Siemens SENTRON - Page 35

Methods of implementing selectivity using circuit breakers
3.5 Zone selective interlocking
Selectivity for 3VA molded case circuit breakers
Configuration Manual, 08/2016, A5E03603181010-01
35
Fault scenario F1
A fault occurs within the protection zone of circuit breaker Q11.
● If the short-circuit current is high enough, it is detected by the trip unit of circuit
breaker Q11.
● Since Q11 does not receive a "block" signal from Q22, the original delay time
t
sd
= 300 ms is replaced by the shortened delay time t
ZSI
= 50 ms.
● Q11 trips after t
ZSI
= 50 ms.
Fault scenario F2
A fault occurs within the protection zone of circuit breaker Q22.
● If the short-circuit current is high enough, it is detected by the trip units of circuit breakers
Q11 and Q22.
● The zone selective interlocking system causes Q22 to temporarily block Q11.
– Q11 retains the original delay time t
sd
= 300 ms.
– Since Q22 does not receive a "block" signal, its original delay time t
sd
= 200 ms is
replaced by the shortened delay time t
ZSI
= 50 ms.
● Q22 trips after t
ZSI
= 50 ms.
Fault scenario F3
A fault occurs within the protection zone of circuit breaker Q32.
● If the short-circuit current is high enough, it is detected by the trip units of circuit
breakers Q11, Q22 and Q32
● As a result of the zone selective interlocking system, Q32 temporarily blocks Q22 which
in turn temporarily blocks Q11.
– Q11 retains the original delay time t
sd
= 300 ms.
– Q22 retains the original delay time t
sd
= 200 ms.
● Q32 trips after t
i
= 10 ms if the short-circuit is within the range of the instantaneous short-
circuit protection (I).
● Q32 trips after t
ZSI
= 50 ms if the short-circuit is within the range of the short-time delay
short-circuit protection (S).
Fault scenario F4
A fault occurs within the protection zone of circuit breaker Q41.
● If the short-circuit current is high enough, it is detected by the trip units of circuit
breakers Q11, Q22, Q33 and Q41.
Since Q41 already trips after t
i
= 10 ms, however, none of the other circuit breakers trip.
● The zone selective interlocking and time selectivity systems have no influence on circuit
breaker operation.