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 MIL-HDBK-419A
b. Use longer ground rods. Rods longer than 10 feet (can be realized by assembling lo-foot sections)
may be used in high resistivity soil in place of a larger number of lo-foot rock. Where the ground water table is
greater than 10 feet below the surface at any season of the year or where the frost line is greater than 10 feet,
use the longer rods to maintain contact with the permanently moist, unfrozen soil. Use Figure 1-4 to estimate
the length needed, given the soil resistivity.
c.
Use horizontal wires or grids instead of vertical rods. Where bedrock or other obstacles prevent the
effective use of vertical rods, horizontal wires, grids, or radials should be used. (See Volume I, Section 2.6.1.2
for design data and equations.)
d.
Lower the soil resistivity through chemical enhancement (salting). Where the above alternatives are
not possible or are not cost effective, chemical enhancement is frequently the only choice left. Consult
Volume I, Section 2.9 before deciding what to do in this regard.
1.2.3 Design Guidelines.
a.
At each facility supplied by electric power, at least one ground rod should be installed near the
service disconnecting means and bonded to the earth electrode subsystem. If the transformer is located on the
site, a bare 1/0 AWG wire or cable should interconnect the ground rod at the transformer with the earth
electrode subsystem at the first service disconnect for lightning protection purposes.
b.
For lightning protection purposes, all facilities large or small or located in areas of low or high
lightning incidence will require an earth electrode subsystem, described in the previous section. Facilities
having structural extensions or equipment protrusions (such as antenna elements or towers) extending above the
surrounding terrain should have a continuous earth electrode subsystem enclosing each facility or should have
individual earth electrode subsystems connected together. See paragraph 5.1.1.3.8.1 of MIL-STD-188-124A.
Most installations will require many interconnected ground rods. The configuration shown in Figure
c.
1-3 is adequate for most facilities. (The number of ground rods actually required at a given location will be
determined by the resistivity of the soil and the configuration of the installation.) Three-meter (ten-foot)
ground rods installed at 20-foot intervals around the perimeter of the structure provide good utilization of the
effective radius of the rod while providing several points of contact with the earth. If longer rods are required
to reach the water level, to make contact with lower resistivity soils, or to penetrate below the frost line,
greater spacings may be employed. The nominal spacing between rods should be between one and two times the
length of the rod; however, it is necessary for a ground rod to be placed near each lightning down conductor, so
spacings should be limited to not more than 50 feet in order to conform to lightning protection requirements
(see Section 1.3.2.2.2).
d.
The rods and interconnecting cable comprising the earth electrode subsystem should be positioned
0.6 to 1.8 meters (2 to 6 feet) outside the drip line of the building or structure to insure that rain, snow, and
other precipitation wets the earth around the rods.
For facilities which do not conform to a rectangular or square configuration, lay out the rod field to
e.
generally follow the perimeter of the structure as illustrated in Figure 1-7.
1-14
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