HT instruments SIRIUS89N - Manual (Page 119)

SIRIUS89N
EN - 117
16.7. Measurement of Short-Circuit Fault Impedance (Z
pn
, Z
pp
)
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 effected in the point where the short circuit current is the highest
possible, usually immediately downstream the RCD to be checked.
The test shall be effected between phase and phase (Z
pp
) in the three phase
installations and between phase and neutral (Z
pn
) in the single-phase installations..
ALLOWABLE VALUES
Three-phase installations: Single-phase installations:
Pi
Zpp
>
400 2
3
* Pi
Zpn
>
230
dove: P
i
= tripping power of the RCD
Z
pp
= impedance measured between phase and phase.
Z
pn
= impedance measured between phase and neutral
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 detect the
prospective phase-PE short-circuit current so the operator can determinate if
magnetotermical protection is coordinated to the protection of indirect contacts.
ATTENZIONE: The resolution of the instrument is 10m
Ω when the impedance value is
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 fulfil:
Z
S
≤ U
o
/ I
a
dove: U
o
= Phase-Earth Voltage.
Z
S
= Impedance Phase-Earth.
I
a
= tripping current of the magnetotermical protection in 5 seconds.