Siemens SENTRON - Page 64

Selectivity with parallel incoming feeders
5.3 Selectivity with bus-couplers
Selectivity for 3VA molded case circuit breakers
64 Configuration Manual, 08/2016, A5E03603181010-01
The following diagrams show the protective response of the transformer and bus-couplers in
reaction to feeder faults in the outer and center sections of the busbar:
① Total short-circuit current
Figure 5-3 Selectivity with an outgoing 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 = 45 kA and
the partial short-circuit currents of the circuit breakers Q1 - Q3, Q5 = 15 kA,
the displacement factor K
t-I
= 3
The displacement factor K
t-I
= 1.5 for the bus-coupler Q4, however, because
the total short-circuit current = 45 kA and
the partial short-circuit current of the bus-coupler 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 Q1 - Q3 and Q5 and are therefore not
shown in the diagram.
Protective response
Fuse F1 trips selectively, the circuit breakers Q1 - Q5 remain closed.