The purpose of the emergency power system is to furnish an immediate, automatic source of electric power to a limited number of selected vital circuits. It includes one or more diesel-driven emergency generators, the emergency switch-boards, and a distribution system, which is separate from the ships service electric plant and distribution system. Emergency feeders run from the emergency switchboards to at least one and usually to two different ships service switch-boards. Emergency power feeders for certain vital auxiliaries are also run to control panels. The emergency power system, with the use of transformers, is also used for furnishing emergency lighting.

Whenever practical, emergency generators and switchboards are installed above the waterline, to minimize danger from flooding. Also, the emergency plant is installed as far away as practical from the ships service plant, to avoid both plants being put out of action by battle damage. On most ships, the emergency generators do not have the same capacity as the ships service plants. Therefore, care must be taken to prevent overloading the emergency generator, which in turn will overload the diesel engine.