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General Information
In this configuration, the injected test current I splits along two paths into I1 (flowing into the connected
grounding 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 (Megger Current Clamp) measures I2 and feeds
this value back to the instrument.
Principle of operation (two-clamp stake-less resistance measurement)
In this example, the electrode under test is connected to a network of other electrodes. It is either impractical
or unsafe to disconnect an individual electrode for testing. Also, there might be insufficient space to perform a
classic three-terminal resistance measurement. The stake-less test method using both Megger Voltage Clamp and
Megger Current Clamp can be used to obtain a measurement for the electrode under test.
A defined test voltage is injected into the system using the Megger Voltage Clamp, inducing a current, I, to flow
and be measured by the Megger Current Clamp. The model shown below can be simplified to the resistance of
the electrode under test, Rx and the resistance of the other electrodes in parallel, i.e. R1 || R2 || … || Rn.
Therefore, the current induced by the test voltage is I=V/[Rx+(R1 || R2 || … || Rn)]. It follows that as the resistance
of the other electrodes in parallel approaches zero, then the resistance measured, approaches the value of the
electrode under test.
Earth electrode
under test
Connection
to rest of system
X (E)
I
I
V
R2
R1
Rx
ICLAMP
VCLAMP
Rn
Schematic for two-clamp stake-less resistance measurement
