HT instruments SIRIUS87 - Manual (Page 81)

SIRIUS 87
EN - 79
11.6. MEASUREMENT OF SHORT CIRCUIT FAULT IMPEDANCE (Z
PN
, Z
PP
)
Purpose of the test is to 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
11.7. FAULT LOOP IMPEDANCE MEASUREMENT (PHASE – EARTH)
Purpose of the test is t check 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 measures fault loop impedance, it detects the prospective phase-PE
short-circuit current. So the operator can determinate if magnetothermical protection is
coordinated to the protection of indirect contacts.
ATTENTION: The resolution of the instrument is 10m
 when the impedance value is
inside (0.01 – 19.99)
, so use the instrument for measurements of
impedance higher of 100m
.
PLANT PARTS TO BE VERIFYED
The test is necessary in TN or IT electrical system without RCDS.
ALLOWABLE VALUES
The following relation has to be fulfill:
Z
S
 U
o
/ I
a
where: U
o
= Phase-Earth Voltage.
Z
S
= Impedance Phase-Earth.
I
a
= Tripping current of the magnetothermical protection in 5 seconds.