Zone Selective Interlocking
5.1 ZSI
3WL/3VL circuit breakers with communication capability - PROFIBUS
106 System Manual, 03/2011, A5E01051353-02
5.1.3 Time selectivity
One method of achieving this aim in the short-circuit or ground fault is time selectivity of the
circuit breakers.
W
VG
PV
W
VG
PV
W
VG
PV
W
VG
PV
Figure 5-1 Example of time selectivity
This means each level of the circuit breaker receives another time delay (t
sd
) that rises in the
direction of the incoming supply. Tripping of the circuit breakers that are further removed
from the short-circuit is thus delayed and the circuit breaker closest to the short-circuit is
given time to switch the short-circuit off.
The disadvantage of such a system, however, is that one short-circuit trip is always delayed
and that there is a long delay in switching off a short-circuit close to the infeed, and the plant
is thus loaded with the short-circuit current for longer than necessary. This can require
increased dimensioning of the system.
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