standards and guidelines for communication sites - Radio And ...

GROUNDING (EARTHING) ELECTRODE SYSTEM COMPONENT AND INSTALLATION REQUIREMENTS CHAPTER 4: EXTERNAL GROUNDING (EARTHING)
4.4.1.2.4 EFFECT OF PARALLEL GROUND RODS
Figure 4-13 below shows the effects of adding additional ground rods (15.9 mm (0.625 in.) diameter by
3 m (10 ft.) long) together in parallel. As seen in the figure, the addition of one ground rod to the first
ground rod (for a total of two rods) significantly reduces the resistance to earth of the ground rod
system. Each subsequent ground rod added in parallel has less of an effect on the resistance to earth of
the ground rod system.
Approximate Resistance to Earth in Ohms
35
30
25
20
15
10
5
0
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
Number of 3 m (10 ft) Ground Rods
Installed 6.1 m (20 ft) apart in 10,000 ohm-cm soil
FIGURE 4-13 RESISTANCE TO EARTH DUE TO PARALLEL GROUND RODS
4.4.1.3 ELECTROLYTIC GROUND RODS
At sites where, due to poor soil conductivity (high resistivity) and/or limited space, an acceptable
grounding (earthing) electrode system resistance cannot be achieved using standard ground rods,
commercially available electrolytic ground rods should be considered. See MIL-HDBK-419A Volume
I, section 2.9.5, and UL 467-2004, section 9.2.7 for additional information. Electrolytic ground rods
(Figure 4-14) are available in straight or L-shaped versions and in various lengths from 3 m (10 ft.) to
6.1 m (20 ft.), or longer as a special order. Electrolytic ground rods are generally constructed of 54 mm
(2.125 in.) diameter hollow copper pipe. This copper pipe is filled with a mixture of non-hazardous
natural earth salts. Holes at various locations on the pipe allow moisture to be hygroscopically extracted
from the air into the salt within the pipe, therefore forming conductive electrolytes. These electrolytes
then leach out of the pipe into the soil, improving soil conductivity. Electrolytic ground rods are inserted
into a pre-drilled hole, or in the case of L-shaped rods, placed into a trench at least 762 mm (30 in.)
deep, and encased in a grounding electrode encasement material. See “Grounding (Earthing) Electrode
Encasement Materials” on page 4-27.
Electrolytic ground rods should be considered for use in grounding electrode systems covered by
concrete or pavement, such as parking lots. By allowing moisture to enter, the design of the electrolytic
ground rod improves the resistance of the grounding electrode system.
4-16 68P81089E50-B 9/1/05