Icu vs Ics vs Icw Ratings for MCCBs Explained
What is the difference between Icu, Ics, and Icw on an MCCB? Icu (rated ultimate short-circuit breaking capacity) and Ics (rated service short-circuit breaking capacity) are both defined in IEC 60947-2, §8.3, but they answer different questions: Icu only certifies that the breaker opens safely once and stays safe to touch, while Ics certifies it can carry rated current again afterward. Confusing the two means a panel spec can pass on paper with a breaker that survives exactly one fault and then needs replacing — expensive on a distribution board you cannot de-energize for long. This article covers the IEC test sequences behind Icu and Ics, how Ics is expressed as a percentage of Icu across ABB, Schneider, and Siemens catalogs, why Icw (short-time withstand) barely exists on MCCB datasheets, and how to read the three numbers together when specifying a breaker.
Icu: The Ultimate Breaking Capacity
Icu is the number printed largest on the nameplate, and the one most engineers quote when they say "this breaker is rated for 65 kA." It is tested with an O-t-CO sequence: open under fault current, wait a specified interval, then close-and-open again under the same fault current. After the sequence, IEC 60947-2 requires the breaker to be electrically safe and to pass a dielectric withstand test. It does not require the breaker to still be fit for normal service.
What we see in the field: procurement teams sometimes size a whole board on Icu alone because it is the bigger, easier-to-find number. That works if the fault is a one-off event followed by a full breaker swap. It does not work if the design assumption is "trip, inspect, re-close, keep running."
Ics: The Service Breaking Capacity
Ics uses a longer test: O-t-CO-t-CO, three operating sequences instead of one. After the third operation, the breaker must pass a temperature-rise check at rated current and complete an overload trip test. In practice, that means Ics is the number that tells you whether the breaker is still doing its job after the fault, not just whether it failed safely.
IEC 60947-2 lets the manufacturer declare Ics as a fixed fraction of Icu: 25%, 50%, 75%, or 100%. Nothing in the standard forces a manufacturer toward the higher classes — it is a design and cost decision, and it shows up directly in the catalog.
How Ics Is Expressed as a Percentage of Icu Across Brands
The relationship is arithmetic, not mysterious.
Formula: Service Breaking Capacity from Percentage Class — Source: IEC 60947-2, §8.3
Ics = (k / 100) × Icu
| Symbol | Description | Unit |
|---|---|---|
| Ics | Rated service short-circuit breaking capacity | kA |
| Icu | Rated ultimate short-circuit breaking capacity | kA |
| k | Manufacturer-declared percentage class (25, 50, 75, or 100) | % |
Where the brands split: ABB and Siemens frequently declare Ics = 100% Icu on their higher breaking-capacity classes, so a breaker rated at, say, the higher end of the Tmax XT or Sentron 3VA2 range keeps its full rated capacity after the standard test sequence. Some Schneider ComPact NSX classes, by contrast, declare Ics at 50-75% of Icu rather than 100%. That is not automatically worse — a lower-Ics breaker still clears the actual fault it was sized for — but the margin for a second event within the same fault clearance is smaller.
This is where catalog comparisons go wrong. A breaker with a lower headline Icu but Ics = 100% can out-perform a higher-Icu breaker at 50% Ics once you look past the first fault. Check the percentage class before comparing raw kA figures across our molded case circuit breakers range, particularly when a customer is replacing one brand with another on an existing board.
Icw: Short-Time Withstand Is Mostly an ACB Rating
Icw (rated short-time withstand current) measures something different again: how much fault current a breaker can carry, without opening, for a defined time — typically 0.5 s, 1 s, or 3 s. It exists to support selectivity schemes where an upstream breaker deliberately waits for a downstream breaker to clear the fault first.
ACBs are built for this by default — a full withdrawable frame with generous thermal mass and short-time-delay (S) settings on the trip unit. MCCBs are generally instantaneous-trip devices at their upper current settings; most standard MCCB datasheets from ABB, Schneider, and Siemens simply do not publish an Icw figure. Larger electronic-trip frames — high-end Tmax XT7, ComPact NSX630, Sentron 3VA2 — can offer a short-time-delay setting and a limited Icw rating, but it is the exception, not the norm, and it is usually well below what an equivalent-frame ACB carries.
