DC load.