Methods of implementing selectivity using circuit breakers
3.5 Zone selective interlocking
Selectivity for 3VA molded case circuit breakers
Configuration Manual, 08/2016, A5E03603181010-01
37
It is clear from this diagram that selective tripping behavior cannot be implemented using the
highest possible setting of the instantaneous short-circuit release (I) of the upstream
3VA2 630A molded case circuit breaker. While the tripping characteristics in the short-circuit
zone of the two circuit breakers only touch with short-circuits of 5 kA or higher so that partial
selectivity up to 5 kA is afforded, the required protective functions are not provided.
Setting the 3VA2 630A molded case circuit breaker:
● The value of I
kmaxA
has been calculated as 20.1 kA at the installation point A.
The 3VA24 molded case circuit breaker for a rated operational current I
rated
= 630 A has a
rated ultimate short-circuit breaking capacity I
cu
= 55 kA at 380 - 415 V AC, which means
that this selection is correct.
● The value of I
kmaxB
has been calculated as 7.0 kA and the value of I
kminB
as 2.3 kA at the
installation point B.
However, the setting of the instantaneous short-circuit release of the 3VA2 630 A molded
case circuit breaker is higher than the value of I
kminB
(due to the partial selectivity up to
5 kA), and it is not possible to interrupt this I
kminB
value of 2.3 kA within 5 s.
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