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 MIL-HDBK-419A
3.2.1.3 Equipments Containing Both Lower and Higher Frequency Circuits. Some types of equipment will
necessarily contain both lower and higher frequency signal circuits in the same equipment case because of
specific design or operational requirements. (For example, a typical VHF or UHF receiver will require both a
higher frequency input from the antenna and a lower frequency output to audio or IF amplifiers as illustrated in
Figure 3-10.) If the lower and higher frequency circuits are functionally independent and can be separated, the
lower frequency signal ground should be designed and installed in accordance with Section 3.2.1.1 and the higher
frequency signal grounds should conform to Section 3.2.1.2.* However, in equipments where both lower and
higher frequency circuits must share a common signal ground because of design or construction requirements,
both signal circuits should be grounded as in higher frequency equipments. The higher frequency interfaces to
all transitional type equipment should be constant impedance, shielded lines with the shield grounded around its
periphery to the chassis or cabinet. The lower frequency interfaces may be shielded, and should be balanced,
twisted pair lines, as illustrated in Figure 3-10, with the shield grounded at one end only.
3.2.2 Fault Protection. Connect each unit or piece of electrical and electronic equipment and all exposed
conductive parts to other nearby conductive objects, such as the building structural steel, to prevent hazardous
voltages from existing on the equipment relative to its surroundings. Also, ground each equipment to the earth
electrode subsystem through a low impedance path to provide a return path for fault currents back to the ac
distribution transformer.
Ground exposed noncurrent-carrying metal parts of electrical and electronic equipment for fault
a.
protection in accordance with the requirements of MIL-STD-188-124A.
Convenience outlets which are provided as an integral part of a piece of equipment must be
b.
grounded in accordance with the NEC. Connect the ground terminal of the receptacle in such outlets to the
equipment case, racks, or cabinet in which the outlet is mounted with a ground conductor equal in size to the
green wire specified by the NEC. (This ground conductor is to be installed in addition to any grounding
connection made by the yoke of the receptacle.)
The ground terminal of all convenience outlets and plugmolds provided with the equipment or
c.
equipment cabinet shall be connected to the fault protection subsystem via the grounding (green) wire. In
addition, the green wire shall be connected to the equipment case and rack. The size of the conductor shall be
in accordance with Table 1-21 or Article 250 of the National Electrical Code. Wire mold or plugmold strips and
convenience outlets which depend upon serrated fingers for ground shall not be used.
d.
Accessible conductive extensions from each equipment must be grounded to the equipment case to
prevent such parts from becoming electrically energized in case of a power line fault or component failure. For
example, metal control shafts must be grounded with close-fitting gaskets, with metal finger stock, or through
the mounting hardware.
*At facilities where lower frequency equipments are installed over the equipotential plane, the single-point
ground should be connected to the equipotential plane.
3-14
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