MI 3290 Earth Analyser Tests and Measurements
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11.2.5 2 Clamps Measurement
This measurement system is used when measuring earth impedances of grounding rods,
cables, under- earth connections, etc. The measuring method needs a closed loop to be able
to generate test currents. It is especially suitable for use in urban areas because there is
usually no possibility to place the test probes.
E a r t h
H
E
MI 3290 Earth Analyser
Ze4
G
V
A
A
C
C
Ze3
Ze2
Ze1
>20 cm
Grounding Rod
fset
Ic
Figure 11.24: 2 Clamps example
The driver (generator) clamp injects a voltage in the earthing system. The injected voltage
generates a test current in the loop. If the total loop earth impedance of the electrodes Z
e1
,
Z
e2
, Z
e3
and Z
e4
connected in parallel is much lower than the impedance of tested electrode
Z
e4
, then the result can be considered as Z
e4
. Other individual impedance can be measured
by embracing other electrodes with the current clamps.
According to the example individual earth impedance is measured:
where:
Z
e1-e4
................................. Earth impedance
I
c
........................................ Measured current with Iron clamp
U
H-E
................................... Test voltage between H and E terminal
N ....................................... Driver (generator) clamp transformation ratio
(depending on the clamp model)
f
set
..................................... Test frequency
Note:
2 Clamps earth resistance test is sometimes called “loop resistance test”.
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