COMBI521
EN - 133
12.7. RA TEST IN TN SYSTEMS
Protection against indirect contacts in TN systems must be guaranteed by means of a
protection device against overcurrents (typically MCB or fuse) which switches off the
power supply of the circuit or the electrical equipment in case of faults between an active
part and a ground mass or a protection conductor within a interval not exceeding 5s,
sufficient for the equipments.
Parts of the system to be checked
The test must be performed at the point in which the minimum short-circuit current is
possible, normally immediately downstream of the protection device to be checked.
The test must be performed between phase and PE (Z
L-PE
) and between phase and
neutral (Z
L-N
) in three-phase systems or single-phase systems.
Allowable values
The measurement is aimed at ensuring that, in every point of the system, the following
relationships are satisfied:
Z
L-PE
≤
Z
LIM
(1)
Z
L-N
≤
Z
LIM
(2)
where:
Z
L-PE
= Impedance measured between phase and PE
Z
L-N
= Impedance measured between phase and neutral
Z
LIM
=
Maximum limit impedance depending on type (MCB or Fuse) and tripping time
of the selected protection (values depending on countries)
The following selections (*) are available on the instrument:
MCB curve B
3A, 6A, 10A, 13A, 15A, 16A, 20A, 25A, 32A, 40A, 45A, 50A, 63A,
80A,100A,125A,160A,200A
MCB curve C
0.5A, 1A, 1.6A, 2A, 3A, 4A, 6A, 10A, 13A, 15A, 16A, 20A, 25A, 32A,
40A, 50A, 63A, 80A,100A,125A,160A,200A
MCB curve D, K
0.5A, 1A, 1.6A, 2A, 3A, 4A, 6A, 10A, 13A, 15A, 16A, 20A, 25A,
32A, 40A, 45A, 50A, 63A, 80A,100A,125A,160A,200A
Fuse gG
2A, 4A, 6A, 8A, 10A, 12A, 13A, 16A, 20A, 25A, 32A, 35A, 40A, 50A, 63A,
80A, 100A, 125A,160A, 200A, 250A, 315A, 400A, 500A, 630A, 800A, 1000A, 1250A
Fuse aM
2A, 4A, 6A, 10A, 12A, 16A, 20A, 25A, 32A, 35A, 40A, 50A, 63A, 80A,
100A, 125A,160A, 200A, 250A, 315A, 400A, 500A, 630A
Time of fault extinction by the protection selectable among:
0.1s, 0.2s, 0.4s, 1s, 5s
(*) The values could be subject to variations
