Selectivity with parallel incoming feeders
5.4 Sources of information and other documentation
Selectivity for 3VA molded case circuit breakers
68 Configuration Manual, 08/2016, A5E03603181010-01
The transformer and bus-couplers in the multiple incoming feeder configurations of industrial
networks are normally connected in series with other over-current protective devices. As a
result, the inclusion of bus-couplers in the selective time grading system may unduly extend
the clearance time for near-to-infeed short-circuits.
Since a user may have reservations about implementing zone selecting interlocking (ZSI) for
reasons of cost, the alternative solution is to dispense with time grading of the S releases of
the transformer and bus-couplers.
Apart from setting identical time delays t
sd
, it is also usual practice to completely eliminate
over-current releases from the bus-couplers. In this case, selectivity is afforded as of the
range 12 kA and 100 ms - 200 ms if tolerances of +10 % for the circuit breakers are taken
into account.
5.4 Sources of information and other documentation
Hartmut Kiank, Wolfgang Fruth: Planning Guide for Power Distribution Plants,
Publicis Publishing
ISBN 978-3-89578-359-3
available from Publicis Publishing, www.publicis.de/books
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