MI 3290 Earth Analyser
Tests and Measurements
94
11.4 Impulse Impedance [Zp]
The impulse impedance of an earthing system is a useful parameter, to predict the behaviour
in transient conditions, as it provides a direct relationship between the peak potential rise and
the peak current rise.
11.4.1 Impulse Measurement
The three pole method or the fall of potential method test configurations are typically used for
this type of tester. 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
ES
E
MI 3290 Earth Analyser
Z
p
Guard
R
p
Vp
Ap
S - probe
H - auxiliary
earth rod
R
c
Ie
10/350 µs
shielded cable
Figure 11.49: Impulse Measurement example
During the measurement a current impulse (10/350 µs) 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 Rc can be decreased by using more
probes in parallel. A higher injected current impulse improves the immunity against spurious
earth currents. The voltage peak is measured by potential probe (S). The impulse impedance
Zp is determined from the voltage peak /current peak ratio.
In the example following impulse impedance is measured:
𝑍
𝑝
=
𝑈
𝑝𝑒𝑎𝑘
𝐼
𝑝𝑒𝑎𝑘
− 𝑍
𝑖𝑛
R
X
L
i
Zp
where:
Z
p
...................................... Impulse impedance
Z
in
.....................................
Internal impedance of the instrument (typ. 1 Ω)
U
peak
.................................. Peak voltage
I
peak
................................... Peak current
Note:
The current probe Rc and potential probe Rp are measured using 3-Pole measurement at a
fixed frequency 3.29 kHz @ 40 Vac open-terminal test voltage.
