MI 3143 Euro Z 440 V
Test and Measurements
19
𝑍 =
𝑈
𝑈𝑁𝐿𝑂𝐴𝐷𝐸𝐷
− 𝑈
𝐿𝑂𝐴𝐷𝐸𝐷
𝐼
𝑡𝑒𝑠𝑡
=
𝛥𝑈
𝐼
𝑡𝑒𝑠𝑡
where:
Z ........................................ Impedance
U
UNLOADED
............................ Measured voltage [no load]
U
LOADED
............................... Measured voltage [with load]
ΔU ...................................... Voltage dip
I
test
...................................... Test current
[A]
[s]
[V]
U unloaded
[s]
U loaded
Itest
ΔU
Figure 6.2: Z line
mΩ measurement voltage and current waveforms examples
6.1.2
Z loop mΩ Measurement
Loop impedance is the impedance within the fault loop when a short - circuit to exposed
conductive parts occurs (conductive connection between phase conductor and protective earth
conductor).
C1
P1
P2
C2
MI 3143 Euro Z 440 V
E a r t h
L1
L2
L3
N
Pe
R1
4 wire (Kelvin)
V
A
S
u
p
p
ly
R
Pe
Itest
ΔU
Z
R2
Figure 6.3:
Z loop mΩ measurement example
During the measurement, an internal resistance is connected between C1 and C2 for a periode
of a half cycle. The instrument internal shunt resistor measures the current (I
test
). A voltmeter
measures the open circuit voltage with no load (U
UNLOADED
), followed by the second reading with
a load (U
LOADED
). The impedance Z is determined from the voltage dip / current ratio. In the
example following impedance is measured:
𝑍 =
𝑈
𝑈𝑁𝐿𝑂𝐴𝐷𝐸𝐷
− 𝑈
𝐿𝑂𝐴𝐷𝐸𝐷
𝐼
𝑡𝑒𝑠𝑡
=
𝛥𝑈
𝐼
𝑡𝑒𝑠𝑡
