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Breaker selectivity

What selectivity is and how to achieve it: current and time selectivity, choosing upstream and downstream breakers.

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What is selectivity

Selectivity (discrimination) is the ability of protection to isolate only the faulty part. The breaker nearest to the fault must trip.

Current selectivity

Achieved by choosing breakers with different ratings: the upstream breaker must be at least 1.6× the downstream breaker (by rated current).

Downstream breakerUpstream breaker (min)
10 A16 A
16 A25 A
25 A40 A
32 A50 A

Time selectivity

Achieved by a delay on the upstream breaker (selective breakers with adjustable time delay), used in distribution and at intakes.

1.6 rule: the upstream/downstream ratio must be at least 1.6 for current selectivity.

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Data is for reference. Final design decisions must follow current regulations and a qualified engineer.