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 MIL-HDBK-419A
1.11.2.2.2.1.2 Earth Electrode Subsystem, Single Ground Rod. A single driven ground rod connected to the
equipment via an interconnecting cable may be used, if the design resistance value is achievable. The rod
should generally be between 1.8 and 3.0 meters (6 and 10 feet) long. Selection of the required type of ground
rod should be based upon the expected soil conditions at the tactical site location. The resistance between the
rod and earth should be measured in accordance with Section 2.7.2 of Volume I. Where measured resistance is
not low enough, a saline solution (see 1.11.1.3.4) should be added to the soil adjacent to the rod to reduce ground
resistance.
1.11.2.2.2.1.3 Earth Electrode Subsystem, Multiple Ground Rods. Where soil resistance cannot be reduced by
chemical means additional electrodes may be connected in parallel with the given ground rod. The
interconnecting cable should be 1/0 AWG bare copper cable, and the ground rods should be spaced 1.5 to 2 rod-
length's apart to minimize overlapping shells (see 1.11.1.1.1.5).
1.11.2.3 Stand-Alone Shelter.
1.11.2.3.1 General Description. A stand-alone shelter is comprised of equipment housed in a mobile metallic
shelter and typically, is not situated close enough to other equipments to merit construction of a common
extensive earth electrode subsystem between its interfacing systems. Power supplied to the shelter may come
from a power generator or a commercial source. Interfacing with the shelter may be through the power cable.
The need for grounding stand-alone shelters is to provide a ground for (a) the fault protection subsystem, (b) to
"bleed off" static charges or EMI from interfacing signal cables, (c) the signal reference subsystem, and (d) the
(signal reference subsystem), lightning protection subsystem.
1.11.2.3.2 Interconnection of Subsystems. The signal reference and fault protection subsystems are connected
to the earth electrode subsystem because of the following reasons: (1) the skin of the shelter generally serves
as the equipotential plane for the signal reference subsystem, (2) the electronic equipment systems are
connected directly to the skin of the shelter by the shortest route possible, and (3) the fault protection
subsystem is connected to the grounding bus in the power entrance panel, and in turn, to the earth electrode
subsystem. Since the power entry panel is bonded to the skin of the shelter, no loops are formed, and
everything within the shelter will remain at the same potential in the event of power faults, EMP, lightning or
EMI. If the lightning activity in the deployment area warrants additional shelter protection, air terminals
should be installed atop the shelter as per 1.11.1.1.3.1. If the installation is long-term, lightning protection
shall be mandatory.
1.11.2.4 Collocated Military Mobile Equipments.
1.11.2.4.1 General Description.
Collocated mobile equipments are equipments operating individually but
housed together within a single transportable enclosure, e.g., tarpaulin. Typically, these equipments are not
rack mounted and may be situated on the ground. Metallic shelter enclosures are not considered here
(reference 1.11.2.5). Intra-enclosure communication links may exist among equipments, but normally links are
established between an equipment and an external system. Basic operational characteristics of collocated
equipments are similar to stand-alone equipments. Grounding requirements are primarily for personnel safety
from lightning and power faults.
1-187
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