14 CFR § 27.1357

Protective devices, such as fuses or circuit breakers, must be installed in each electrical circuit other than—

The main circuits of starter motors; and

Circuits in which no hazard is presented by their omission.

A protective device for a circuit essential to flight safety may not be used to protect any other circuit.

Each resettable circuit protective device (“trip free” device in which the tripping mechanism cannot be overridden by the operating control) must be designed so that—

A manual operation is required to restore service after trippling; and

If an overload or circuit fault exists, the device will open the circuit regardless of the position of the operating control.

If the ability to reset a circuit breaker or replace a fuse is essential to safety in flight, that circuit breaker or fuse must be located and identified so that it can be readily reset or replaced in flight.

If fuses are used, there must be one spare of each rating, or 50 percent spare fuses of each rating, whichever is greater.