Siemens SENTRON 3WL10 Series Manual page 36

Description
2.4 Electronic trip unit ETU
Selectivity with 3WL10 air circuit breakers
The 3WL10 circuit breaker is designed for selective (discriminating) tripping behavior in
conjunction with both high breaking capacity and high current carrying capacity.
Selectivity tables are available on the Internet for simple implementation of selective trip
combinations (see 3WL10 communication guide
(https://support.industry.siemens.com/cs/WW/en/view/109750691)).
● Tripping value settings of the trip unit
● Tripping and break times
The selective behavior of circuit breakers can be implemented technically by a variety of
selectivity concepts:
● Current selectivity
The selectivity can be calculated in the overload range by comparing the time/current
characteristics. In the short-circuit range, this comparison results in values that are too
low. The reason for this is that the trip unit behaves differently in the case of short-circuit
currents compared with its long-term behavior, e.g. in the case of overload.
If the short-circuit currents differ sufficiently at the points where two circuit breakers are
mounted, the instantaneous short-circuit releases can normally be set such that if a short-
circuit occurs behind one of the circuit breakers as seen from the other circuit breaker,
only the circuit breaker nearest to the fault will trip.
If the short-circuit currents are approximately the same at the points where the circuit
breakers are mounted, the time grading of the operating currents of the short-circuit
releases only enables selectivity up to a specific short-circuit current.
● Time selectivity (characteristics of the 3WL10 air circuit breaker)
Up to the operating values of the instantaneous short-circuit releases, selectivity can be
achieved by time selectivity. To achieve this, the upstream circuit breaker requires a time
delay provided by a time-delayed short-circuit release so that, in the event of a fault, only
the circuit breaker nearest to the fault has time to disconnect the part of the installation
with the fault from the power system.
Both the tripping delays and the tripping currents of the short-circuit releases are graded.
You will find more information in the manual on selectivity (see Product-specific information
(Page 7)).
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Air circuit breaker 3WL10
Manual, 11/2017, L1V30499596002-01