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 MIL-HDBK-419A
f.
Where two or more structures or facilities are located in the same general area (less than 200 feet)
and are electrically interconnected with signal, control, and monitor circuits, either provide a common earth
electrode subsystem, or interconnect the separate earth electrode subsystems with two buried bare cables. A
common example of an installation where two separate structures are involved is a radar or communications
site where the equipment shelter is adjacent to the antenna tower. Signal cables (both coaxial and waveguide),
control cables, and power lines typically run between the tower and the shelter. The tower, being taller than
the shelter, is more susceptible to lightning strikes. To minimize voltage differentials between the two
structures, the facilities should effectively share a common earth electrode subsystem. Separate structures
spaced closer than 6 meters (20 feet) should have a common earth electrode subsystem installed that encircles
both facilities as shown by Figure 1-8. Figure 1-9 shows the recommended arrangement when separations equal
to or greater than 6 meters (20 feet) but less than 60 meters (200 feet) are encountered. One of the
interconnecting buried bare cables may also serve as a guard for buried signal or power cables. A typical site
installation involving three structures separated less than 200 feet is illustrated in Figure 1-10. Structures or
facilities having no interconnecting cables and separated by a distance greater than 60 meters (200 feet)
generally do not require their earth electrode subsystems be interconnected.
There may be a number of incidental, buried, metallic structures in the vicinity of the earth
g.
electrode subsystem. These structures should be connected to the subsystem to reduce the danger of potential
differences during lightning or fault protection; their connection will also reduce the resistance to the earth of
the electrode subsystem. Such additions to the earth electrode subsystem should include the rebar in concrete
footings, and buried tanks and pipes.
To minimize resistance variations caused by surface drying of the soil and by the freezing of the soil
h.
during winter and to minimize the possibility of mechanical damage to ground rods, connections, and
interconnecting cables, the tops of ground rods should be at least 0.3 meters (1.0 foot) below grade level. Bury
the bare 1/0 AWG interconnecting cable at least 0.45 meters (1.5 feet) below grade level. The recommended
practices are Illustrated in Figure l-11.
If the subsystem is installed after foundations are poured, cables are installed, utility pipes installed,
i.
etc., make proper provisions for performing the needed interconnections between the water system, lightning
down conductors, structural steel, buried lines and cables, and the electrodes.
Access to the earth electrode subsystem should be provided through the installation of one or more
j.
grounding wells at each site. Two acceptable types of grounding wells are illustrated in Figures 1-12 and 1-13.
Either clay pipe or poured concrete may be used. Removable access covers must be provided. In very large
structures, particularly those in which grounding grids are installed underneath, the grounding well or wells may
be located inside the building in an accessible location. More than one grounding well may be necessary
depending upon the size of the facility, the extent of the electrode subsystem, and the degree of accessibility to
the electrodes deemed desirable. Locate at least one of the ground wells in an area with access to open soil so
that resistance checks of the earth electrode subsystem can be made once the building is in use. The top view
of a representative ground rod installation shown in Figure 1-14 illustrates the required connections to the
signal reference subsystem, the lightning protection subsystem, and the facility ground network.
1-15
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