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16.7.
Measurement of Short-Circuit Fault Impedance (Z
PURPOSE OF THE TEST
Check that the tripping power of the RCD is higher than the maximum fault current of
the installation.
INSTALLATION PARTS TO BE CHECKED
The test shall be performed in the point where the short circuit current is the highest
possible, usually immediately downstream of the RCD to be checked.
The test shall be performed between phase and phase (Z
installations and between phase and neutral (Z
ALLOWABLE VALUES
Three-phase installations:
400
>
Pi
Zpp
Note:
P
= tripping power of the RCD
i
Z
= impedance measured between phase and phase.
pp
Z
= impedance measured between phase and neutral
pn
16.8.
Fault Loop Impedance Measurement (Phase – Earth)
PURPOSE OF THE TEST
The fault loop is the circuit of the current when there is a bad isolation of the electrical
system toward earth. The fault loop is composed:
Transformer coil impedance.
-
The impedance of the line from the transformer to the fault.
-
-
The impedance of the protective conductor from the conductive part to the neutral of
the transformer.
When the instrument measure the impedance of the fault loop, the instrument will detect
the prospective phase-PE short-circuit current so the operator can determine if the
overload protection is appropriate.
The resolution of the instrument is 10m Ω when the impedance value is
ATTENTION:
inside (0.01 – 19.99) Ω , so use the instrument for measurements of
impedance higher of 100m Ω .
POINTS TO BE CHECKED
The test is necessary in TN or IT electrical system without RCDS.
ALLOWED VALUES
The following relation has to be fulfilled:
Note:
U
= Phase-Earth Voltage.
o
Z
= Impedance Phase-Earth.
S
I
= tripping current of the overload protection in 5 seconds.
a
2
*
3
≤ U
Z
S
Page 118 of 131
, Z
)
pn
pp
) in the single-phase installations..
pn
Single-phase installations:
230
>
Pi
Zpn
/ I
o
a
VR2250
) in the three phase
pp
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