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 MIL-HDBK-419A
1.4.2 Equipment Fault Protection Subsystem Composition. One of the major shortcomings in grounding
systems is the interconnection and reversal of ac neutral and protective wires of the ac power distribution at
various power distribution panels and at equipment throughout a facility. These installation errors result in
additional electrical noise and ac currents in the ground system. The equipment fault protection subsystem
should generally follow a configuration from a central or main ground point which, ideally, should be at the
primary power station transformer ground point; or, it should be bonded directly to the earth electrode
subsystem at the communications building, if a protective wire is not available to the main ground point. The
configuration consists of a central main or trunk lead from the power source with protective conductors to the
various intermediate power panels and equipment. The protective wire is carried along with the phase and
neutral wires from the main ground point to the main circuit breaker panel, from there to intermediate circuit
breaker panels to the equipment panels, and finally to the equipment.
1.4.3
Configuration of the Equipment Fault Protection Subsystem.
The equipment fault protection subsystem consists primarily of the grounding conductors of the
a.
interior ac power distribution system. The grounding conductors are green insulated or bare wires running in
the same conduit or duct with the neutral and phase conductors. Covered grounding conductors are preferred to
reduce EMI. Figure l-46 illustrates a typical equipment fault protection subsystem. Key points to be noted
are:
(1) The conduit is grounded to the power panel at each end, but it is not used in lieu of a grounding
conductor which continues through the conduit to the protective bus bar.
(2) The ac neutral lead can be grounded at the first service disconnect means. In this case, the ac
neutral also serves as a protective conductor back to the source. For best results, the ac neutral (grounded) and
green (grounding) wire should be grounded at the service transformer and the first service disconnect means
through the five-wire distribution system.
b.
To protect personnel from exposure to hazardous voltages, all exposed metallic elements of
electrical and electronic equipment shall be connected to ground. In the event of inadvertent contact between
the "hot" lead and chassis, frame, or cabinet through human error, insulation failure, or component failure, a
good, direct, known fault current path will be established to quickly remove the hazard. The neutral lead shall
be grounded for fault protection preferably at the distribution transformer and if the transformer is outside the
building, at one additional point outside the building. It should never be grounded on the load side of the first
service disconnect means.
c.
Grounding conductors and ground connections for transformers, switchgears, motors etc., shall
comply with the requirements of the NEC Articles 250-92 and 250-95.
Metal boxes, fittings, and noncurrent-carrying metal parts of other fixed equipment do not require
d.
additional protection if metallically connected to the grounded cable armor or bonded to the grounded members
of the building. MIL-STD-188-124A provides that the path to ground for circuits, equipment, and conductor
enclosures be permanent and continuous. The path must have (1) the capacity to conduct safely any fault
current likely to be imposed upon it, and (2) sufficiently low impedance to limit voltage to ground and to aid the
operation of circuit protective devices.
1-100
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