16
Digital Ground Resistance Tester Model 4500
C) The only component remaining is the resistance of the surround
-
ing earth. The electrode can be thought of as 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; hence,
the greater its resistance. The farther away the shells are from the
electrode, the greater the surface of the shell; hence, 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 for-
mula:
R =
ρ
Resistance = Resistivity x
L
A
Length
Area
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
Thickness of Shell
Area
In the case of ground resistance, uniform earth (or soil) resistivity through
-
out the volume is assumed, although this is seldom the case in nature.
The equations for systems of electrodes are very 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 =
ρ
2
π
L
{(In 4L) -1}
r
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 ohms-cm
