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 MIL-HDBK-419A
2.8.2.3 Minimum Electrode Size. The necessity to hold the surface temperature below boiling temperatures
establishes a minimum amount of electrode material.
The minimum length of a single ground rod is
(2-72)
The value of I is approximately 1000 for both lightning stroke currents and power system fault currents, so
for satisfactory energy dissipation the minimum rod length is specified by
If the earth is moist soil with a
of approximately
ohm-cm, the limit becomes
(2-74)
ohm-cm, the limit becomes
In granite with a
of approximately
(2-75)
If 2-cm rods are used, the safe dissipation of heat in granite would require at least 80 rods, each 2 meters long.
For moist earth, only 8 rods, each 2 meters long, would be required for heat dissipation.
2.9 ELECTRODE ENHANCEMENT.
2.9.1 Introduction. Sites may be encountered where acceptable and practical numbers of driven rods, buried
cables, and other available materials will not achieve the desired low resistance to earth for special
communication systems, i.e., HF transmitters. In such situations, enhancement of the resistivity of the soil
around the electrodes may be necessary to lower the resistance to the desired value. While enhancement of the
resistivity may be required in certain situations, discretion of its use should be exercised due to the reduced life
span of the earth electrode subsystem.
The resistance to earth of an electrode is directly proportional to soil resistivity and inversely proportional to
the total area of contact established with the soil. For fixed land areas, additional vertical rods or horizontal
cables produce diminishing returns because of increased mutual coupling effects. The most straight forward
enhancement method is to reduce soil resistivity. The parameters which strongly affect soil resistivity are
moisture content, ionizable salt content, and porosity; the latter determining the moisture retention properties
of the soil. Thus two recommended techniques for reducing earth resistivity are water retention and chemical
salting.
2-59
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