1.4.6 Power Distribution Systems.
The neutral of an ac power distribution system is to be grounded to the earth electrode subsystem at
the first service disconnect as well as to the ground terminal at the secondary distribution transformer. For
separate facilities served by a common distribution system, each disconnecting means should be grounded to the
earth electrode subsystem common to the facility. This connection may be lifted if noise or hum problems are
encountered. At separate facilities having a common earth electrode subsystem, each service disconnecting
means should be grounded to the closest point on the earth electrode subsystem.
All distribution neutrals are to be isolated from equipment and structural elements except for the
connection at the first service disconnect,
Connect the ground terminals of convenience outlets to the facility ground system with the green
wire specified by the NEC. Do not use wire mold or plug mold distribution strips which depend upon serrated or
toothed fingers for grounding. Effectively ground the ground terminals on such strips with auxiliary grounding
conductors equivalent to the green wire requirements of Table 250-95 of the NEC.
For a dc power system, ground one leg with a single connection to the earth electrode subsystem.
The size of the grounding conductor should conform to the requirements of the NEC. Whether grounded at the
source or at the load, provide a dedicated current return conductor from the load to the source to assure that
the dc load current in the facility ground system or the lower frequency signal ground network is minimized.
1.4.7 Standby AC Generators. The frames and housing of ac generators should be grounded as prescribed by
the NEC. Ground the neutral to the facility main ground plate or to the earth electrode subsystem, whichever
is closest. When generators are connected in parallel, interconnect the neutrals and ground them to the
facility's earth electrode subsystem with a common grounding conductor.
1.4.8 Equipment Fault Protection Subsystems for Transportable Equipment.
To protect personnel from exposure to hazardous voltages, all exposed metal elements of equipment
and supporting structures shall be interconnected by a green wire from the ac power distribution system and
referenced back to the power source. The grounding requirements of a transportable facility installed in the
field and operating from transportable engine generators is relatively simple. The primary requirement is to
ensure that (1) all vans, vehicles, trailers, and engine generator units are interconnected through a protective
(green wire) network, and (2) the power neutral is grounded from a common bus that is connected to an earth
electrode at the generator. Where parts are movable or subject to vibration, metal straps may be used in lieu
of the green wire.
When transportable facilities are powered from a commercial base ac source or are integrated into a
permanent installation with nontransportable facilities, personnel protection requirements become more
complex. When part of a fixed installation, the transportable system shall be integrated into the facility ground
system by extending the earth electrode subsystem to provide connections for the transportable facility. All
metallic components of the facility shall be interconnected through the equipment fault protection subsystem
and bonded to the earth electrode subsystem at the main power panel, or back to the primary power source
through the ground conductor of the power distribution cable.