HT instruments MACROTEST G3 - Manual (Page 95)

MACROTESTG3 - COMBIG3
EN - 94
12.6. VERIFY OF PROTECTION AGAINST INDIRECT CONTACTS IN TN SYSTEMS
Purpose of the test
The protection against indirect contacts in the TN systems must be guarantee by means a
protection device against the overcurrents (typically MCB or fuse) which swich off the
power supply of the circuit or the electrical equipment in case of fault between an active
part and a ground mass or a protection conductor within a interval not exceeding at 5s,
sufficient for the equipments, or in compliance with the times declared in the following
Uo [V] Trip out time of protection [s]
50  120 0.8
120  230 0.4
230  400 0.2
>400 0.1
Table 5: Tripping times for protection devices
Uo = nominal AC voltage refer to ground of the system
The above conditions is satisfied by the following relationshisp:
Zs * Ia  Uo
where:
Zs = Fault Loop P-PE impedance which includes the phase winding of the
transformer, the line conductor up to the fault point and the protective
conductor from the fault point to the star center of the transformer
Ia = Tripping current of the protection device within the specified time in Table 5
Uo = nominal AC voltage refer to ground
CAUTION
The instrument must be used to measure fault loop impedance values at
least 10 times higher than the resolution value of the instrument in order to
minimize errors.
Parts of the system to be checked
The test must necessarily be performed on TN and IT systems not protected by differential
devices.
Allowable values
The measurement is aimed at ensuring that in every point of the system the relationships
derived from standard EN60909-0 are satisfied:
PE P
NOM
PE P
MIN PE P MIN
Z
U
C I Ia



  
Measured voltage U
NOM
C
MIN
230V-10% < Vmeasured < 230V+ 10% 230V 0.95
230V+10% < Vmeasured < 400V- 10% Vmeasured 1.00
400V-10% < Vmeasured < 400V+ 10% 400V 0.95