Figure 4: schematic for three-terminal resistance measurement with lead null
In this diagram, the C1 and 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.
Figure 5: schematic for three-terminal resistance measurement without lead null
Principle of operation (three-terminal resistance measurement using ART)
The classic three-terminal test method has a disadvantage, namely that the electrode under test must be disconnected from
the system it is supposed to protect in the event of a power system fault. The reason for this is that the injected test current
will take all possible routes to ground and not all of it will necessarily flow through the electrode under test. In this case, the
instrument will make a reading of the entire earthing network, not just the individual electrode.
By using a current transducer (the Megger MCC CLAMP) to measure the current flowing through the electrode under test as
a fraction of the total test current injected, the instrument can determine the individual resistance. This arrangement is shown
in Figure 6.
Figure 6: schematic for three-terminal resistance measurement using ART without lead null
In this configuration, the injected test current I splits along two paths into I1 (flowing into the connected earthing system) and
I2 (flowing into the electrode under test, i.e. I=I1+I2. The resistance of the electrode under test is calculated as R=V/I2 or R=V/
(I-I1). The current transducer (MCC CLAMP) measures I2 and feeds this value back to the instrument.
MCC
