METREL MI 3290, Instruction Manual

Page: 73 / 162

MI 3290 Earth Analyser Tests and Measurements
73
11.2.2 3
– pole Measurement
The three-pole measurement is the standard earthing test method. It is the only choice if
there is no well earthed auxiliary terminal available. The measurement is performed with two
earthing probes. The drawback if using three wires is that the contact resistance of E
terminal is added to the result.
E a r t h
H
S
E
MI 3290 Earth Analyser
R
p
G
V
A
S - probe
H - auxiliary
current probe
R
c
Z
e
fset
Ie
Figure 11.12: 3
– pole measurement example
During the measurement a sinusoidal current
I
e
is injected into the earth through an auxiliary
current probe (H). The impedance of the auxiliary probe (H) should be as low as possible in
order to inject a high test current. The impedance
R
c
can be decreased by using more probes
in parallel. A higher injected current improves the immunity against spurious earth currents.
The voltage drop is measured by auxiliary potential probe (S). The earth impedance
Z
e
is
determined from the voltage/current ratio.
In the example following earth impedance is measured at a set frequency:
𝑍
𝑒
=
𝑈
𝑆−𝐸
[𝑉]
𝐼
𝑒
[𝐴]
= [𝛺]
where:
Z
e
...................................... Earth impedance
R
e
..................................... Earth resistance (excluding reactance)
R
c
...................................... Impedance of auxiliary current probe (H)
R
p
..................................... Impedance of auxiliary potential probe (S)
I
e
........................................ Injected test current
U
S-E
.................................... Test voltage between S and E terminal
f
set
..................................... Test frequency
Refer to Appendix C
– Functionality and placing of test probes
for more information
how to place the earth auxiliary current (H) and potential probe (S).