14 CFR 27.1351 - General.
(a)Electrical system capacity. Electrical equipment must be adequate for its intended use. In addition -
(1) Electric power sources, their transmission cables, and their associated control and protective devices must be able to furnish the required power at the proper voltage to each load circuit essential for safe operation; and
(2) Compliance with paragraph (a)(1) of this section must be shown by an electrical load analysis, or by electrical measurements that take into account the electrical loads applied to the electrical system, in probable combinations and for probable durations.
(b)Function. For each electrical system, the following apply:
(1) Each system, when installed, must be -
(i) Free from hazards in itself, in its method of operation, and in its effects on other parts of the rotorcraft; and
(ii) Protected from fuel, oil, water, other detrimental substances, and mechanical damage.
(2) Electric power sources must function properly when connected in combination or independently.
(3) No failure or malfunction of any source may impair the ability of any remaining source to supply load circuits essential for safe operation.
(4) Each electric power source control must allow the independent operation of each source.
(c)Generating system. There must be at least one generator if the system supplies power to load circuits essential for safe operation. In addition -
(1) Each generator must be able to deliver its continuous rated power;
(2) Generator voltage control equipment must be able to dependably regulate each generator output within rated limits;
(3) Each generator must have a reverse current cutout designed to disconnect the generator from the battery and from the other generators when enough reverse current exists to damage that generator; and
(4) Each generator must have an overvoltage control designed and installed to prevent damage to the electrical system, or to equipment supplied by the electrical system, that could result if that generator were to develop an overvoltage condition.
(d)Instruments. There must be means to indicate to appropriate crewmembers the electric power system quantities essential for safe operation of the system. In addition -
(1) For direct current systems, an ammeter that can be switched into each generator feeder may be used; and
(2) If there is only one generator, the ammeter may be in the battery feeder.
(e)External power. If provisions are made for connecting external power to the rotorcraft, and that external power can be electrically connected to equipment other than that used for engine starting, means must be provided to ensure that no external power supply having a reverse polarity, or a reverse phase sequence, can supply power to the rotorcraft's electrical system.