Selectivity with parallel incoming feeders
5.1 Selectivity with two identical incoming feeders
Selectivity for 3VA molded case circuit breakers
Configuration Manual, 08/2016, A5E03603181010-01
59
5.1.1 Symmetrical distribution (ideal scenario)
Symmetrical distribution of the short-circuit current occurs when the following conditions are
fulfilled:
● The load feeder is positioned exactly in the center.
● The load currents in other feeders are disregarded.
In this ideal scenario, the total short-circuit current is evenly distributed between both
incoming feeders.
As a result, the tripping characteristic of the circuit breakers Q1 and Q2 can be shifted by the
characteristic displacement factor K
t-I
= 2 to the right on the current scale to the basis for the
fault scenario I
kΣ
.
By moving the tripping characteristic to the right, it becomes possible to implement time
selectivity and improve the quality of current selectivity.
5.1.2 Asymmetrical distribution
If the incoming and outgoing feeders are not arranged symmetrically on the busbar, there will
be variations in the distribution of the short-circuit current.
Depending on the impedance ratio of the transformers and incoming feeders, the
characteristic displacement factor K
t-I
can be calculated as follows:
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