Humboldt H-4385 Resistivity Meter - Manual (Page 7)

3-Electrode Application
The 3-Electrode Application can be used to measure the resistance-to-earth of a buried
electrode, such as a buried ground rod or a buried anode.
In this case, two of the electrodes are pins driven into the earth and the third
electrode
is the test electrode itself, for example, a ground rod or an anode.
A resistance-to-earth measurement will actually comprise 3 components; the resistance
of the electrode itself (the resistance of the ground rod or anode material, for example)
including test leads, the electrode-earth contact resistance and the resistance of the
surrounding earth. Typically, the resistance of the surrounding earth is the largest
component of the resistance-to-earth measurement.
The three electrodes are positioned in a straight line (ideally) as indicated in Figure 3
below.
As indicated in Figure 3, a jumper wire is connected between the C1 and P1 terminals on
the H-4385 and test leads connect the “Potential Electrode” and the “Current Electrode”
to terminals P2 and C2, respectively. Finally, a test lead connects the electrode under test
(a ground rod or an anode, for example) to the C1 terminal.
With this configuration, the H-4385 passes a current between the test electrode and the
“Current Electrode” and generates a resistance reading based on the voltage dropped
between the electrode under test and the “Potential Electrode”.
The magnitude of the resistance reading will be a function of the separation (distance)
between the electrode under test and the “Potential Electrode”, with respect to a fixed
position for the “Current Electrode”.
A characteristic “resistance versus distance” plot is illustrated in Figure 4 below.
Figure 3