MCCB Discrimination and Selectivity Coordination
What is discrimination (selectivity) in an MCCB protection scheme? Discrimination is the coordination of two or more series-connected circuit breakers so that only the breaker closest to a fault opens, per IEC 60947-2 Annex A, while every upstream breaker stays closed and keeps supplying the rest of the installation. Get it wrong and a fault on one feeder blacks out an entire switchboard instead of one circuit. This article covers current discrimination (separating Ir and Isd bands), time discrimination (staggering the Isd delay), total versus partial discrimination up to a stated limit current, zone-selective interlocking and energy discrimination, and how to read a manufacturer's discrimination table without over-trusting it.
Why Discrimination Is Not Automatic
Two MCCBs in series do not discriminate just because one has a higher current rating. An XT4 250 A set at Ir = 200 A sitting above an XT2 160 A set at Ir = 140 A will still both see the same fault current on a downstream short circuit, and if both trip curves overlap in the instantaneous region, both breakers open. Discrimination has to be engineered into the trip settings, not assumed from frame size.
What we see in the field: panel builders often assume a bigger upstream breaker is automatically selective with a smaller downstream one. It works for overload discrimination almost every time, because Ir bands are easy to separate. It fails for short-circuit discrimination far more often, because instantaneous elements and thermal-magnetic curves converge at high fault current unless someone deliberately checks the table.
Current Discrimination: Separating Ir and Isd
Current discrimination relies purely on setting gaps, no added time delay. The upstream Ir (long-time pickup) is set well above the downstream Ir so overload discrimination is trivial across almost the full range. The harder part is Isd (short-time pickup): if the upstream Isd threshold is set high enough that it never picks up for any fault current the downstream breaker can clear on its own instantaneous trip, the two devices discriminate without any time delay at all.
This only works up to a limit, because at very high fault current the downstream breaker's own let-through current can exceed the upstream Isd pickup, and both breakers see "instantaneous" conditions simultaneously. That ceiling is the discrimination limit current, and it is always lower than the downstream breaker's Icu.
Time Discrimination: Staggering the Isd Delay
Time discrimination adds a deliberate short-time delay (tsd) to the upstream breaker's Isd stage so it waits before tripping, giving the downstream breaker a window to clear the fault first. This needs an electronic trip unit with an adjustable short-time delay — Ekip on ABB Tmax XT2/XT4 and up, Micrologic 5/6/7 on Schneider ComPact NSX, ETU on Siemens 3VA2 — because fixed thermal-magnetic units (TMD, TM-D, 3VA1 TM) cannot be graded this way.
Formula: Time-Grading Margin for Discrimination — Source: IEC 60947-2 Annex A, standard grading practice
tup ≥ tdown + Δt
| Symbol | Description | Unit |
|---|---|---|
| tup | Upstream breaker short-time delay (Isd) setting | s |
| tdown | Downstream breaker total clearing time, including arcing time, at the same fault current | s |
| Δt | Grading margin — typically 100 to 150 ms for static/electronic trip units, wider for older electromechanical relays | s |
Every step of upstream delay leaves the upstream breaker carrying full fault current for longer, which raises its own I²t stress and thermal duty. That is the real cost of time discrimination: it buys selectivity at the price of extra let-through energy on the device that stays closed, so it is checked against the downstream breaker's short-time withstand, not just its trip curve. This is also why time discrimination is used sparingly on MCCB pairs — most MCCBs carry a limited or no genuine Icw rating, unlike ACBs, so the delay has to stay short.
Total vs Partial Discrimination Up to a Limit Current
Manufacturer tables classify a breaker pair one of two ways. Total discrimination means the downstream breaker clears every fault up to its own full breaking capacity (Icu) without the upstream breaker ever opening. Partial discrimination means selectivity holds only up to a stated limit current (Is), below the downstream breaker's Icu — above Is, both breakers trip together.
Total discrimination across the entire prospective short-circuit current is the exception, not the rule, for MCCB-to-MCCB pairs — it usually needs a real frame-size and breaking-class gap (an XT7 above an XT2, for instance), or an ACB upstream instead of an MCCB. Most published MCCB-MCCB tables state partial discrimination values in kA, and that number moves with every combination of frame, trip unit setting, and voltage. A pair that discriminates to 40 kA at one Isd setting can drop to 15 kA if the downstream Ir/Isd is changed later without rechecking the table.
Zone-Selective Interlocking and Energy Discrimination
Zone-selective interlocking (ZSI) is a wired signal between trip units: when a downstream breaker's short-time or ground-fault element picks up, it sends a restraining signal upstream telling the upstream unit "I am handling this fault, do not trip instantaneously." If no restraining signal arrives, the upstream unit assumes the fault is in its own zone and trips without delay. ZSI is available on higher-end electronic trip units — Ekip Touch/Hi-Touch, Micrologic 6E/7E-class units with comms, ETU with ZSI wiring — and it gets close to total discrimination with far less added delay than a blanket time-grading scheme, because the upstream unit only waits when it actually needs to.
