Icu is the Ultimate Breaking Capacity. Consider this to be the very maximum fault current that a circuit breaker will interrupt—once—without annihilating itself. If the fault current is this maximum, the breaker will open and end the flow, but it’s not necessarily in functioning condition afterward. It’s performed its function, but it may not be suitable for reuse.

This rating is paramount when you’re choosing a breaker for a system where you understand the potential fault current is high. For instance, if your network has the capability to create a fault current of 36 kA at some location, then you require a breaker that possesses an Icu of equal to or more than 36 kA.

Manufacturers usually quote Icu as a figure of 25 kA, 36 kA, or 50 kA at a given voltage level (e.g., 415 V AC). It’s the hard limit. If fault current is greater than Icu, the breaker won’t trip safely, or worse—explode.

Ics – The Service Short-Circuit Breaking Capacity

If the top end is Icu, the usable one is Ics. Ics is Service Breaking Capacity, and it’s a percentage of Icu—usually 25%, 50%, 75%, or 100%. This is how much fault current the breaker can interrupt repeatedly and still be functioning.

Suppose you have a breaker with an Icu rating of 36 kA and an Ics rating of 27 kA (or 75% of Icu). That is to say, it can safely interrupt a 27 kA fault multiple times and still perform as required. But if it had to interrupt a fault at its maximum Icu rating of 36 kA, it can trip properly but may not be available for service again.

The greater the Ics rating in comparison to Icu, the better the breaker performs in actual application. In industrial settings where faults may occur more than once during the life of the installation, this is extremely significant.

Most quality breakers will have an Ics that’s 100% of Icu, providing you with the confidence that the breaker can withstand whatever is thrown at it without having to be replaced after one fault condition.

Icw – Short-Time Withstand Current

Icw is where it gets exciting. This rating has nothing to do with breaking or interrupting existing at all—it’s about surviving it. Icw is short for Short-Time Withstand Current, and it gives you how much current a piece of equipment (or the busbars on the switchboard) can survive without opening or failing, for some amount of time—usually between 0.2 to 1 second.

Why would a breaker ever have to survive fault current without opening?

Because in selective coordination schemes, we prefer that upstream breakers remain closed long enough for downstream breakers to clear the fault. If upstream breakers too quickly trip, they may initiate unnecessary outages to undisturbed portions of the installation.

Icw is particularly applicable in main switchboards and submains. For example, if you have a 630 A-rated main incomer with an Icw rating of 25 kA for 1 second, then you can be sure that it will not melt or get damaged by a short-circuit current of up to 25 kA for a duration of up to one second.

Notably, Icw ratings do also extend to busbar systems. Busbars need to withstand the fault current mechanically and thermally without bending, melting, or loosening. That’s the reason why an effective coordination study doesn’t only examine the breakers but the physical switchboard components as well.

How These Ratings Work Together

Let’s put this all into context. Imagine you’re designing a switchboard for a commercial building where the calculated prospective fault current is 36 kA at the main distribution point.

You choose a main breaker with the following ratings:

Icu = 36 kA
Ics = 36 kA (100% of Icu)
Icw = 25 kA for 1 second

Here’s what that tells you:

The breaker can safely interrupt a fault of 36 kA once (Icu).
It is able to interrupt a 36 kA fault repeatedly and continue in service (Ics).
If there is a fault somewhere downstream and 0.5 seconds are taken for the clearing of the fault, the breaker and the busbar assembly can handle up to 25 kA during this time delay without getting harmed (Icw).

This coordination is aimed at ensuring protection along with dependability. The downstream breaker will be the one that trips first, and the upstream breaker will not be affected unless the fault is not cleared within time.

Why These Ratings Matter

The majority of electrical installations are based on the AS/NZS 3000 wiring regulations, and fault protection design hinges on kA ratings. Whether developing a domestic main switchboard, a shopping centre distribution board, or an industrial MCC, getting to know these ratings guarantees that the system is capable of surviving the worst-case electrical faults without disastrous collapse.

Supply authorities can also require minimum values for Icu for service protection devices, and manufacturers will typically offer coordination tables or test certificates to demonstrate that the ratings of the device meet the anticipated short-circuit levels of the network.

Mistakes to Avoid

Taking Icu to be equal to Ics
Not every breaker has 100% Ics. If you do not verify this, you may end up installing a breaker that will not last through a fault more than once.

Disregarding Icw in panel design
Busbar systems should be coordinated with the breaker’s Icw rating. Under-rated busbars may deform or fail even if the breaker trips correctly.

Applying breakers with an inadequate Icu
Always compare the breaker’s Icu against the potential fault current at the point of installation. The fault level may be computed with software programs or quoted by the utility.