Selectivity for 3VA molded case circuit breakers
Configuration Manual, 08/2016, A5E03603181010-01
47
Selectivity implemented by combinations of
protective devices
4
4.1 Molded case circuit breakers and downstream fuse
Note
Tolerance ranges for fuses
If the selectivity properties of a protective device combination comprising circuit breaker and
fuse, it is clear that a permissible tolerance range of ±10 % must be applied to the current-
time tripping characteristics of the circuit breakers.
The tolerance range is reduced to ±6 % if Siemens type 3NA LV HRC fuses are used.
Circuit breakers with LI or LSI releases behave selectively in the overload zone with respect
to a downstream fuse if a safety margin t
A
of around 1 s is provided between the top edge of
the tolerance band of the time-current characteristics and the lower tripping characteristic of
the L release.
The times specified are empirical values and also take into account the data for the products
of other manufacturers.
Circuit breaker without an S release
① Dynamic selectivity zone
② Tripping characteristic
③ Time-current characteristic
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