30 CFR § 18.51 Electrical protection of circuits and equipment.
(a) An automatic circuit-interrupting device(s) shall be used to
protect each ungrounded conductor of a branch circuit at the junction with
the main circuit when the branch-circuit conductor(s) has a current carrying
capacity less than 50 percent of the main circuit conductor(s), unless the
protective device(s) in the main circuit will also provide adequate
protection for the branch circuit. The setting of each device shall be
specified. For headlight and control circuits, each conductor shall be
protected by a fuse or equivalent. Any circuit that is entirely contained in
an explosion-proof enclosure shall be exempt from these requirements.
(b) Each motor shall be protected by an automatic overcurrent device.
One protective device will be acceptable when two motors of the same rating
operate simultaneously and perform virtually the same duty.
(1) If the overcurrent-protective device in a
direct-current circuit does not open both lines, particular attention shall
be given to marking the polarity at the terminals or otherwise preventing the
possibility of reversing connections which would result in changing the
circuit interrupter to the grounded line.
(2) Three-phase alternating-current motors shall have an
overcurrent-protective device in at least two phases such that actuation of a
device in one phase will cause the opening of all three phases.
(c) Circuit-interrupting devices shall be so designed that they can
be reset without opening the compartment in which they are enclosed.
(d) All magnetic circuit-interrupting devices shall be mounted in a
manner to preclude the possibility of their closing by gravity.
