MI 3290 Earth Analyser
Tests and Measurements
102
11.7.1 Potential Measurement
Local potential differences can be simply measured using 3
– pole wiring and setting up step
size (m or ft), test frequency and direction
ϕ
.
H
S
E
MI 3290 Earth Analyser
G
V
A
fset
E a r t h
Ie
1
S - probe
H - auxiliary
earth rod
Step Size
2
S - probe
...
S - probe
d
Figure 11.67: Potential 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).
𝑈
𝑆
= 𝑈
𝑚
[𝑉] ∗
𝐼
𝑓𝑙𝑡
[𝐴]
𝐼
𝑒
[𝐴]
= [𝑉]
where:
U
S
.......................... Potential voltage normalized to a fault current
U
m
.......................... Measured Potential voltage
I
e
............................. Injected test current
I
flt
............................ Fault current
Step Size ................ Distance between neighbouring measurement points [fixed value]
d ............................. Distance between E point and measuring (S probe) point
ϕ
............................ Direction of potential measurement or angle (0
° – 360°)
E a r t h
H
step
d
Distance
P
ot
en
tia
l v
ol
ta
ge
Potential gradient
E
S 1
S 2
S n+1
Us1
Us2
Us
n+1
T
ra
ns
m
is
si
on
T
ow
er
(
4
le
g)
Step Size
Grounding Rod
Us1
Us2
Us n+1
Direction ϕ = 0°
Us1
Us2
Di
re
cti
on
ϕ
= 4
5°
D
ire
ct
io
n
ϕ
=
9
0°
Us1
Us2
Us n+1
Us n+1
ϕ
ϕ3
S
te
p
S
iz
e
Figure 11.68: Potential gradient example
(straight line)
Figure 11.69: Potential gradient
example (around the building)
