Description
2.8 Electronic trip unit ETU
3VA27 molded case circuit breakers & 3WL10 air circuit breakers
Manual, 10/2018, L1V30499596002-01
49
Switching devices connected in series, e.g. circuit breakers and fuses, can work together in
a selective or discriminating manner to ensure graded tripping in the event of a fault. The
upstream switching device closest to the fault must trip in the event of a short-circuit. The
other switching devices in the circuit remain closed. This minimizes the effects of a fault in
terms of its duration (selectivity) and the area affected.
Short-circuit location
Selectivity tables for the 3WL10 are available on the Internet for simple implementation of
selective trip combinations (see Selectivity tables
(https://support.industry.siemens.com/cs/ww/en/view/109748621
)).
The parameters relevant to selectivity are:
● Tripping value settings of the trip unit
● Tripping and break times
● Let-through current values
● Modes of switching of the relevant circuit breakers
– Zero-current interrupter (3WL10)
– Current limiter (3VA27)
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