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3. The only component remaining is the resistance of the surrounding earth.
Think of the electrode being surrounded by concentric shells of earth or
soil, all of the same thickness. The closer the shell to the electrode, the
smaller its surface and the greater its resistance. The farther away the shells
are from the electrode, the greater the surface of the shell and the lower
the resistance. Eventually, adding shells at a distance from the grounding
electrode will no longer noticeably affect the overall earth resistance
surrounding the electrode. The distance at which this effect occurs is
referred to as the effective resistance area and is directly dependent on the
depth of the grounding electrode.
In theory, the ground resistance may be derived from the general formula:
R =
This formula clearly illustrates why the shells of concentric earth decrease in
resistance the farther they are from the ground rod:
R = Resistivity of Soil x
In the case of ground resistance, uniform earth (or soil) resistivity throughout
the volume is assumed, although this is seldom the case in nature. The
equations for systems of electrodes are complex and often expressed only as
approximations. The most commonly used formula for single ground electrode
systems, developed by Professor H. R. Dwight of the Massachusetts Institute of
Technology, follows:
R = resistance in ohms of the ground rod to the earth (or soil).
L = grounding electrode length.
r = grounding electrode radius.
ρ = average resistivity in Ωcm.
22
L
ρ
Resistance = Resistivity x
A
ρ
R =
π
2
Digital Ground Resistance Tester Models 4620 & 4630
Thickness of Shell
Area
4L
In
r
L
Length
Area
-1
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