The EPSUs control and monitor the emergency lighting system. They are installed in the ceiling of the passenger compartment. To operate the different emergency lights during usual conditions, each EPSU converts 28V DC to 6V DC and 6V DC to 115V AC. If power from the DC essential busbar is not available, the battery pack, installed in each EPSU, gives power to all the emergency lights. The batteries give sufficient power to the lights for a minimum of 10 minutes.