Selectivity with parallel incoming feeders
5.2 Selectivity with three identical incoming feeders
Selectivity for 3VA molded case circuit breakers
Configuration Manual, 08/2016, A5E03603181010-01
61
5.2.2 Fault between transformer terminals and incoming feeder circuit breaker
Symmetrical fault current distribution
The instantaneous short-circuit release (I release) must be set in such a way that its value is
higher than the partial short-circuit currents I
k-Ti
of a transformer. This means that only that
incoming feeder circuit breaker carrying the total short-circuit current
kΣ
ever trips
instantaneously on a secondary-side terminal short-circuit on the transformer. The circuit
breakers in the incoming feeders without a fault remain closed because of the delay time t
sd
set on the S release.
K
t-I
= 30 kA / 15 kA = 2 is the ideal characteristic displacement factor for the transformer
circuit breakers Q2 and Q3 of the fault-free incoming feeders.
① Characteristic displacement factor
② Total short-circuit current
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