Breaker Selectivity Margin
Screen upstream and downstream breakers for selectivity limit, margin and verdict.
Step-by-step
- Enter valid values to begin.
Simplified screening check only. The downstream maximum instantaneous trip is taken at the top of its band (B 5, C 10, D 20, MCCB 10 times In). For a fuse upstream the pickup field is read as the selectivity limit multiple of In. A 50 ms allowance covers breaker opening and arc time. Rules of thumb do not replace the manufacturer’s selectivity tables, and cascading (back-up) protection is not selectivity.
Breaker Selectivity: Making Only the Nearest Device Trip
Core Engineering Principles
Selectivity, also called discrimination, means a fault is cleared by the protective device closest to it and nothing upstream moves. When a 16 A lighting circuit shorts, the 250 A feeder breaker should stay closed, so the rest of the board keeps its power. Don’t confuse it with back-up or cascading, where a current-limiting upstream device lets a lower-rated downstream breaker survive a fault. In a cascaded pair both usually open, so you save money and get no continuity.
We get selectivity three ways. Current discrimination relies on the upstream instantaneous pickup sitting above the highest fault the downstream device can see. Time discrimination gives the upstream trip unit a short-time delay, so the downstream device clears first, provided the upstream breaker withstands the fault for the delay (its Icw). Energy discrimination uses the let-through energy of the downstream breaker, which is what manufacturer tables are built on. Zone-selective interlocking refines time discrimination: the downstream device signals upstream to hold, so a fault at the upstream breaker itself still clears fast.
Time check: tdn + 0.05 s ≤ tsd • Margin = Is / Ik (selective if ≥ 1)
NEC & Standard References
IEC 60947-2 defines the trip characteristics of circuit breakers, and its annex A covers coordination between breakers in series, including selectivity and back-up. IEC 60364-5-53 covers coordination of protective devices. In the US, NEC 700.32 requires selective coordination for emergency systems, 701.32 for legally required standby, and 708.54 for critical operations power systems. Verify clause numbers in the adopted edition. Rules of thumb never replace the manufacturer’s tested selectivity tables.1. Rating ratio = 250 / 63 = 3.97, above 1.6.
2. Downstream maximum instantaneous trip = 10 × 63 = 630 A.
3. Is = 10 × 250 = 2500 A, above 630 A, so some selectivity exists.
4. Margin = 2500 / 4000 = 0.63. Below 1, so selectivity is partial: reliable only up to 2.5 kA, and at 4 kA both breakers may trip.
5. Fix: use a trip unit with short-time delay. A delay of 0.1 s passes the check, since 0.01 + 0.05 = 0.06 s, a time margin of 1.67. The instantaneous must then rise to 16 × In (4000 A) or be switched off, and the breaker’s Icw must cover 4 kA.
- Overlapping curves are not selectivity. Trip bands are wide; above the instantaneous point only tested tables count.
- Let-through energy matters. Both breakers can open together if the downstream device does not limit I²t enough.
- Instantaneous override. Many trip units have a fixed override above Icw that ignores your delay.
- Mixing manufacturers. Tables cover one maker’s tested pairs; other brands usually have no published value.
- Field changes. Someone dials a trip unit down to stop nuisance trips and the study is void. Record settings and re-check.