Description
2.8 Electronic trip unit ETU
3VA27 molded case circuit breakers & 3WL10 air circuit breakers
50
Manual, 10/2018, L1V30499596002-01
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.
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 leads to values that are too low.
The reason for this is that the trip unit behaves differently in the case of short-circuit
currents compared to 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 (strong characteristic 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 closest to the fault location 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 Reference documents
(Page 10)).