Understanding breaking capacity: kA ratings explained

The kA figure on a circuit breaker is the most commonly misread number in a low-voltage panel. It is not a rating of how much current the breaker carries, it is not a single quantity, and two devices marked with the same number can have been tested to different standards. This guide sets out what each symbol means and what you have to check.

Start with the fault current, not the breaker

Breaking capacity is a requirement derived from the installation, so the first number you need is not on any product page. It is the prospective short-circuit current: the current that would flow at a point if a bolted fault of negligible impedance occurred there.

It is set almost entirely by the impedance between the source and that point: the supply transformer’s rating and percentage impedance, and the length and cross-section of every conductor in between. Because cable impedance accumulates with distance, the prospective fault current is highest at the origin and falls as you move outwards. A main board and a final circuit board fed from it can differ by an order of magnitude, so specifying every board for the incomer’s fault level is expensive and unnecessary.

That figure comes from the supply authority’s declared value plus a calculation through the installation, or from a measurement at an existing board. It is not something a distributor can supply, and not something a catalogue can tell you.

Icu and Ics: interrupt once, or interrupt and carry on

The ultimate capacity, I_cu, is verified by a sequence written O – t – CO: the breaker opens on a fault, an interval passes, then it closes onto the fault and opens again. Afterwards it must have interrupted safely, passed a dielectric verification, and still have a functioning overload release. What it need not be is fit for continued service — a breaker that has cleared a fault at its ultimate capacity may legitimately be at the end of its life.

The service capacity, I_cs, is verified by the longer sequence O – t – CO – t – CO, and the device must afterwards still meet its temperature-rise and tripping requirements — that is, go back into normal service. IEC 60947-2 gives preferred values for I_cs as 25 %, 50 %, 75 % or 100 % of I_cu.

The consequence is direct. Where an unplanned outage is costly, you want the prospective fault current at that point at or below I_cs, not merely below I_cu. Choosing on I_cu alone is choosing a breaker permitted to be scrap after one operation.

Icn: the IEC 60898 marking

Devices to IEC 60898-1 are marked with I_cn, conventionally shown inside a rectangle on the case. One defined test sequence and one set of acceptance criteria are exactly why the marking works for an installer: two 60898 devices marked with the same I_cn have been through the same test. The confusion to avoid is a different one — an MCB’s I_cn and an MCCB’s I_cu come from different standards, and comparing them directly is meaningless.

Icw and Icm: withstanding and closing

I_cw, the rated short-time withstand current, is what the device can carry without opening for a stated time — typically declared for 0.05 s, 0.1 s, 0.25 s, 0.5 s or 1 s. This is what makes time-based discrimination possible: an upstream breaker with a short-time delay waits so a downstream device clears the fault first, and I_cw guarantees that waiting will not destroy it. IEC 60947-2 classifies breakers as category A (no intentional delay) or category B (with one, and a declared I_cw). Air circuit breakers and larger moulded-case frames are commonly category B.

I_cm, the rated short-circuit making capacity, is a peak current rather than an r.m.s. one, and it describes closing onto an existing fault. Closing is the harsher duty: the contacts must survive the electrodynamic forces of the peak asymmetric current without welding. I_cm relates to I_cu through a factor depending on the power factor of the test circuit — the lower the power factor, the higher the peak for a given r.m.s. value. The datasheet states I_cm directly.

Current limiting and let-through energy

A fault current rises along a curve set by the circuit’s impedance and inductance. A current-limiting device — a fuse, or a breaker with a fast repulsion-driven contact mechanism — opens its contacts and develops an arc voltage before the current reaches its prospective peak, so the current that actually flows is substantially less.

Two published quantities describe this. The peak let-through current is the highest instantaneous current the device allows through. The let-through energy (I²t, in A²s) is the integral of the square of the current over the clearing time, and it determines the thermal stress on everything downstream — cable insulation, busbar, contactor contacts. It is what you check a cable’s short-circuit withstand against, and it is the mechanism behind cascading.

Cascading, and why the tables are not optional

Cascading — back-up protection — allows a device to be installed where the prospective fault current exceeds its own breaking capacity, because an upstream current-limiting device acts with it to clear the fault. It is legitimate, standardised, and can substantially reduce the cost of a board.

What it is not is a calculation. The enhanced rating is a type-tested result for that exact pair, published as a table. No formula derives it, and it does not transfer between manufacturers or necessarily between ranges. If a combination does not appear in a published table, the correct conclusion is that it has not been tested — not that it is probably fine.

A worked sequence

1. Establish the prospective short-circuit current at the point.
2. Note the circuit’s rated operational voltage — every kA figure is quoted against a voltage and falls as voltage rises.
3. Require I_cu (or I_cn) at that voltage to be at least the prospective fault current, unless a published cascading table permits otherwise.
4. If continued service after a fault matters, require I_cs at that voltage to meet it too.
5. Check I_cw if the device will hold a short-time delay, and I_cm if it may be closed onto a fault.
6. Verify the downstream cable against the device’s published I²t let-through.

How to check this on the product page

The specification table under each breaker on this site carries the breaking-capacity fields as separate, labelled values rather than one merged “kA” figure, because they are separate quantities. Look for:

• Rated operational voltage (Ue) — read this first; everything below is quoted against it.
• Icu and Ics for a 60947-2 device, or Icn for a 60898 device.
• Icw where declared — its presence tells you the device is category B.
• Standard, which tells you which set of symbols applies.

Each value is clickable and filters the catalogue to everything sharing it. Where a field is absent, we do not hold a verified figure for it — take it from the manufacturer’s datasheet and ask us if you would like it confirmed. Let-through curves and cascading tables are published per device pair and are not reproduced in the catalogue; we can send you the document for a specific combination. Add your shortlist to a quote request and the sales desk will confirm stock, lead time and price.

A closing note on responsibility

Breaking capacity is one input to a protection design that also has to satisfy disconnection times, cable thermal withstand, earth-fault protection, discrimination and the installation standard in force. The prospective fault current at each point is a property of the installation and must be established for it. Final selection and verification are the responsibility of the designing engineer, working from actual installation data and the manufacturer’s published data and type-test tables for the specific devices. RMS can assist with selection, supply datasheets and published cascading and discrimination tables, and check a proposed combination against them — but that assistance supports competent design, it does not replace it.