Methods of implementing selectivity using circuit breakers
3.3 Current selectivity
Selectivity for 3VA molded case circuit breakers
18 Configuration Manual, 08/2016, A5E03603181010-01
3.2.3 Tripping characteristics of circuit breakers
Figure 3-3 Current-time diagram: tripping characteristics of 100 A and 250 A circuit breakers with
selectivity in the overload zone
In order to achieve successful selectivity in the overload zone, the overload tripping
characteristics of the two circuit breakers must meet the following criteria:
● No contact or overlapping in the overload zone, including tolerance ranges.
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).
● There is a sufficient distance between all tolerance ranges in branched power distribution
boards.
The distances are dependent on the overload protection setting values of the circuit
breakers in the other branches.
The tripping characteristics fulfill these criteria in the overload zone. Selectivity is thus
assured in this zone.
The selectivity limit current (I
s
) equals 1000 A. When the current reaches this limit, both
circuit breakers trip simultaneously. As a result, the selectivity in the overload zone is only
partial.
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