Principle of Operation
Typically grounded distribution system may be simulated by the basic
circuit shown in Fig. 20 or an equivalent circuit, shown in Fig. 21. If volt-
age (V) is applied to any measured grounding electrode Rx through a
special transformer, current (I) flows through the circuit, thereby
establishing the following equation:
1
V
= Rx +
Σ
1
I
n
Ri
i=1
Therefore, V/I = Rx is established. If I is detected and measured with V
kept constant, the measured grounding electrode resistance Rx can be
obtained. A signal is fed to a special transformer via a power amplifier
from a 1.689 kHz constant voltage oscillator. The resulting current is
then sensed by a detection CT. An active filter is used to dampen earth
current at commercial frequency and high-frequency noise.
Example: If we clamp around any grounding electrode in a multi-grounded
system, the measured value of the electrode under test will be the resis-
tance of that particular rod in series with the equivalent parallel resistance
value that the rest of the multi-grounded system represents. If we had an
electrical system that had 101 grounding electrodes and each had a
resistance value of 25 Ω, and we were to clamp around any electrode in
the system, the measured value would be 25 Ω in series with the equiva-
lent parallel resistance or 0.25 Ω. The displayed value would be 25.2 Ω
(instrument resolution to 0.1 Ω).
V/I = 25 Ω + 0.25 Ω
Rx = 25.2 Ω
In most field applications, the number of electrodes that make up a
multi-grounded system would be higher, therefore the equivalent
parallel resistance is negligible with respect to the rod under test.
Ground Resistance Tester Models 3710/3730
»
where, usually
Rx
1
Σ
1
n
Ri
i=1
21