Methods of implementing selectivity using circuit breakers
3.4 Time selectivity
Selectivity for 3VA molded case circuit breakers
Configuration Manual, 08/2016, A5E03603181010-01
27
3.4.2 Tripping characteristics of circuit breakers
Figure 3-7 Current-time diagram: Tripping characteristics in time selectivity applications
In order to achieve successful time selectivity, the characteristics of the two circuit breakers
must meet the following criteria:
● No contacts between the characteristics including tolerance ranges until the operating
current of the I release at the upstream 250 A circuit breaker is reached.
For the purpose of the selectivity evaluation, the characteristics of the upstream circuit
breaker must be in the top, right-hand area (blue characteristic) and those of the
downstream circuit breaker in the bottom, left-hand area (green characteristic).
As a result of the changes to the parameters of the I release or S release, a significantly
higher degree of selectivity is achieved as compared to pure overload zone selectivity
(see "Selectivity in the overload zone (Page 15)"), but selectivity in the ove
rload zone is also
assured.
The selectivity limit current (I
s
) has been raised to 5000 A. Selectivity would still be only
partial if the short-circuit current were to rise above 5000 A.
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