In traction substations, batteries are used to provide DC power for switchgear protection. The DC bus on traction substations is 110Vdc (55 x 2V cells). This voltage is used to induce a trip condition on the breaker when there is a fault, or overcurrent condition.
The same DC bus is also used for closing the breaker to ensure power supply. Due to the critical nature of switchgear protection, reliable batteries with a long service life are required.