IEC 60947-2 — Circuit-Breaker Short-Circuit Ratings
IEC 60947-2 defines what a low-voltage circuit-breaker's ratings mean and how they are verified by test. For the engineer it answers one question: which rating does my calculated fault current have to be compared against — and the answer is different for breaking, for withstanding and for discriminating.
What is IEC 60947-2?
IEC 60947-2 is the product standard for low-voltage circuit-breakers. It specifies their rated characteristics, the marking that must appear on them, and the test sequences that prove each rating — including the short-circuit sequences of clause 8.3 which a manufacturer must pass before a number may be printed on the label.
This matters for a calculation because the ratings are not interchangeable. A breaker with Icu = 36 kA is not a breaker that can carry 36 kA for a second, and a device that discriminates with the one downstream needs a rating that many breakers do not have at all.
The short-circuit ratings
| Rating | Clause | What it means |
|---|---|---|
| Icu rated ultimate short-circuit breaking capacity |
4.3.6.2 | The largest prospective current the breaker can break, at the corresponding rated operational voltage. After breaking at Icu the device is not required to remain in service — it is required to have interrupted safely. |
| Ics rated service short-circuit breaking capacity |
4.3.6.2 | The current the breaker can break and remain serviceable afterwards, declared by the manufacturer, commonly as a percentage of Icu. This is the rating that matters where the installation must keep running after a fault. |
| rated short-circuit making capacity | 4.3.6.3 | The current the breaker can close onto. For a.c. it shall be not less than Icu multiplied by the factor n of Table 2 — which is how the standard ties the making duty to the peak current of the fault. |
| Icw rated short-time withstand current |
4.3.6.4 | The current the breaker can carry, closed, for a stated short time without tripping or damage — the rating that makes time-based discrimination possible. |
I_cw and the minimum values of Table 3
Icw exists so an upstream breaker can wait while a downstream one clears the fault. Where a breaker declares an Icw, IEC 60947-2 Table 3 sets the minimum value it may declare.
| Rated current In | Minimum Icw |
|---|---|
| In ≤ 2 500 A | 12 In or 5 kA, whichever is greater |
| In > 2 500 A | 30 kA |
The withstand duty is a thermal one, so it is compared as energy rather than current: the fault's Joule integral over the actual clearing time against Icw² × t of the device.
with the right-hand side from IEC 60909-0 Clause 14, Formula (108).
Selectivity categories A and B (clause 4.4)
Category B is a breaker with a rated short-time withstand current; category A is one without. Only category B can be graded by time delay against a downstream device, because only category B is specified to stay closed while the fault current flows.
A category A breaker trips instantaneously on a large fault, so discrimination with it relies on current or energy rather than time. The distinction runs through the test sequences too: sequence IV, the short-time withstand sequence, applies only to breakers that declare an Icw.
Which calculated current goes against which rating
| Check | Compare | Source of the current |
|---|---|---|
| Can it break the fault? | Icu ≥ I″k at the point of installation, maximum regime | IEC 60909-0, 7.2.1 Formula (33), with cmax |
| Will it survive being closed onto the fault? | Making capacity ≥ n · Icu per Table 2, against the peak ip | IEC 60909-0, 8.1.1 Formulas (56) and (57) |
| Can it wait for the downstream device? | Icw² · t ≥ the fault's Joule integral | IEC 60909-0, Clause 14 Formula (108) |
| Will it operate at all on an earth fault? | Ia from the tripping curve ≤ I″k1, minimum regime | IEC 60909-0, 7.5 Formula (54) under 7.1.2; requirement in IEC 60364-4-41, 411.4.4 |
The last row is the one most often missed. A breaker can have ample breaking capacity and still fail to clear a remote earth fault, because the current that reaches it is the minimum one — computed with cmin, without impedance correction factors and with the conductor resistance at its final temperature.
Selection workflow
- Size the cable and the load current first — IEC 60364-5-52 gives Iz, and the breaker must protect that cable.
- Calculate the maximum fault current at the point of installation, and the peak.
- Pick a device whose Icu exceeds it, and decide whether the installation needs Ics close to Icu for post-fault service.
- If the device has to discriminate by time, choose category B and check Icw²t against the fault's Joule integral over the delay you intend to set.
- Calculate the minimum earth-fault current and check it against the device's operating current Ia for the disconnection time required by IEC 60364-4-41 Table 41.1.
- Verify the cable's own withstand against the energy the device lets through — the k²S² check of IEC 60364-5-54, or IEC 60949 for a non-adiabatic case.
Icu, Ics, Icw, the let-through energy and the tripping curve are manufacturer data: no IEC standard tabulates them for a particular device. A calculation tool can check them, but it cannot invent them — which is why the calculators here ask for those values and say where they come from.
IECCalc calculators that use these ratings
- Algorithm LV cable selection algorithm The coordination conditions of IEC 60364-4-43, 433.1 in their place in the cable selection sequence.
- #012 Short circuit — LV up to 1 kV, with the device check Computes I″k3, the Joule integral and the minimum I″k1 at the fault busbar, then checks them against the Icu, Icw and Ia you enter from the datasheet.
- #002 Short circuit — MV 1 to 33 kV Fault level at every busbar, for switchgear selection upstream of the low-voltage boards.
- #008 LV cable sizing The cable the breaker has to protect, sized to IEC 60364-5-52.
This page explains the ratings of IEC 60947-2 in its own words and cites clause and table numbers so they can be checked against your own copy; the standard's text is not reproduced. Reference: IEC 60947-2:2016 (with Corrigendum 1:2016), Low-voltage switchgear and controlgear — Part 2: Circuit-breakers.