Siemens SENTRON - Page 46

Methods of implementing selectivity using circuit breakers
3.6 Dynamic selectivity
Selectivity for 3VA molded case circuit breakers
46 Configuration Manual, 08/2016, A5E03603181010-01
Dynamic selectivity in the short-circuit zone
① Dynamic selectivity zone
The diagram shows an example with two 3VA2 molded case circuit breakers (Q1 and Q2)
connected in series.
The tolerance bands of the tripping characteristics of the L and I releases do not touch until
the current reaches or exceeds 3600 A. In other words, current selectivity is provided up to
3600 A.
In the 3600 A short-circuit current zone or above, it is not possible to reach any clear
conclusion about selectivity based on the tripping characteristics. For this reason, circuit
breakers that support dynamic selectivity at short-circuit currents higher than 3600 A must be
used.
The new 3VA2 molded case circuit breakers have been developed to meet the requirements
of dynamic selectivity and do so with short-circuit currents of 3600 A or above. In this
example, they also guarantee selectivity in the zone above 3600 A until the selectivity limit
current (I
s
) is reached.
You can find the selectivity tables as "FAQs
(https://support.industry.siemens.
com/cs/de/de/view/97493202)" in the Siemens Industry
Online Support (SIOS).
According to chapter 2 of these tables, the total selectivity in this example is listed, i.e. the
selectivity limit current I
s
corresponds to the relevant rated ultimate short-circuit breaking
capacity I
cu
of the downstream 3VA2 160 A molded case circuit breaker.