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§120.376 Grounded distribution systems (Neutral grounded).
(a) If a grounded distribution system is provided, there must be only one connection to ground, regardless of the number of power sources. This ground connection must be at the switchboard or at the common ground plate, which must be accessible.
(b) Each propulsion, power, lighting, or distribution system having a neutral bus or conductor must have the neutral grounded.
(c) The neutral or each grounded generation and distribution system must be grounded at the generator switchboard and have the ground connection accessible for checking insulation resistance of the generator to ground before the generator is connected to the bus, except the neutral of an emergency power generation system must be grounded with:
(1) No direct ground connection at the emergency switchboard;
(2) The neutral bus permanently connected to the neutral bus on the main switchboard; and
(3) No switch, circuit breaker, or fuse in the neutral conductor of the bus-tie feeder connecting the emergency switchboard to the main switchboard.
(d) On a metallic vessel, a grounded alternating current system must be grounded to the hull. On a nonmetallic vessel, the neutral must be connected to the common ground, except that aluminum grounding conductors must not be used.