Siemens SENTRON 3VL series - Transformer Protection on the Primary Side

Application planning
5.3 Transformer protection on the primary side
3VL IEC molded case circuit breakers
System Manual, 11/2013, 110 0110 - 02 DS 03
59
5.3
Transformer protection on the primary side
The molded case circuit breaker as transformer protection on the primary side
When switching on low-voltage AC transformers, the extremely high inrush current peaks
place special demands on the trip unit or on the making capacity of the molded case circuit
breakers if these are also used to switch the transformer.
For most applications, an inrush current of 20 to 30 times the rated operating current is
expected in practice and must be taken into account when selecting the molded case circuit
breakers.
The maximum short-circuit current I
k
of the 3VL molded case circuit breakers is 11 x I
n
(rated
current). A molded case circuit breaker in the lower setting range must therefore be used for
transformer protection on the primary side.
Example: A transformer with 500 A rated current; 20 times the inrush current
Selected: ETU with I
n
= 1000 A; setting range 0.4 - 1 x I
n
= 400 A to 1000 A
50% of I
n
= 500 A; I
i
= 11 x I
n
= 1000 A x 11 = 11000 A = 22 x current setting
Note
Disconnection of molded case circuit breaker
It is imperative to ensure that the minimum short
-circuit current I
kmin
in accordance with
VDE
0100 can be disconnected using a protection facility (e.g. molded case circuit breaker).
The 3VL molded case circuit breaker can be disconnected using the short-time-delayed
short-circuit release (S), e.g. a 3VL with an ETU20, where it is possible to set the delay time
to up to 500 ms depending on the duration of the inrush current.
The short delay "bridges" the inrush current peak and the short-circuit protection can then
respond at low current values after a delay.
CAUTION
Molded case circuit breaker with phase failure protection
Molded case circuit breakers with phase failure protection must not be used. Their trip units
have protection against unbalanced network load which cannot be deactivated and can
lead to unintentional trips.