Cascading And Discrimination Selectivity In Circuit Breakers How To Prevent Total Switchboard Shutdowns

Published by the Korlen Engineering Team · System Coordination · 9 min read

Category: Circuit Breakers

What Discrimination (Selectivity) Actually Means

Discrimination — also called selectivity — is the ability of a protection system to isolate a fault at the precise point where it occurs, and nowhere else. If a motor branch shorts out, only that branch’s breaker should trip. The feeder breaker above it, and the main incomer, must stay closed so the other 47 circuits keep running.

When selectivity is absent, the fault current is high enough to operate both the branch and the upstream breaker — and the upstream one wins the race to clear it, blacking out everything downstream. In a hospital operating theatre or a server hall, that “upstream trip” is a catastrophe, not a nuisance.

Why Total Switchboard Shutdowns Happen

Three typical root causes:

• No rating grading. Branch and feeder breakers share the same frame size and curve, so both see the fault as “their” trip.
• No time delay upstream. The feeder has no selectivity delay, so it reacts as fast as the branch.
• Undeclared Icu mismatch. The upstream breaker’s breaking capacity is exceeded and it trips to disconnect rather than clear.

Current Grading vs Time Grading

Selectivity is achieved two ways, often combined:

• Current (amplitude) selectivity: each downstream breaker has a clearly lower rating than the one above, so only the nearest device senses “its” fault level. Simple, but needs wide rating steps.
• Time selectivity: the upstream breaker adds an intentional time delay (e.g., a selectivity-rated MCCB or ACB with a delay trip), letting the branch trip first. Enables closer rating steps and full coordination.

Full vs partial selectivity: “full” means selectivity holds for every fault current up to the upstream Icu; “partial” holds only up to a defined lower current. Partial selectivity is common and cost-effective where the maximum likely fault is bounded.

Cascading: Current-Limiting Coordination

Cascading (sometimes called “discrimination by energy limitation”) is a smarter trick. A current-limiting upstream MCCB clips the fault current so drastically that the downstream MCB — even if it would normally see a destructive let-through — only experiences a small, safe current. The downstream device is thus “backed up” by the upstream limiter and need not be sized for the full short-circuit level.

The payoff: you can use smaller, cheaper MCBs downstream while still meeting the board’s overall breaking requirement, provided the manufacturer’s cascade table is respected. Korlen publishes cascade coordination between its MCCBs and MCBs so panel builders can verify the combination.

Selectivity Selection Quick Table

Korlen Range for Coordinated Boards

Korlen’s MCCB and ACB families are built for exactly this hierarchy — current-limiting MCCBs up top, selective grading through the feeder, compact MCBs at the branch.

Korlen KNM3 — high-breaking MCCB for feeder / selectivity layer

Korlen KNW1 — universal intelligent ACB for main incomer coordination

FAQ

What is the difference between selectivity and cascading?

Selectivity keeps the upstream breaker closed by grading; cascading uses an upstream current-limiting breaker to slash the fault energy the downstream device sees. They are often combined in one board.

Is partial selectivity acceptable?

Yes, for many commercial boards, provided the maximum credible fault stays within the partial-selectivity current. Full selectivity is reserved for critical loads (hospitals, data centers).

How do I verify a cascade combination?

Use the manufacturer’s published cascade/coordination table — never assume. Korlen provides MCCB–MCB coordination data for its range.

Does an ACB help with selectivity?

Yes. An ACB like Korlen’s KNW1 at the incomer adds precise time grading and high Icu, making full selectivity down to the feeders practical. See KNW1.

Why did my whole board trip on one fault?

Almost always a missing rating or time grade between branch and feeder. Re-grade the feeder with a selectivity delay or a clearly higher frame size.

Conclusion

Selectivity is what turns a fault from “one circuit down” into “one circuit down, everything else fine.” Design the grade deliberately — current, time, or cascade — and verify it against the manufacturer’s coordination tables before the board is built.

Related Resources

• Korlen MCCB / ACB coordination catalog
• Korlen KNM3 / KNW1 selectivity models

© Korlen Electric. For B2B specification reference. Always verify selectivity against the latest manufacturer coordination tables and local standards.