Siemens SIRIUS 3RH Series, System Manual

Page: 393 / 786

SIRIUS 3RV2 motor starter protectors
4.5 Configuration
SIRIUS Innovations
System Manual, 01/2011, A8E56203870002-03
393
4.5.6 Transformer protection
Protection against inrush currents
When control transformers are protected on the primary side, the high inrush currents
generated at the time the transformers are switched on often cause spurious tripping in the
protective devices. 3RV24 MSPs for transformer protection therefore have short-circuit
releases which are factory-set to a fixed value of approx. 20 times the rated current (upper
value on the setting scale). This enables transformers whose inrush current peak values can
reach up to 30 times the rated current to be protected on the primary side by means of motor
starter protectors.
4 AM control transformers
This version of motor starter protector is not necessary in the case of 4 AM control
transformers with low inrush current (e.g. Siemens control transformers). In such cases
3RV20 motor starter protectors can be used for motor protection.
4.5.7 Main switch
3RV2 motor starter protectors satisfy the features of main switches according to
IEC 60947-2.
3RV20 and 3RV21 motor starter protectors are also suitable for protecting systems.
To prevent premature tripping caused by phase loss sensitivity, the load applied to each of
the three current paths must always be the same. With single-phase loads, the current paths
must be connected in series.
4.5.8 Use in IT systems
3RV2 motor starter protectors are suitable for use in IT systems according to IEC 60947-2.
In the event of a 3-pole short circuit, they respond in exactly the same way as other systems:
For this reason, the same short-circuit breaking capacity I
cu
and I
cs
applies.
Function
In the case of IT systems the first fault (ground fault) does not necessarily lead to the
immediate disconnection of the system. A second unrelated fault (ground fault) can reduce
the switching capacity of the motor starter protector.
This is the case specifically if both ground faults occur in different phases and if one of the
two ground faults is on the motor starter protector's input side and the other is on its outgoing
side.
To maintain the motor starter protector's short-circuit-protection function even in the case of
two separate ground faults (known as a double line-to-ground fault), the reduced short-circuit
breaking capacity for double ground faults in IT systems I
cuIT
has to be taken into account.
Detecting a ground fault when it occurs (ground-fault monitoring) and dealing with it quickly
can significantly reduce the risk of a double ground fault and, as a result, that of a reduced
short-circuit breaking capacity I
cuIT
.
Reference
More information ... Can be found in the chapter titled ...
About short-circuit breaking capacity Technical data (Page 453)