Siemens SENTRON 3WA1 - Selection Criteria

Applications
3.1 Use in systems with isolated transformer neutral point (IT systems)
3WA1 air circuit breaker
200 Equipment Manual, 02/2023, 92310000002-06
3.1.2 Selection criteria
The devices are always dimensioned and selected irrespective of the type of system in
which they will be used. The circuit breaker is selected based on the maximum number
of short-circuit currents that occur in a given system type.
The circuit breaker is generally selected based on the applicable I
cu
-/I
cs
values. If the
operator of an IT system ensures that a ground fault cannot occur on the infeed and
outgoing sides of the circuit breaker, or if the residual risk of a double ground fault is
negligible as the result of defined measures, the double ground fault and the suitability
of the circuit breaker according to Annex H for the double ground fault can be
disregarded provided the operator assumes responsibility for this.
If this is not the case, only circuit breakers which have been successfully tested
according to Annex H under operational voltage may be used. Standard IEC 60947-2
requires testing with a current of 12 x I
n
or 0.8 x I
cw
(depending on which value is
greater), but not more than 50 kA.
All circuit breakers are tested with an I
IT
of 50 kA.
The breaking capacities of the 3WA circuit breakers in IT systems in the event of a
double ground fault are listed in Chapter Technical specifications of 3WA circuit
breakers (Page 489).
Circuit breaker markings
The following markings can be found on the 3WA circuit breakers:
• 690 V
Suitable for IT systems up to 500 V AC according to IEC 60947-2 Annex H (3WA
circuit breakers with breaking capacity N, S, M, H or C)
• 1000 V
Suitable for IT systems up to 690 V AC according to IEC 60947-2 Annex H (3WA
circuit breakers with breaking capacity E)
Note
3WA circuit breakers can
also be used in IT systems (isolated systems) with operational
voltages > 690
V AC if the occurrence of a double ground fault can be reliably excluded.
Article number coding for IT system capability and breaking capacity
3WA11
3WA12
3WA13
Breaking capacity
N
S
M
E
S
M
H
C
E
H
C
E
IT system
Article number digit 8
2
3
4
8
3
4
6
7
8
5
6
8