Abstract: The invention relates to an electrical power supply system for powering electronic equipment. The system uses an AC main power supply under normal conditions and uses a DC secondary power supply to supply power if the main power supply fails. The system has a means for detecting the failure of the main supply and for switching over to the secondary supply in a back-up mode. Each piece of electrical equipment has a distinct power converter that delivers internal power required by each piece of equipment. The power converter of each is powered directly by the main power supply under normal conditions and directly by the secondary power supply in the back-up mode. The system of the instant invention is less costly and produces less loss due to conversions than the prior art systems.