MI 3290 Earth Analyser Tests and Measurements
81
11.2.6 HF-Earth Resistance (25 kHz) Measurement
If the tested objects have in addition to a local earthing also a remote earthing, like overhead
grounding wires (OHGW) or both side grounded shielded underground cable or any other
galvanic connection to remote grounding, a measurement of local earthing with classic 3 or 4
pole method is not possible without disconnecting the remote grounding. Typical example is
a pillar of the power line.
In such cases advanced measurement methods have to be used, one of them is the HF-
Earth Resistance measurement.
The HF method is based on the effect, where, due to the very high frequency (25 kHz) of the
test signal, the chain impedance of the OHGW becomes so high that the current through
OHGW is very low and can be ignored (EN 50522, Annex L.2.2.e).
E a r t h
H
S
ES
E
MI 3290 Earth Analyser
Guard
Rp
G
V
A
S - probe
H - auxiliary
earth rod
Rc
25 kHz
shielded cable
tower
nex tower
overhead grounding wire
Igw
Re
Igw
LGW
nex tower
Ie
RGW
LGW
Ih
Figure 11.27: HF-Earth Resistance (25 kHz) example
During the measurement a (25 kHz) sinusoidal current I
h
is injected into the earth through an
auxiliary 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 resistance R
e
is
determined from the voltage/current ratio. In the example following earth resistance is
measured:
where:
R
e
..................................... Earth resistance (excluding reactance)
Z
e
...................................... Earth impedance
R
c
...................................... Impedance of auxiliary current probe (H)
R
p
..................................... Impedance of auxiliary potential probe (S)
I
h
........................................ Injected test current
I
e
........................................ Measured test current
U
S-E
.................................... Test voltage between S and E terminal
I
gw
...................................... Overhead grounding wire current
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