46 C.F.R. § 111.30-17 – Protection of instrument circuits.

(a) Each circuit that supplies a device on a switchboard, except a circuit under paragraph (b) of this section, must have overcurrent protection.
(b) A circuit that supplies a device on a switchboard must not have overload protection if it supplies:
1. (1) An electric propulsion control;
2. (2) A voltage regulator;
3. (3) A ship's service generator circuit breaker tripping control; or
4. (4) A device that creates a hazard to the vessel if deenergized.
(c) If short circuit protection is used in any of the circuits listed in paragraph (b) of this section, it must be set at not less than 500% of the expected current.
(d) A secondary circuit of a current transformer must not be fused, and the circuit from a current transformer to a device that is not in the switchboard must have a high voltage protector to short the transformer during an open circuit.