Description
2.10 Zone-selective interlocking ZSI
3WA1 air circuit breaker
Equipment Manual, 02/2023, 92310000002-06
197
Fault scenario B
• The -Q3 and -Q1 circuit breakers detect the short-circuit. Via the ZSI system, the -Q1
circuit breaker is informed of the pick-up of -Q3. This prevents tripping of -Q1 after
the delay t
ZSI
.
• The -Q3 circuit breaker does not receive any pick-up information from a downstream
circuit breaker at the ZSI interface port A. Therefore, the -Q3 circuit breaker trips
after the set delay t
ZSI
0.2 s for the short-time-delayed short-circuit protection ST.
• If the fault current cannot be switched off by the -Q3 circuit breaker, tripping of -Q1
as back-up protection takes place after the delay time t
sd
.
Fault scenario C
• The -Q1 circuit breaker detects the short-circuit and does not receive any information
about the pick-up of a downstream circuit breaker.
Therefore -Q1 trips after the delay t
ZSI
0.2 s.
It can also be seen in this example that when fuses and circuit breakers are used, the
load on the power distribution system in the event of a short-circuit is significantly
reduced by the ZSI system.
This reduction in load can also be seen in the two grading diagrams. On the left, the
grading times for a fault in B and on the right for a fault in C are shown.
For both faults, tripping takes place after the delay t
ZSI
.
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