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ZSI Module
To use the ZSI function with the
WL Circuit Breaker, the external
CubicleBUS ZSI module must be
implemented.
The zone selective interlocking (ZSI)
module provides the complete
range of selectivity with the
short delay time of t
= 50 ms,
ZSI
irrespective of the number of
levels and the location of the short-
circuit in the distribution system.
Its benefits become even more
apparent, the higher the number
of levels in large systems and the
longer the resulting delay times.
By shortening the time, the ZSI
module significantly reduces stress
and damage in the event of a short-
circuit in the switchgear.
Operating principle
If the ZSI module is used in a
distribution system comprising
several levels, each circuit breaker
affected by a short-circuit
interrogates the circuit breaker
directly downstream, to ascertain
whether the short-circuit also
occurred in the next level below:
Graphic 2-6
This graphic illustrates the operating principle of the ZSI function using an example in a power distribution system. It is also a
connection diagram that shows how the ZSI module must be wired if the WL Circuit Breakers are used.
WL PROFIBUS Communication and Electronic Accessories • July 2004
Communication-capable Circuit Breakers
• If the short-circuit did occur in the
downstream level, the upstream
circuit breaker delays tripping to
ensure that the circuit breaker
directly upstream of the short-
circuit has enough time to
interrupt the short-circuit.
• If the circuit breakers in the
downstream level do not report
a short-circuit, the short-circuit
occurred between the two levels
in question. In this case, one of
the two upstream circuit breakers
interrupts the short-circuit once
the programmed delay time of
t
= 50 ms has elapsed.
ZSI
Example as illustrated in Graphic 2-6.
This shows a section of a power
distribution system that has been
installed with the ZSI module. WL
Circuit Breakers are implemented
at different levels.
WL Circuit Breaker
Short-circuit at 3:
Circuit breakers -Q5, -Q3, and -Q1
establish that a short-circuit has
occurred. -Q5 blocks -Q3 by means
of the ZSI signal and, as a result,
-Q1 too, so that they do not trip in
50 ms. Since -Q5 does not receive a
blocking signal from a subordinate
circuit breaker, it is responsible for
interrupting the short-circuit as
quickly as possible. If this does
not take place, because the circuit
breaker is no longer operational due
to an overcurrent, -Q3, as a backup,
trips after the time-discriminating
response time of 150 ms.
Short-circuit at 2:
-Q1 and -Q3 establish that a short-
circuit has occurred; -Q5 does not.
For this reason, -Q3 does not
receive a blocking signal from -Q5,
but provides a blocking signal for
-Q1. This information tells -Q3 that
it is closest to the short-circuit and
trips with a delay of t
= 50 ms
S
instead of t
= 150 ms. Time
sd
saved = 100 ms.
2/30
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