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 MIL-HDBK-419A
Article 250-91 of the NEC describes the types and materials used for equipment grounding
e.
conductors. Types include solid and stranded (insulated or bare) wire or other shapes, such as metallic tubes,
pipes, and conduit. The grounding conductor types permitted by the NEC also include various metal ducts,
cable trays, and raceways however these types shall not be used in lieu of the equipment grounding conductors.
The NEC also permits/allows certain types of armored cable sheath be used as grounding conductors.
Experience with military C-E facilities has proven that a low-noise, low-impedance equipment fault
f.
protection subsystem can be maintained over a prolonged period of time if separately designed and installed
ground conductors are provided. Therefore, a separate equipment fault protection conductor shall be included
with the ac power distribution if not provided in the power cable. A grounding (green) wire should be used and
installed in the same conduit as the other ac wires. When ferrous ducts or conduits are used to protect or shield
the neutral and phase conductors, the lowest impedance will result when this grounding conductor is installed in
the same duct or conduit. The impedance can be further decreased if the grounding conductor is wrapped
around the other conductors and bonded to the duct or conduit at both ends. In a correctly installed power
distribution system, there should be no power current on the grounding conductor, except during a fault
condition. It should be noted that there are two types of faults causing overcurrent devices to operate. The
first is an overload condition in equipment. In this case, fault current is on the neutral and phase leads. The
second fault is where a phase or hot lead is inadvertently grounded. The fault current in this case is on the base
lead and the grounding conductor. In both cases, the overcurrent protective device, usually a circuit breaker, is
opened in the phase or hot lead. Due to the fault currents that can flow either on the phase leads, neutral, or
grounding conductor, it is recommended that a 2-inch separation be maintained between power runs and signal
runs when neither is in conduit.
1.4.4 Pipes and Tubes.
All metallic pipes
and tubes (including conduit) and their supports should be electrically continuous
a.
facility ground system at least at one point. If any run of metal pipes or tubes
and are to be bonded to the
exceeds 3 meters (10 feet) in
length, it should be bonded to the facility ground system at each end. Also, longer
runs should be bonded to the
facility ground system at intervals of approximately 45 meters (150 feet).
At indoor locations, these bonds may be made with clamps which provide continuous pressure. Pipes
b.
installed out of doors should be bonded to the facility ground system at entry point or wherever feasible by
welding or brazing. Compatible stainless steel straps may be used with stainless steel pipe. In the event that a
direct bond cannot be made, zinc-plated hose clamps or stranded, bare copper, untinned bond straps may be
used. All bonds should be adequately protected against corrosion in humid or corrosive environments.
c.
Joints in metal pipes and tubes should have a dc resistance no greater than 5 milliohms. In the case
of threaded joints, the threads should be cleaned and firmly tightened (200 ft-lbs for hard wall conduit) and
protected against corrosion. Leaded or caulked joints, flared and other compression fittings, and O-ring fittings
should all be measured to verify that the joints have a dc resistance no greater than 5 milliohms.
1.4.5 Electrical Supporting Structures. Electrical supporting structures such as conduit, cable trays or
raceways, wiring system enclosures, and metallic power cable sheaths should be electrically continuous and are
to be bonded to the facility ground system at multiple points. In the event of a fault, this continuity will
prevent these structures from rising to a hazardous potential.
1-102
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