What breaking capacity means
A circuit breaker does two jobs: it carries current during normal operation and it interrupts current when something goes wrong. Breaking capacity is the measure of the second job. It states the largest short-circuit current the device can interrupt safely without being destroyed or becoming a hazard itself.
That number matters because a fault current can be many times the rated current of the circuit. If the breaker's breaking capacity is lower than the fault current available at its terminals, the device may fail violently instead of clearing the fault. Choosing a rating that covers the prospective fault level is therefore a safety decision, not a convenience.
The prospective fault current depends on the supply and on the impedance of the path back to the source. It is highest at the point closest to the supply and falls as you move downstream, because cable impedance adds up along the way.
Icu and Ics defined
Two ratings describe breaking capacity, and they answer slightly different questions.
Icu is the rated ultimate short-circuit breaking capacity. It is the highest fault current the breaker can interrupt, after which it may no longer be suitable for continued service without inspection or replacement.
Ics is the rated service short-circuit breaking capacity. It is a lower value at which the breaker interrupts the fault and remains fit to carry its rated current afterwards, so the circuit can be restored and put back into service.
In short, Icu is the limit of what the device can survive, and Ics is the limit of what it can survive and then keep working. Both are expressed in kiloamperes.
Both figures appear on the device nameplate and in the manufacturer's catalogue, and they are meant to be read together. They tell you not only whether the breaker will clear the calculated fault, but whether it can be put straight back into service afterwards. A device whose Ics matches its Icu keeps its full rating through a fault; one with a low ratio is close to a single-use item at its limit.
The difference between the two ratings
The table below summarises the working difference between Icu and Ics.
| Aspect | Icu | Ics |
|---|---|---|
| Meaning | Ultimate breaking capacity | Service breaking capacity |
| After the fault | May need inspection or replacement | Remains fit for continued service |
| Typical level | The higher of the two | A defined fraction of Icu |
| Used for | Surviving the worst fault | Repeated, restorable operation |
Many manufacturers state Ics as a percentage of Icu, and a device with a high ratio keeps more of its rating after a fault. On a board that must be restored quickly after an incident, that ratio is worth checking.
Why the fault level sets the bar
The breaking capacity of a device has to be at least as large as the prospective fault current at the point where it is installed. That is why the same breaker type appears in different ratings across a project: a device near the supply faces a much higher fault current than one at the end of a long final circuit.
This is the reason a design always starts from a fault-current calculation. Without it, a breaker rated for 6 kA might be installed where the available fault current is 10 kA, leaving no safety margin at all. The calculation is straightforward but it depends on accurate source data and on the impedance of every cable in the path.
Checks against IEC 60364 tie the device rating to the installation conditions, and a sound project records both the calculated fault current and the chosen device rating side by side.
Selecting a breaking capacity
Selection is a short sequence once the fault level is known.
- Calculate the prospective fault current at the device location.
- Choose a standard rating at or above that value, with the manufacturer's data confirming it.
- Check the ratio of Ics to Icu if the circuit must be restored after a fault.
- Confirm the rating suits the system voltage and the number of poles.
- Record both the fault level and the device rating in the project documentation.
Where a high fault current meets a low-rated device, coordinated back-up between two devices can be used, but it must be verified for the specific pair rather than assumed.
Where designers go wrong
The same mistakes recur on real projects.
- Reusing a rating from a previous project without recalculating the fault level.
- Reading Icu as if it were Ics, and expecting a device to keep working after a fault it only just survived.
- Ignoring the effect of a new upstream transformer, which can raise the fault level across a whole building.
- Assuming back-up coordination applies between devices that were never tested together.
Keeping the fault current and the device ratings in one place is the simplest way to avoid all four, because a change upstream becomes visible downstream.
Working with breaking capacity in GorkyCAD
In GorkyCAD the protective devices live in the same electrical model as the cables and boards. Fault-current figures and device selections flow from that one model, and the toolset includes short-circuit and selectivity calculations together with checks against IEC 60364.
Because the diagram, the board and the cable schedule share a source, the fault level that drives a breaking-capacity choice is the same one shown on the drawing. There is no parallel worksheet to fall out of step when the design changes.
Next steps
Icu and Ics are not decoration on a datasheet — they are the guarantee that a breaker clears a fault safely, and in the case of Ics, that it can do so and still be trusted afterwards. Match them to the calculated fault level and record the choice.
You can open the browser-based editor and work through a fault-current and device-selection exercise free of charge, with no card required.
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FAQ
Is Icu the same as Ics?
No. Icu is the ultimate breaking capacity the device can survive once; Ics is the lower value at which it clears the fault and remains fit for continued service. Ics is usually a fraction of Icu.
How do I know the fault current at a breaker?
Calculate it from the supply data and the impedance of the path back to the source. It is highest near the supply and falls downstream as cable impedance accumulates.
Can I use a lower-rated breaker with back-up?
Coordinated back-up can let two devices share the duty, but only for a pair verified together. It should never be assumed from similar devices or from a previous project.
Why record the rating in the documentation?
So a reviewer can see that the device covers the calculated fault level, and so a later change upstream can be checked against the ratings already chosen downstream.