MCB breaking capacity: 6 kA or 10 kA?

Two miniature circuit breakers, same 16 A rating, same C curve, same width on the rail. One costs twice as much as the other. The difference is printed in a small rectangle: 6000 or 10000, meaning 6 kA or 10 kA of short-circuit current that the device can interrupt and still be usable afterwards.

This number has nothing to do with the load. It is entirely about the supply.

What the number means

Breaking capacity is the prospective short-circuit current the breaker can safely interrupt. The test is brutal: the breaker is closed onto a fault at its rated breaking capacity and must clear it without the arc escaping the case, without the case bursting, and - in the case of the service rating - while remaining fit for further service.

Two ratings appear on IEC devices:

• Icn (IEC 60898-1) - the rated short-circuit capacity. After the test the breaker must still be safe, but is not required to carry current again.
• Icu / Ics (IEC 60947-2) - rated ultimate and rated service breaking capacity. Ics is the value at which the device must remain serviceable; it is usually 50-100 % of Icu.

A breaker marked "6000" on an IEC 60898 body means 6 kA. On an IEC 60947-2 device the marking is usually Icu, with Ics somewhere in the data sheet. Reading only the front of the device is how a 6 kA device ends up on a board with 10 kA available.

How much fault current is actually there

The prospective fault current at a distribution board depends on the transformer and the cable between it and the board:

• Domestic service, 100 m from a 400 kVA transformer - typically 1-3 kA.
• Small commercial board, own transformer - 4-8 kA.
• Industrial board close to a 1,000 kVA transformer - 15-25 kA.
• Sub-distribution downstream of long cable runs - falls quickly; impedance matters more than transformer size.

You do not estimate this. The utility supplies the fault level at the point of connection, and the designer calculates downstream values from cable impedance. Where no data exists, it is measured with a loop impedance tester - a five-minute job that settles the question.

What happens if the rating is too low

The breaker does not simply fail to trip. At a fault above its rating:

• The arc is not extinguished inside the arc chamber, and the case ruptures. On a DIN rail inside a distribution board, the result is ionised gas, molten metal and a flash that can arc to adjacent devices and to the enclosure door.
• The contacts may weld closed, so the circuit stays live after the event.
• The upstream device may trip instead, which protects the installation but leaves a destroyed breaker in the panel and no clear indication of why.

This is why breaking capacity is a safety specification, not a cost optimisation.

Back-up protection

There is a legitimate way to use a 6 kA device where 10 kA is available: back-up protection. An upstream current-limiting device - a higher-rated MCB, an MCCB, or fuses - clears high faults fast enough to limit the let-through energy, and the manufacturer publishes a table stating which combinations are tested and at what prospective current.

Two conditions, both mandatory:

1. The combination must be in the manufacturer's back-up table. Two devices from different manufacturers do not have a tested combination, whatever the arithmetic suggests.
2. The upstream device must be in the same panel and rated for the full fault level at its own position.

Without the table it is an assumption, and the assumption fails exactly in the case the breaker was bought to protect against.

Energy limitation class

IEC 60898-1 also classifies devices by how much energy they let through in the first half-cycle: class 1, 2 or 3. Class 3 limits let-through energy most. It matters for coordination with downstream devices and for cable protection in the first few milliseconds, and it is almost never specified by anyone buying a breaker.

For most installations it changes nothing. Where it does matter: long cable runs protected only by the breaker, and coordination with a residual current device downstream.

A practical selection rule

Then apply back-up protection only where the manufacturer's table supports it, and prefer the higher rating when the difference is a few euros. The device that has to interrupt a fault once in its life is not the place to save.

One more number people ignore

Icn describes one operation. After clearing a fault at rated capacity, an IEC 60898 breaker is not required to be reusable. A device that has actually cleared a heavy fault should be replaced, not reset and put back into service - the arc chamber and contacts have absorbed energy they were only designed to absorb once.

If a breaker trips with a bang, smells of burnt plastic, or shows a discoloured case, replace it. Resetting it and hoping is how the second fault becomes a fire.