METREL MI 3290, Instruction Manual

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MI 3290 Earth Analyser Tests and Measurements
71
11.2.1 2
– pole Measurement
The two-pole measurement can be used if there is a well-grounded auxiliary terminal
available (e.g. source/ distribution earthings via the neutral conductor, water pipeline
…). The
main advantage of this method is that no test probes are needed for the test. The method is
fast and relatively reliable.
E a r t h
H
E
MI 3290 Earth Analyser
G
A
Z
e
Water pipeline
H - auxiliary
current probe
R
c
fset
V
Ie
Figure 11.8: 2
– pole measurement example
During the measurement, a sinusoidal current
I
e
is injected into the earth through an auxiliary
probe (H). The impedance of the auxiliary probe (H) should be as low as possible. The
impedance Rc can be decreased by using more probes in parallel or using an auxiliary
earthing system as the auxiliary probe. A higher injected current improves the immunity
against spurious earth currents. The earth impedance
Z
e
is determined from the
voltage/current ratio. Usually the impedance R
c
is much lower than Z
e
. In this case the result
can be considered as

Z
e
.
𝑍
𝑒
=
𝑈
𝐻−𝐸
[𝑉]
𝐼
𝑒
[𝐴]
= [𝛺]
where
𝑍
𝑒
>> 𝑅
𝑐
Z
e
...................................... Earth impedance
R
e
..................................... Earth resistance (excluding reactance)
R
c
...................................... Impedance of auxiliary current probe (H)
I
e
........................................ Injected test current
U
H-E
................................... Test voltage between H and E terminal
f
set
..................................... Test frequency
Refer to Appendix C
– Functionality and placing of test probes
for more information
how to place the auxiliary current probe (H).