(a)
Direct current distributing systems. The neutral of three-wire
direct current systems shall be grounded at one or more supply stations, but
not at individual services nor within buildings served. One side of a two-wire
direct current system may be grounded, but at one station only. In three-wire
systems the neutrals entering any junction box should be bonded together, but
the box should not be specially grounded. In two-wire systems the grounded side
of the circuit should be insulated from ground except at the station ground
connection.
(b)
Alternating
current distribution systems. All secondary distribution systems shall
be grounded at the building services or near the transformer (or transformers)
either by direct ground connection or by the use of a system ground wire to
which are connected the grounded conductors of many secondary mains and which
is itself effectually grounded at intervals which fulfill, for any secondary
utilizing the system ground wire, the resistance and current carrying
requirements of §
39.69 (relating to ground
resistance). Single-phase, three-wire distribution systems shall be grounded at
the neutral conductor. Two-wire, single-phase systems shall be grounded at the
neutral point or on either conductor. Two-wire, single-phase and two or three
phase systems shall, in general, be grounded at that point of the system which
brings about the lowest voltage from ground of unguarded current carrying parts
of connected devices and also permits most convenient grounding. If one phase
of a two or three phase system is used for lighting, that phase should be
grounded and at the neutral conductor, if one is used. In the absence of direct
grounds at all building services, ground connections shall be made to the
grounded neutral or other grounded conductor of a secondary system supplying
more than one utilization equipment, at intervals that will fulfill the
resistance and current-carrying requirements of §
39.69. If the secondaries of
transformers are supplying a common set of mains, the fuses shall be installed
only at such points as will not cause the loss of the ground connections after
the fuses in the transformer circuits or mains have been blown. Multiple
grounds are preferable in all cases, because of the assurance provided against
loss of the protection afforded by the chance disconnection of any ground
connection. Grounds other than the single ground connection at the building
service shall not be made to alternating current secondaries within buildings.
Reference should be made to §
39.66 (relating to nature of
ground connection).
(c)
Lightning arresters. The connection to a lightning arrester
shall be at such a point that its ground conductor is as short and straight as
practicable. Ground conductors for lightning arresters shall not pass through
iron or steel conduits unless electrically connected to both ends of such
conduits.
(d)
Equipment and
wire runways. The point at which the ground conductor is attached to
equipment or wire runways shall, if practicable, be readily
accessible.
