Soil Resistivity Measurements (Model 4610 Only) - AEMC 3640 User Manual

4.9 Soil Resistivity Measurements
Why make soil resistivity measurements?
Soil resistivity measurements have a threefold purpose. First, such data
are used to make sub-surface geophysical surveys as an aid in identifying
ore locations, depth to bedrock and other geological phenomena. Second,
resistivity has a direct impact on the degree of corrosion in underground
pipelines. A decrease in resistivity relates to an increase in corrosive activ-
ity and therefore dictates the protective treatment to be used. Third, soil
resistivity directly affects the design of a grounding system, and it is to that
task that this discussion is directed. When designing an extensive ground-
ing system, it is advisable to locate the area of lowest soil resistivity in
order to achieve the most economical grounding installation.
Resistivity measurements are of two types, the two point and the four point
method. The two point method is simply the resistance measured between
two points. For most applications, the most accurate method is the four
point method, which is used by the AEMC
Ground Tester. The four point method, as the name implies, requires the
insertion of four equally spaced, and in-line, electrodes into the test area.
A known current from a constant current generator is passed between the
outermost electrodes. The potential drop (a function of the resistance) is
then measured across the two innermost electrodes. The Model 4610 is
calibrated to read directly in ohms.
b < a
20
Digital Ground Resistance Tester Model 3640 and 4610
X
C1
GROUND RESISTANCE TESTER
MODEL 4610
Press To
Measure
X electrode Xv electrode
a
(Model 4610 Only)
Instruments Model 4610
®
X Y Z
v
P1 P2 C2
X-Z Fault
225
Ω
Xv-Y Hi Resistance
Xv-Y Hi Noise
AUTORANGING
!
REFER TO USER MANUAL
FOR FAULT WARNING LIGHT
EXPLANATIONS
®
I N S T R U M E N T S
Y electrode
a
Figure 20
Z electrode
a
33