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 MIL-HDBK-419A
e.
The ac neutral, i.e., the white wire, must not be connected to any noncurrent-carrying metal parts
of the equipment. (This restriction means that the use of ac/dc type of power circuit design is prohibited and
that a power transformer is required for each equipment or subsystem.)
3.2.3 Cabinet Grounding. The case or cabinet of each individual unit or piece of equipment must be
electrically bonded to the cases of other nearby equipments and to the nearby equipotential plane or fault
protection subsystem to minimize noise voltages produced by stray currents.
Each unit or piece of equipment that is not rack mounted should have its case or enclosure
a.
connected to the nearest point on the equipment ground network with a low resistance ground cable. To ensure
that this ground cable has a sufficiently low resistance, the size of the ground cable should provide at least 2000
cmil per running foot. To determine the necessary wire size, first compute its required cross-sectional area
from:
Required area in circular mils = Bun length in feet x 2000 cmil per foot.
Using either Table 5-1 in Volume I or a standard wire table, determine the standard AWG size having the
required (or larger) cross-sectional area. For example, assume the run length is 20 feet. The minimum
necessary cross-sectional area is then
20 ft x 2000 cmil/ft = 40,000 cmil.
The wire tables show that a No. 4 AWG wire has a cross-sectional area of 41,740 circular mils and should be
used for a run of 20 feet.
If the equipment is mounted in a rack, frame, or cabinet, the equipment case must be directly
b.
bonded to the rack, frame, or cabinet in accordance with the recommendations of Section 3.3. The rack, frame,
or cabinet must then be grounded to the nearest point on the fault protection subsystem within the facility with
a ground conductor providing 2000 cmil per running foot. Figure 3-11 provides a typical equipment cabinet
grounding detail.
3-16
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