49 C.F.R. § 193.2445 – Sources of power.

(a) Electrical control systems, means of communication, emergency lighting, and firefighting systems must have at least two sources of power which function so that failure of one source does not affect the capability of the other source.
(b) Where auxiliary generators are used as a second source of electrical power:
1. (1) They must be located apart or protected from components so that they are not unusable during a controllable emergency; and
2. (2) Fuel supply must be protected from hazards.