WL Circuit Breakers - Siemens

External CubicleBUS Modules
WL Circuit Breaker
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 ZSI = 50 ms, 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
Graphic
2-7
WL PROFIBUS/MODBUS Communication and Electronic Accessories
short-circuit also occurred in the next level
below:
• 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 ZSI = 50 ms
has elapsed.
Example as Illustrated in
Graphic 2-7
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.
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 tS = 50 ms instead of
tsd = 150 ms. Time saved = 100 ms.
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.
SECTION 2
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