Soil Resistance

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MIL-HDBK-419A
b.
Tactical Facilities.
Tactical earth electrode subsystems are connected to existing buried low
resistance facilities, if available, or to driven ground rods or ground-rod configurations. Total resistance to
earth is affected by the type of soil, contact resistance between ground rod and soil, and by the connector
resistance.
1.11.1.1.1.3 Soil Resistance.  Resistivity of the soil into which the earth electrode subsystem is buried
constitutes a basic constraint on achieving low ground resistance.  Soil resistivity, measured in ohm-m, is
defined as the electrical resistance of a cube of homogeneous material (soil). Table l-30 contains sample
resistivity values of various soil types. Soil temperature and moisture content are other variables affecting
ground resistance. Ground resistance increases inversely as the soil temperature with only slight changes in
ground resistance occurring above 32 degrees Fahrenheit. Soil resistance also varies widely as a function of
moisture content.  Additional information on soil resistance is contained in Sections 1.2.2 and 1.2.3 of this
Volume, and Section 2.3 of Volume I.
1.11.1.1.1.4 Ground Rod Resistance.  Ground rod resistance is primarily a function of the depth the rod is
driven into the earth and the soil Theoretically, the resistance (R) of a ground rod driven vertically
into uniform soil is:
(l-3)
where ρ is the resistivity of the soil,  and d are the rod's length and diameter, respectively. Figure l-96
illustrates the measured effect of rod length on total ground resistance.
1.11.1.1.1.5 Ground Resistance Shells.  Associated with a driven ground rod injected with current are
imaginary ground resistance shells. The concentric shells of resistance outward from the rod are a function of
the earth's resistance to flow of current. The shell having the smallest cross-sectional area closest to the
ground rod will exhibit the largest incremental resistance. Approximately 90 percent of the shells of total
resistance to the earth occur, on the average, within a radius of two rod lengths from the electrode.
1.11.1.1.2  Power Ground. Power supplied to tactically-deployed equipments and systems may be derived from
three sources; transportable ac power generators, commercially-available ac power, and battery supplied dc
power. The ac neutrals are always floated on the load side of the first service disconnect within the shelter.
The 5-wire system described in 1.11.1.1.2.1 is preferred for new systems. If multiple shelters are serviced from
a single power source or or if hum is encountered, the neutral conductor should be
grounded at the source only.
1.11.1.1.2.1 Three-Phase Power Distribution System.  Transportable power generators presently used with
military mobile equipments are 3-phase, 4-wire, 120/240 V ac wye distribution systems. Ground points of a
S-phase wye system are illustrated in Figure l-97. Five-wire ac power grounding requires that the neutral
(white) or grounded conductor be connected to an earth ground at the source (generator or and
again at the supply side of the first service disconnect/power entry panel (PEP). The grounding (green)
conductor should also be connected to earth ground at the first service disconnect and at the transformer.
l-178