HT instruments SPEED418 - Manual (Page 79)

400 Series
EN - 78
14.6. MEASUREMENT OF SHORT-CIRCUIT IMPEDANCE
14.6.1. Purpose of the test
Check that the tripping power of the protection device is higher than the maximum fault
current of the installation.
14.6.2. 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.
14.6.3. Allowable values
Three-phase installations: Pi
Zpp

400 2
3
*
Single-phase installations: Pi
Zpn

230
where: P
i
= tripping power of the protection device
Z
pp
= impedance measured between phase and phase
Z
pn
= impedance measured between phase and neutral
14.7. FAULT LOOP IMPEDANCE MEASUREMENT
14.7.1. 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 measures fault loop impedance, it detects the prospective fault
current. So the operator can determinate if magnetothermical protection is coordinated to
the protection of indirect contacts.
CAUTION
The instrument must be used to measure a fault loop impedance value
which is at least ten times higher than the instrument’s resolution, so as to
minimize possible errors.
14.7.2. Installation parts to be checked
The test is necessary in TN or IT electrical system without RCDs.
14.7.3. Allowable values
The following relation has to be fulfill:
Z
S
 U
o
/ I
a
ove: U
o
= phase to earth voltage.
Z
S
= impedance measured between phase and earth.
I
a
= tripping current of the magnetothermical protection in 5 seconds.