Some engineers argue this makes MCCBs unsuitable for any selective scheme. In practice that is too strong: MCCBs achieve selectivity through current discrimination or zone-selective interlocking on electronic trips, not necessarily through Icw. Reach for an ACB, or a category B MCCB with a confirmed Icw rating, specifically when the scheme requires the upstream device to stay closed and carry full fault current for a set time.
Icu vs Ics vs Icw at a Glance
| Criteria | Icu | Ics | Icw |
|---|---|---|---|
| Defined in | IEC 60947-2, §8.3 | IEC 60947-2, §8.3 | IEC 60947-2, §8.3 |
| Test sequence | O-t-CO | O-t-CO-t-CO | Rated time at Icw, no opening |
| What it proves | Opens safely once, stays safe to touch | Opens safely and remains serviceable afterward | Carries fault current without tripping for a set duration |
| Typical MCCB availability | Always published | Always published, 25-100% of Icu | Rare, limited to select high-frame electronic-trip models |
| Primarily used on | MCCB and ACB | MCCB and ACB | ACB (large frames), some category B MCCB |
Specifying a Breaker: Which Number to Design Around
For a fault-current study, start with the prospective short-circuit current at the point of installation and check it against Ics, not Icu. If Ics comfortably covers the calculated fault level, the breaker will still be usable after clearing it. If only Icu covers it, the breaker will survive the event but the spec should assume replacement follows.
This depends on duty cycle, too. A breaker protecting a rarely-faulted lighting feeder tolerates a lower Ics percentage class better than one protecting a main incomer that has already tripped twice this year. Where budget allows, matching or exceeding the prospective fault current with a 100%-Ics-class breaker removes the "was this breaker still good after the last trip?" question from every incident report going forward.
None of this replaces a proper short-circuit study. Icu, Ics, and Icw only matter once the fault-current envelope at each board is known; see the MCCB engineering guide for how that envelope feeds into frame and rating selection, and how MCCB breaking capacity ratings are structured for the letter-class system ABB and Siemens use alongside Icu/Ics.
Frequently Asked Questions
Is Ics always lower than Icu?
Ics is always less than or equal to Icu — IEC 60947-2 allows manufacturers to declare it at 25%, 50%, 75%, or 100% of Icu. It is never rated higher than Icu.
Which rating should I use for panel design, Icu or Ics?
Size around Ics when the breaker needs to remain in service after clearing a fault, which is the normal assumption for most distribution boards. Use Icu only if the design already assumes replacement after any fault-current event.
Do all MCCBs have an Icw rating?
No. Icw is mostly an ACB rating tied to short-time-delay settings used in selective coordination. Most standard MCCB datasheets do not list one; only some large, electronic-trip frames offer a limited short-time-delay setting with a stated Icw.
Why does Schneider rate Ics lower than ABB or Siemens on some frames?
It reflects a different design and cost trade-off on those specific classes, not a blanket rule across the whole range. Some Schneider ComPact NSX classes declare Ics at 50-75% of Icu, while ABB and Siemens often declare 100% Icu on their higher classes — always check the specific frame and class, not the brand as a whole.
What happens if an MCCB interrupts a fault right at its Icu rating?
It is certified to open safely and stay safe to touch, but the standard makes no promise that it can still carry rated current afterward. Inspect it, and budget for replacement, before re-energizing the circuit.
Conclusion
Icu tells you the breaker will fail safely once. Ics tells you it will still be doing its job afterward, at whatever percentage of Icu the manufacturer declares. Icw is a different rating altogether, mostly reserved for ACBs and a handful of large MCCB frames running short-time-delay settings. Check all three against the actual fault-current study for the board, not just the biggest number on the datasheet — see the MCCB selection checklist and IEC 60947-2 standards overview for how these ratings fit into a full specification.
