Siemens SENTRON 3VL - Page 342

Appendix
B.1 Selectivity
3VL molded-case circuit breakers
342 System Manual, 03/2009, 110 0110 - 02 DS 01
● For circuit breakers with LI and LSI characteristics, it is important to ensure that the
appropriate I
i
and/or I
sd
protection settings fulfill the protection function in TN/TT/IT
networks. This means the I
i
or I
sd
protection setting must switch off a short-circuit within
5 s (for fixed loads) or 0.4 s (for mobile loads).
NOTICE
I
i
and I
sd
protection settings
The I
i
and I
sd
protection settings also influence the selectivity behavior of upstream and
downstream devices.
● Current selectivity:
In general, only partial selectivity can be achieved using current grading (current
selectivity through the use of LI releases)! That is, complete selectivity can only be
achieved up to the setpoint value of the instantaneous short-circuit release of the
upstream protection device I
i
Q1 minus 20%, rather than up to the maximum short-circuit
current I
kmax
! (See EN 60947 T2)
● Circuit breakers with an adjustable time delay are often required to achieve total
selectivity between two protective devices. To calculate the selectivity limit values, the
time delay t
sd
of the LSI releases is always set to the first time level and I
d
to the
maximum value.
● Time selectivity:
If the maximum short-circuit currents at the mounting points are approximately the same
(e.g. on the main distribution board), the upstream circuit breaker (Q1) requires a delayed
short-circuit release (S release). It must not have an instantaneous release (I release)
unless the I function must be switched "Off". The microprocessor-controlled "Zone-
Selective Interlocking (ZSI)" feature has been developed by SIEMENS to prevent long,
undesired tripping times in the case of series-connected circuit breakers. ZSI enables the
tripping delay to be reduced to a maximum of 50 ms for the circuit breaker upstream from
the location of the short-circuit.
● More information on this topic:
Additional selectivity limit tables can be obtained on request from our Technical
Assistance. Characteristic curves programs Simaris deSign - configuration software