Energy discrimination is a related but distinct manufacturer concept for current-limiting MCCBs at high fault current: instead of coordinating trip times, it relies on the downstream breaker's current-limiting action cutting the let-through current and I²t so fast and so low that the upstream breaker's instantaneous element never reaches its own pickup threshold in the first place. It only holds within the specific current-limiting range the manufacturer tested and published — it is not a general substitute for a discrimination table.
Reading a Manufacturer Discrimination Table
A discrimination (selectivity) table lists breaker pairs — upstream model/rating/setting against downstream model/rating/setting — and gives either "Total" or a kA figure for partial discrimination at a stated voltage, usually 400 V. Three things trip people up when reading one.
The value is setting-specific
The table entry assumes the exact Ir/Isd (and tsd, if time-graded) combination printed next to it. Change either breaker's setting after commissioning and the published value no longer applies without rechecking.
Voltage and frequency matter
Tables are normally built at 400 V/415 V, 50 Hz. Higher system voltage or different frequency is not automatically covered — some publishers include separate 690 V columns, some do not.
"Total" has a ceiling too
"Total" in the table means up to the downstream breaker's Icu at that voltage, not an unlimited value. If the installation's prospective short-circuit current exceeds the downstream Icu, discrimination is irrelevant — the downstream breaker itself is under-rated for the job, a separate check entirely from selectivity.
Discrimination vs Cascading: Not the Same Thing
Discrimination and cascading solve different problems and get confused constantly. Discrimination is about continuity of supply: keep the upstream breaker closed so unfaulted circuits stay energized. Cascading (back-up protection) is about breaking capacity: let a lower-Icu downstream breaker rely on a higher-Icu upstream breaker's current-limiting action to survive a fault that exceeds its own standalone rating, with the tradeoff that the upstream breaker also trips.
Some engineers treat a cascaded pair as if it were also selective, because both are published as tested combinations in manufacturer literature. In practice the two ratings are usually mutually exclusive on the same pair at the same fault level — a combination optimized for cascading typically trips both breakers, which is the opposite of what discrimination is trying to achieve. For the full mechanics of current-limiting back-up protection, see our article on MCCB cascading and back-up protection.
Frequently Asked Questions
Is discrimination the same as selectivity?
Yes, the terms are used interchangeably in IEC 60947-2 documentation and manufacturer literature. "Selectivity" is more common in North American and some European catalogs, "discrimination" in IEC/British-derived usage; both describe the same coordination goal.
Can two MCCBs from different brands be selective with each other?
Technically yes, if their published trip curves and settings are compatible, but almost no manufacturer publishes mixed-brand discrimination tables. In practice this means testing the combination yourself or treating the pair as unverified, which most panel builders avoid on a production job.
Do thermal-magnetic MCCBs support time discrimination?
No. TMD, TM-D, and 3VA1 TM units have a fixed instantaneous trip with no adjustable short-time delay, so only current discrimination (setting gaps) is possible with them, not time grading or ZSI.
Why does adding time delay to the upstream breaker matter for its rating?
Every millisecond of added delay increases the I²t energy the upstream breaker's contacts and busbar connections absorb during the fault, so the delay has to stay within the breaker's short-time withstand — this depends on duty cycle and how often the installation actually sees faults near that current level, which is a judgment call, not a fixed number.
What does a "0.5 tsd" setting mean on an electronic trip unit?
It is a fraction of the maximum available short-time delay step on that trip unit, not an absolute time in seconds — the actual delay in milliseconds is read from the manufacturer's setting table for that specific unit and Isd combination.
Conclusion
Discrimination sits alongside — and depends on — the trip settings and breaking-capacity work covered elsewhere. Ir/Isd/Ii staging is set per our guide on how to set MCCB trip settings before any discrimination check is meaningful, and the let-through energy tradeoff behind time discrimination and energy discrimination is explained in depth in our article on MCCB current limiting and let-through energy. Icu/Ics selection, which caps what "total" discrimination can ever mean for a given breaker, is covered in our Icu vs Ics vs Icw ratings guide. For the underlying frame and trip-unit families referenced throughout — Tmax XT, ComPact NSX, Sentron 3VA — see the molded case circuit breakers collection and the MCCB engineering guide.
Discrimination between MCCBs is a settings problem first and a table-lookup problem second. Current discrimination costs nothing but a wider Ir/Isd gap and holds only to a limit current; time discrimination and ZSI buy a higher limit at the cost of extra let-through energy on the breaker that stays closed. Total discrimination across the full fault range is uncommon between two MCCBs and usually needs a frame-class jump or an ACB upstream. Whatever combination gets specified, the discrimination table entry is only valid for the exact settings, voltage, and fault-current range it was published for — and it answers a different question from cascading, which trades breaking capacity, not continuity of supply.
