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Ground Resistance Tester Models 3705
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Function Access Summary
Function
Push Button(s)
Used
Function Setting
Saved @ Power Down
On/Off ON/OFF
Ω
Function
ON/OFF Default
Setting
Display Hold
HOLD
No
Auto-Off On/Off
ON + HOLD
No
Principle of Operation
Typically grounded distribution system may be simulated by the basic
circuit shown in Fig. 9 or an equivalent circuit, shown in Fig. 10. 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:
V
I
= Rx +
1
Σ
n
i=1
1
Ri
Rx »
1
Σ
n
i=1
1
Ri
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 2003Hz 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 25
Ω
, and we were to clamp around any
electrode in the system, the measured value would be 25
Ω
in series with
the equivalent 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.
where, usually