MI 3290 Earth Analyser
Tests and Measurements
79
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
Z
e4
G
V
A
A
C
C
Z
e3
Z
e2
Z
e1
>
20
c
m
G
ro
un
di
ng
R
od
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:
𝑍
𝑒4
+ (𝑍
𝑒1
||𝑍
𝑒2
||𝑍
𝑒3
) =
𝑈
𝐻−𝐸
[𝑉] ∗
1
N
𝐼
𝑐
[𝐴]
= [𝛺]
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
”.
