Siemens SENTRON 3VL160X System Manual page 70

Application planning
5.8 Use in series connection
This current is referred to as the selectivity limit.
If the values determined by the short-circuit current calculation (e.g. according to
IEC / EN 60909, DIN VDE 0102) at the installation point of the downstream circuit breaker
are below the selectivity limit listed in the respective table for the selected combination,
selectivity is guaranteed for all possible short-circuits at the installation point.
If the calculated short-circuit current at the mounting point is higher than the selectivity limit,
selective tripping by the downstream circuit breaker is only ensured up to the value listed in
the table. The engineer must judge whether the value can be considered to be sufficient
because the probability of, for example, the maximum short-circuit occurring is low.
Otherwise, a circuit breaker combination should be chosen whose selectivity limit lies above
the maximum short-circuit current.
Time selectivity
Selectivity can be achieved by time selectivity up to the threshold values of the
instantaneous short-circuit release. To achieve this, the upstream circuit breaker requires
delayed short-circuit releases, so that in the event of a fault, only the downstream circuit
breaker will disconnect the faulted system component from the supply.
Both the tripping delays and the tripping currents of the short-circuit releases are staggered.
Zone-selective interlocking - ZSI
Zone-selective interlocking (ZSI) has been developed by SIEMENS for the 3VL molded case
circuit breakers to prevent long, undesired release times when several molded case circuit
breakers are connected in series.
ZSI enables the tripping delay to be reduced to 50 ms for the circuit breaker upstream from
the location of the short-circuit.
When selecting, ensure that the molded case circuit breaker can handle the initial balanced
short-circuit current I
The following are required for the ZSI function:
● A COM20 or COM21 communication module
● A communication-capable ETU
You can find further details in the following manuals:
● "SENTRON WL and SENTRON VL circuit breakers with communication capability -
PROFIBUS" (Order No. A5E01051347)
● "SENTRON WL and SENTRON VL circuit breakers with communication capability -
Modbus" (Order No. A5E02126886)
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at the installation point.
K
3VL IEC molded case circuit breakers
System Manual, 11/2013, 110 0110 - 02 DS 03