MCB Discrimination and Selectivity Explained
What is discrimination between MCBs? Discrimination (selectivity) is the coordination of two circuit breakers in series so that only the one closest to a fault opens, per the discrimination principles in IEC 60947-2, while every upstream device stays closed and unaffected circuits stay energized. Get it wrong and a fault on one final circuit drops the whole distribution board. This article covers current discrimination and the rating-ratio rule between MCB tiers, the fault-current point where MCB-to-MCB selectivity stops working, why an upstream MCCB or fuse restores it, and how to read a manufacturer's discrimination table instead of guessing at a ratio.
What Discrimination Means When MCBs Are in Series
Take a simple feed: a main incomer, a distribution board main breaker, and a final-circuit MCB downstream of it. A fault on the final circuit should trip only the final-circuit MCB. The main breaker should never see it. That outcome is total discrimination — full protection across the whole prospective fault-current range at that point. Partial discrimination means the pair only coordinates correctly up to some lower current; above it, both devices can open. Neither is automatic. It has to be engineered into the pairing, and for MCBs that means picking ratings and curves specifically, not just stacking breakers by convenience.
Current Discrimination: the Rating-Ratio Method
MCBs don't have adjustable settings. No short-time delay, no field-selectable pickup — the thermal-magnetic curve is fixed at manufacture. That leaves current discrimination as the only lever available between two MCBs: separating them by rated current and curve so the upstream device's magnetic trip threshold sits well above the downstream device's total let-through at fault level. The common field rule of thumb is a rating ratio of roughly 1.6:1 or higher between adjacent devices on the same curve — a 32A C-curve downstream of a 50A C-curve, for instance, rather than a 40A. It's a starting heuristic, not a guarantee. The number that actually matters is the discrimination limit published for that specific pair.
Formula: Current Discrimination Limit — Source: manufacturer discrimination tables, coordination principles per IEC 60947-2
Selectivity holds only while If ≤ Is
| Symbol | Description | Unit |
|---|---|---|
| If | Prospective fault current at the point of the downstream MCB | A (or kA) |
| Is | Discrimination limit — max fault current up to which the downstream device alone clears the fault | A (or kA) |
| Im,us | Instantaneous magnetic trip threshold of the upstream device | A |
| In,us / In,ds | Rated current, upstream / downstream device | A |
Is is not something you calculate from the ratio alone. Curve shape, tolerance band, and the specific pair's breaking capacity all feed into it, which is why manufacturers publish it as a table rather than a formula you can run yourself.
Where MCB-to-MCB Discrimination Fails
Above Is, both breakers can trip. The mechanism is straightforward: past a certain fault current, the magnetic element in both devices picks up almost simultaneously — within a few milliseconds of each other — and there's no delay built into an MCB to let the downstream one clear first. The upstream breaker doesn't wait around to see if its neighbor handles it. It can't; it has no timer to wait with.
Curve mismatch makes this worse. Two C-curve MCBs of similar rating have overlapping magnetic trip bands (5-10x In for both), so a rating ratio that looks fine on paper can still fail at high fault levels. A B-curve downstream of a C-curve upstream, or a bigger rating step, buys more separation — but even then, discrimination is only guaranteed up to the published Is, not the switchboard's full prospective fault current.
What we see in the field: installers often assume any two MCBs of different amperage discriminate against each other. They don't, automatically. On a board close to the transformer with prospective fault current in the several-kA range, a 1.6:1 ratio between two 6 kA MCBs can still cross-trip well below that fault level if neither device's discrimination limit reaches it.
Time Discrimination Doesn't Apply to Standard MCBs
True time discrimination relies on a short-time delay setting: the upstream device holds off tripping for a set number of milliseconds so the downstream device gets first chance to clear the fault. That function exists on MCCBs and air circuit breakers with adjustable trip units, not on a fixed thermal-magnetic MCB. An MCB's only behavior at fault level is instantaneous. So when two MCBs are in series, the discrimination available is current-based only, bounded by the curves and ratings involved, and typically limited to a modest kA figure well short of what a board near a large transformer can deliver.
Using an Upstream MCCB or Fuse to Restore Discrimination
When an MCB-to-MCB pair can't discriminate across the board's full fault-current range, moving to a different device type upstream usually closes the gap. An MCCB with an adjustable short-time delay or a settable instantaneous pickup gives you a real margin to place above the MCB's response, rather than relying on curve separation alone. A gG fuse works differently but gets a similar result: its melting time-current characteristic is inherently slower at moderate overcurrents and gets progressively faster at higher currents, and sized correctly it lets the downstream MCB clear first across a wider current band than a same-tier MCB would.
