METREL MI 3290, Instruction Manual

Page: 77 / 162

MI 3290 Earth Analyser Tests and Measurements
77
11.2.4 Selective (Iron Clamp) Measurement
This measurement is applicable for measuring selective earth resistances of individual
earthing points in an earthing system. The earthing rods do not need to be disconnected
during measurement. 4-pole wiring is used for this measurement.
E a r t h
S
ES
E
MI 3290 Earth Analyser
R
p
G
V
A
S - probe
H - auxiliary
current probe
R
c
A
C
C
Z
e2
Z
e1
Z
e3
fset
H
Ie
Ic
Figure 11.20: Selective (Iron Clamp) 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) and (ES) terminal. The
selective current
I
c
is measured through the earthing electrode ( Z
e1
) selected by the user.
The selected earth impedance Z
sel
is determined from the voltage/current (external current
clamp
–
I
c
) ratio.
According to the example selective (individual) earth impedance is measured:
𝑍
𝑠𝑒𝑙
=
𝑈
𝑆−𝐸𝑆
[𝑉]
𝐼
𝑐
[𝐴]∗N
=
𝑈
𝑆−𝐸𝑆
[𝑉]
𝐼
𝑍𝑒1
[𝐴]
= [𝛺] 𝐼
𝑐
=
𝑍
𝑒1
||𝑍
𝑒2
||𝑍
𝑒3
𝑍
𝑒1
∗ 𝐼
𝑒
= [𝐴]
where:
Z
sel
.................................... Selected Earth impedance
Z
e1-3
................................... Earth impedance
R
c
...................................... Impedance of auxiliary current probe (H)
R
p
..................................... Impedance of auxiliary potential probe (S)
I
e
........................................ Injected test current
I
c
........................................ Measured current with Iron clamp
U
S-ES
.................................. Test voltage between S and ES terminal
N ....................................... Turn ratio of current clamps (depending on the model)
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).