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 compliance with MIL-STD-188-124, Grounding, Bonding and Shielding for Common
Long Haul/Tactical Communications Systems, General Requirements. Conduits,
boxes, all of the metallic piping, tubes, and their supports should be
electrically continuous and bonded to the facility ground system, along with
the metal structures they are attached to. A shielded enclosure with a single
point ground, needs to be solidly grounded to the same facility ground system
as the power system and the facility structural members. The single point
ground stud is best located close to the power filter penetrations. For
isolated shielded enclosures, typical for TEMPEST requirements, there is no
green wire ground or conduit grounding of the enclosure, and the neutral is
normally provided a power filter with the same attenuation requirements as for
the energized phases. Without solid grounding to the facility ground system
dangerous differences of potential could exist between the enclosure and
facility structural members, contributed in part by the capacitive reactive
currents of the power filters. Fault clearance and breaker action also
requires this solid single point grounding for proper functioning.
2.9.3
Signal Reference Ground Subsystem. The signal reference subsystem
can be divided into two subclasses, one for higher frequencies (above 300 kHz)
and one for lower frequency signal networks. The higher frequency grounding
is composed of three primary components, an equipotential plane, equipment
grounding conductors, and structural steel elements, electrical supporting
structures and utility piping, with all of the foregoing connected to the
earth electrode subsystem. When these three components are contained with a
metal shielded enclosure with a single point ground system, they all come
together at the single point ground stud. The higher frequency ground system
is comprised of a multipoint equipotential plane to provide a minimum
grounding impedance (short ground lead connection) for equipment cabinets.
Equipment chassis are then connected to the cabinets and signal return leads
are connected to the equipment chassis. The connection of the equipotential
plane to the earth electrode subsystem is made to assure personnel safety and
to provide a low impedance path for the lower frequency signals. Within a
shielded enclosure the floor of the metal enclosure can be utilized as an
equipotential ground plane for the higher frequency grounding. When a raised
access flooring system is installed in the shielded enclosure, it may more
conveniently be utilized for the multipoint grounding connections. It should
be constructed with conductive stringers and panels conforming to
MIL-F-29046(TD), Flooring, Raised, maximum resistance requirements. This
raised flooring system should be grounded at intervals, (10 ft or less) around
its periphery to the parent shielding material utilizing a welded connection
to the wall shielding in a welded enclosure or bolted to seam bolts (with the
bolts replaced with a threaded rod with nuts and washers) in a bolted
enclosure, as shown in Figure 12. Equipment cabinets can then be conveniently
connected to stringers with short grounding leads.
The low frequency grounding system approach references the signal
circuits to a single-point ground which is then connected to the facility
ground, or to its own buried ground electrode system. Ideally, a separate
conductor would be extended from the single-point ground to the return side of
each of the numerous circuits located throughout a facility. This would
require an extremely large number of conductors, and so a compromise approach
uses a ground bus network in the form of a tree. This system is described in
MIL-HDBK-419, and consists of feeder ground cables attached to branch ground
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