Types Of Resistivity Measurements - AEMC 4620 User Manual

3. 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 grounding system, it is advisable to locate the area of lowest
soil resistivity in order to achieve the most economical grounding
installation.
5.4.2

Types of Resistivity Measurements

There are two types of resistivity measurements:
• 2-Point method
The 2-Point method is simply the resistance measured between
two points.
• 4-Point method
For most applications, the most accurate method is the 4-Point
method, which is used by the AEMC
and 4630 Ground Testers. The 4-Point method, as the name
implies, requires the insertion of four equally spaced, in-line elec-
trodes into the test area. A known current from a constant current
generator is passed between the outermost electrodes (X and Z).
The potential drop (a function of the resistance) is then measured
across the two innermost electrodes (Xv and Y). The Models 4620
and 4630 are calibrated to read directly in ohms.
A
b<
20
Digital Ground Resistance Tester Model 4620 and 4630
X electrode Xv electrode
A
Instruments Model 4620
®
Y electrode
A
Figure 18
Ω
Z electrode
A
33