Siemens SENTRON - Page 38

Methods of implementing selectivity using circuit breakers
3.5 Zone selective interlocking
Selectivity for 3VA molded case circuit breakers
38 Configuration Manual, 08/2016, A5E03603181010-01
Setting the 3RV circuit breaker for motor protection (motor starter protector):
● The value of I
kmaxB
has been calculated as 7.0 kA at the installation point B.
The 3RV motor starter protector with a rated operational current I
rated
= 45 A has a rated
ultimate short-circuit breaking capacity I
cu
= 50 kA at 400 V AC, which means that this
selection is correct.
● The value of I
kminC
has been calculated as 1.0 kA at the installation point C.
The setting of the instantaneous short-circuit release of the 3RV motor starter protector is
correct because – despite the current selectivity – the instantaneous short-circuit release
is set to 585 A and the setting is thus lower than I
kmin
. It is also ensured that the 3RV
motor starter protector would interrupt this short-circuit within 5 s.
① Dynamic selectivity zone (more information about dynamic selectivity can be found in chapter
"Dynamic selectivity (Page 40)")
In order to ensure that the two circuit breakers can fulfill the total selectivity conditions and
also guarantee that both circuit breakers will trip within 5 s at the relevant I
kmin
at installation
points B and C, the following must be noted with respect to the upstream 3VA2 molded case
circuit breaker.
The selectivity limit currents I
S
and thus also total selectivity can only be implemented with
the instantaneous short-circuit releases of the upstream circuit breakers set to their
maximum values.
For this reason, the upstream 3VA2 molded case circuit breaker with its ETU 320 must be
replaced by a 3VA2 molded case circuit breaker with an ETU 350. The protection of I
kminB
can then be achieved using the short-time delay short-circuit release (S), and total selectivity
can be implemented by the instantaneous short-circuit release (I) permanently set to its
maximum value in the dynamic selectivity zone.