Principle Of Operation - AEMC 3711 User Manual

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
voltage (V) is applied to any measured grounding electrode Rx through a
special transformer, current (I) flows through the circuit, thereby
establishing the following equation:
= Rx +
I
Σ
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 2403Hz 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 resistance 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 25Ω, and we were to clamp around any
electrode in the system, the measured value would be 25Ω in series with
the equivalent parallel resistance or 0.25Ω. The displayed value would
be 25.2Ω (instrument resolution to 0.1Ω).
V/I = 25Ω + 0.25Ω
Rx = 25.2Ω
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 3711 and 3731
1
1
where, usually
n
Ri
i=1
x »
- 22 -
1
1
Σ
n
i
i=1