Selectivity implemented by combinations of protective devices
4.2 Fuse and downstream molded case circuit breaker
Selectivity for 3VA molded case circuit breakers
Configuration Manual, 08/2016, A5E03603181010-01
51
Example
The selectivity limit current for selectivity implemented between a circuit breaker and an
LV HRC fuse approximately corresponds to the value at which a safety margin t
A
of not less
than around 70 ms is maintained between the lower tolerance band of the time-current
characteristic and the upper tolerance value of the operating time of the instantaneous over-
current release (I release), or the delay time of the short-time delay over-current release
(S release).
① Dynamic selectivity zone
② Tripping characteristic
③ Time-current characteristic
The diagram above indicates that the selectivity limit current (I
s
) is approximately 2200 A.
According to the selectivity table, the selectivity limit value (I
s
) is 7.9 kA (see "FAQs
(https://support.industry.siemens.
com/cs/de/de/view/97493202) chapter 7" in the Siemens
Industry Online Support (SIOS)).
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