| Criteria | Upstream MCB (same tier) | Upstream MCCB (adjustable trip) | Upstream gG Fuse |
|---|---|---|---|
| Discrimination margin at high fault current | Limited — bounded by fixed curve overlap | Wide — settable short-time delay or pickup | Wide — time-current curve separates naturally |
| Adjustability | None (fixed at manufacture) | Field-adjustable trip unit | None (fixed by fuse rating/type) |
| Typical use | Small final-circuit boards, low fault levels | Main incomer, sub-main distribution | Sub-main or transformer-side protection |
| Where it falls short | Cross-trips above Is on high faults | Higher cost, more complex setting selection | No overload/thermal role beyond fuse rating; slower on low overcurrents |
Reading a Manufacturer's Discrimination Table
Manufacturer discrimination tables list the upstream device (rating and curve) down one axis and the downstream device across the other, with the cell giving either a kA value (partial discrimination up to that current) or "Total" (full discrimination across the device's whole breaking capacity). Three things to check before trusting a pair: first, that the table entry actually covers your two specific models and curves, not a generic same-family assumption. Second, that the published Is meets or exceeds your board's calculated prospective fault current — pull that figure from the site's fault-level study, not an estimate. Third, whether the rating is marked "Total" or a specific kA; a partial value below your prospective fault current means the upstream device will open too, above that point.
Discrimination vs Cascading — Different Problems, Different Fixes
These two get conflated constantly, and they solve different problems. Discrimination is about which breaker opens. Cascading (back-up protection) is about whether the downstream breaker survives at all — it lets a downstream device with a breaking capacity lower than the prospective fault current still interrupt safely, because the upstream device limits the let-through energy during the first few milliseconds of the fault. A system can have cascading without discrimination: both breakers might open together, and the downstream one still survives because the upstream device capped the energy it saw. It can also have discrimination without needing cascading at all, if every device's own breaking capacity already covers the local prospective fault current. The mechanics, the sizing rules, and the manufacturer test data behind cascading are covered in depth in the MCB cascading and back-up protection article — worth reading alongside this one if your board has a device with less breaking capacity than the incoming fault level demands.
Putting It Together on a Real Board
Start with the fault-level study for the board in question — the prospective fault current at the main and at each final-circuit point. Compare that to the discrimination table for the specific upstream/downstream pair you're proposing, not a generic rating-ratio guess. If the published Is falls short of the site's fault current, either step up the rating gap further within the same MCB family, or move the coordination problem upstream to an MCCB with a short-time delay or a correctly sized gG fuse. None of this replaces checking each device's own MCB breaking capacity against the fault current it individually sees, and it works alongside curve selection covered in the MCB tripping curves guide. For the underlying framework both IEC 60898-1 and IEC 60947-2 apply to a coordination study, see the IEC standards comparison, and where an MCCB is the better upstream choice, the MCB vs MCCB article covers that trade-off directly. Stoklink stocks the Schneider Acti9 iC60, ABB S200, and Siemens 5SY families referenced in the tables above across the miniature circuit breakers collection, and the full coordination picture — sizing, curves, standards, and selectivity together — sits in the MCB engineering guide.
Frequently Asked Questions
Do two MCBs of different amperage always discriminate against each other?
No. A rating ratio around 1.6:1 or higher is a common starting point, but it doesn't guarantee discrimination at every fault current. The actual limit is the published Is value for that specific pair, and above it both devices can trip together.
What's the difference between discrimination and cascading?
Discrimination determines which breaker in a series opens on a fault, ideally only the one closest to it. Cascading lets a downstream device with lower breaking capacity than the prospective fault current still interrupt safely, because the upstream device limits let-through energy. A board can need either, both, or neither.
Can two MCBs on the same curve ever give full discrimination?
Only up to a limited current, and usually a modest one, because their magnetic trip bands overlap across the same 5-10x In range for C-curve, for example. Full discrimination across a board's whole fault-current range usually needs an upstream device of a different type.
Why can't MCBs use time discrimination like MCCBs do?
Time discrimination needs an adjustable short-time delay in the trip unit, letting the downstream device clear first. Standard IEC 60898-1 MCBs have a fixed thermal-magnetic response with no such delay, so the only discrimination method available between two MCBs is current-based.
Where do I find the actual discrimination limit for two specific breakers?
In the manufacturer's discrimination (selectivity) table for that product family, indexed by upstream and downstream rating and curve. Match your exact models — don't extrapolate from a different family or a different curve pairing.
Does an upstream MCCB always fix a discrimination gap?
It closes the gap in most cases because its short-time delay or adjustable pickup gives a real margin to set above the downstream MCB's response. It still has to be sized and set correctly against the actual fault-level study, not assumed.
Conclusion
Discrimination between MCBs is current-based, bounded by a published Is value, and it stops working above that point — both devices can open on the same fault. A rating-ratio rule of thumb gets you in the right range; the manufacturer's discrimination table tells you the real limit. When that limit falls short of the board's prospective fault current, the fix sits upstream: an MCCB with an adjustable trip, or a correctly sized gG fuse, not a bigger MCB from the same family.
