Selectivity with parallel incoming feeders
5.3 Selectivity with bus-couplers
Selectivity for 3VA molded case circuit breakers
66 Configuration Manual, 08/2016, A5E03603181010-01
The following diagrams show the protective response in reaction to incoming feeder faults in
the outer and center sections of the busbar:
① Total short-circuit current
Figure 5-5 Selectivity with an incoming feeder short-circuit in the outer busbar section
The displacement factor of the tripping characteristics K
t-I
is as follows:
With a total short-circuit current = 30 kA and
the partial short-circuit currents of the circuit breakers Q2, Q3 and Q5 = 15 kA,
the displacement factor K
t-I
= 2
The displacement factor K
t-I
= 1 for the incoming feeder and bus-couplers Q1 and Q4,
because
the total short-circuit current = 30 kA and
the partial short-circuit current of the incoming feeder and bus-couplers Q1 and Q4 = 30 kA
The tripping characteristics of the instantaneous releases are far to the right of the total
short-circuit current in the case of circuit breakers Q2, Q3 and Q5 and are therefore not
shown in the diagram.
Protective response
Circuit breaker Q1 trips instantaneously, the contacts of circuit breakers Q2 - Q5 remain
closed.
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