Siemens SENTRON - Page 25

Methods of implementing selectivity using circuit breakers
3.4 Time selectivity
Selectivity for 3VA molded case circuit breakers
Configuration Manual, 08/2016, A5E03603181010-01
25
Excerpt from IEC 60947-2, 4.4:
"Selectivity category B comprises circuit breakers providing selectivity by having a short-time
withstand current rating and an associated short-time delay according to 4.3.6.4. Selectivity
of circuit breakers of selectivity category B is not necessarily ensured up to the ultimate
short-circuit breaking capacity (e.g. in the case of operation of an instantaneous release) but
at least up to the value specified in Table 3."
"Selectivity category A comprises all other circuit breakers. These circuit breakers may
provide selectivity under short-circuit conditions by other means."
Section 4.3.6.4. of the standard stipulates that circuit breakers must fulfill the following
requirements:
● Circuit breakers with a rated current
I
rated
of < 2500 A must have a rated short-time
withstand current
I
cw
equal to 12x I
rated
, but at least 5 kA.
● Circuit breakers with a rated current
I
rated >
of 2500 A must have a rated short-time
withstand current
I
cw
of at least 30 kA.
● The short-time delay of the rated short-time withstand current must be at least 0.05 s.
This means that the molded case circuit breakers fulfill the requirements of utilization
category A in most cases.
However, 3VA24 circuit breakers with a rated current I
rated
= 400 A or above also fulfill the
requirements of utilization category B because the electronic trip unit (ETU) of the 5 and
8 series allows a short-time delay of between 0.05 and 0.5 s and a rated short-time
withstand current I
cw
of at least 5 kA.
The 3WL air circuit breakers generally fulfill the requirements of utilization category B.