Principle of operation (three-terminal resistance measurement)
The classic “fall of potential” test is used to accurately measure the resistance of an earth electrode using
auxiliary stakes driven into the soil, which form a circuit for the test current injection and voltage
measurement as used for the two-terminal method.
The DET injects an a.c. current of known magnitude into the system under test and measures the voltage
developed across it as shown in Figure 4. The system resistance is a simple ratio as per Ohm’s Law. In this
case, the potential stake is moved by fixed increments in a straight line between the electrode under test
and the current stake. At each location, the resistance is calculated as R=V/I. A graph of resistance versus
potential stake position is plotted and the resistance of the electrode under test is taken to be the point at
which the curve is flattest.
Empirical testing has shown that with suitably positioned stakes, this method can be shortened by placing
the potential stake at a distance of approximately 62% between the electrode under test and the current
stake, i.e. at A = 0.62 x B.
Figure 4: schematic for three-terminal resistance measurement with lead null
In this diagram, the C1and P1 terminals are connected together at the electrode under test. This is the
“three-terminal with lead null” configuration which is only applicable in four-terminal testers. This
configuration allows the resistance of the P1 lead to the electrode under test to be “nulled” out. For three-
terminal testers or when lead null is not required, only the P1 terminal (or X terminal on a three-terminal
instrument) connects to the electrode under test. This is shown in Figure 5.
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