Breaking capacity of an MCB: 3, 6 or 10 kA?

What do 3 kA, 6 kA and 10 kA on a circuit breaker mean?

The kA figure is the breaking capacity: the value of the current the protective device is able to interrupt at a specified voltage and under prescribed conditions of use and behaviour. That is the AREI definition, section 2.6.4. A 3 kA breaker handles a 3,000 A short circuit at its point of installation; a higher short-circuit current destroys it instead of being cleared by it.

The figure has nothing to do with the rating printed on the front. A C16 rated 3 kA and a C16 rated 10 kA protect the same cable against the same overload — they differ only in how much short-circuit energy they themselves survive while clearing the fault.

What matters is therefore not your preference but the short-circuit current to be expected at the point of installation. The AREI states it literally: the breaking capacity of a short-circuit protective device is at least equal to the prospective short-circuit current, as determined according to good practice, at the point where the device is installed (sub-section 4.4.2.1, item 1).

What breaking capacity does the AREI require in a dwelling?

Unlike the rated current, the regulation does not stop at a principle here: for domestic installations, sub-section 5.3.5.5 point e gives figures.

The starting point is a ceiling, not a device property. At the output terminals of the first overcurrent protective devices placed after the general residual current device(s), the prospective single-phase short-circuit current may not exceed 3,000 A. Upstream of those output terminals, minimum values apply:

Two readings fall away. First, “3 kA is always enough”: that holds only as long as the 3,000 A ceiling at those output terminals is actually respected. Second, “6 kA is mandatory”: the 6,000 A concerns the distribution network operator’s device, not your final circuit breakers. Fitting 6 kA in the board is a common and harmless margin, but it is not an AREI requirement for the outgoing ways.

For non-domestic installations the AREI gives no such figures. Only sub-section 4.4.2.1 item 1 applies, and the short-circuit current has to be established at the point of installation.

What is the difference between Icn, Icu and Ics — and which value is on a domestic breaker?

These three symbols do not come from the AREI but from the product standards of the devices. The AREI refers to them explicitly, to the standards approved by the King or registered by the NBN (sub-section 5.3.5.5 point d for fuses, point a for the calibrating components).

• Icn — rated short-circuit capacity under IEC 60898-1, the standard for circuit breakers for household use. This is the value a DIN-rail breaker in a Belgian domestic board carries. It appears as an ampere figure without a unit, inside a rectangle — the “3000” of 5.3.5.5 e is exactly that marking.
• Icu — ultimate short-circuit breaking capacity under IEC 60947-2, the standard for industrial circuit breakers. The device clears the short circuit but need not remain serviceable afterwards.
• Ics — service short-circuit breaking capacity under the same standard: the current after which the device may stay in service. Ics is expressed as a percentage of Icu (for example 50 %, 75 % or 100 %).

The practical consequence for the drawing: when you plan a dwelling, you document Icn. Icu and Ics only appear once the board contains a circuit breaker to IEC 60947-2 — as the main switch of a larger installation, for instance.

The AREI does have its own term for the reinforced case: where the downstream protective device is a circuit breaker and the upstream device a fuse or a circuit breaker, that technique gives the downstream breaker a reinforced ultimate breaking capacity (sub-section 4.4.1.3).

What determines the short-circuit current at the point of installation?

The prospective short-circuit current is not a property of your installation alone. It follows from the impedance of the entire loop, from the supplying transformer to the fault, and drops with every metre of cable:

• Rating and short-circuit voltage of the supplying transformer — the larger the substation, the higher the short-circuit current on the low-voltage side.
• Length and cross-section of the service connection from the distribution cable to the meter box.
• Length and cross-section of the cables inside the installation — which is why the value at a final circuit breaker is always lower than at the origin.
• Number and nature of the connections in series — terminals, busbars, comb busbars.

The AREI mirrors this in its documentation duties. For non-domestic installations, the characteristics of the sources (apparent power, nominal voltages, nominal currents, impedances) must be stated, together with the installation method, the number and cross-section of the conductors and the length of the electrical cables — and explicitly the maximum prospective short-circuit currents above 3,000 A at the origin of the installation and at each distribution and switchgear assembly (sub-section 3.1.2.2 point b). Those are precisely the quantities that feed the calculation.

The dependable figure for your service connection comes from the distribution network operator. It depends on the substation rating and the connection length; a rule of thumb for “the Belgian domestic connection” is no design basis. Ask for the number instead of estimating it – the AREI requires a determination “according to good practice” (sub-section 4.4.2.1, item 1).

What if the breaking capacity at the point of installation is not enough?

The AREI explicitly permits a breaker with insufficient breaking capacity — under one condition. Sub-section 4.4.1.3 states that the use of a protective device whose breaking capacity is lower than the prospective short-circuit current at the point where it is installed is permitted, but does not release you from installing another device upstream that has at least the necessary breaking capacity. The upstream device then lets through no more energy than the downstream device and the protected cables can withstand without damage.

Where both devices in series are circuit breakers, the AREI calls this additional protection back-up protection — filiation in the RGIE. And it says at once where the design data comes from: to determine the characteristics of back-up protection, the back-up protection tables of the protective devices must be requested, drawn up in accordance with the product standards for circuit breakers (sub-section 4.4.1.3).

That is the decisive point: back-up protection cannot be calculated and cannot be inferred from datasheets; it follows only from the tested combination of two specific device types from the same or an approved manufacturer. How this plays out in an installation with a sub-board per storey is covered in Electrical planning across several floors.

Where does the breaking capacity appear on the single-line diagram, and why does the inspection want to see it?

The single-line diagram of a domestic installation must state the type and characteristics of the overcurrent protective devices (sub-section 3.1.2.2 point a). Breaking capacity is such a characteristic; the boxed ampere figure therefore belongs beside the rated current and the curve, at the breaker.

For non-domestic installations this is more explicit still. There sub-section 3.1.2.2 point b lists the characteristics of the protective devices individually and names “the rated current; the breaking capacity; the nature and characteristics of the interruption including the settings”. And the marking of each distribution and switchgear assembly carries there — besides identification number, supply voltage and earthing system — the maximum prospective short-circuit current at the assembly and, where applicable, the use of the back-up protection technique under 4.4.1.3 (sub-section 3.1.3.3 point b). The inspector can read the target value off the board and compare it with the breakers.

There is a second reason. Exceeding the permissible short-circuit power of the installed equipment is named by the AREI as an example of a substantial modification or extension (section 2.11.2) — and a substantial modification triggers a fresh conformity inspection. So when the service connection is upgraded or the substation changes, review the breakers as well – otherwise a formal problem joins the technical one.

On the device itself the marking is prescribed in any case: “3000” inside a rectangle on small circuit breakers, and on RCDs without overcurrent protection rated In ≤ 40 A the indication “3000 A, 22,5 kA2s” — together on the same face and visible after installation (sub-section 5.3.5.5 point e). With the figure on the diagram, that visual comparison on site takes seconds.

How does breaking capacity differ from rated current and the B/C/D curve?

Three separate properties, three separate rules — and a recurring confusion:

Table 4.11 limits only the rated current per conductor cross-section — 1.5 mm² at most 16 A, 2.5 mm² at most 20 A, 6 mm² at most 40 A and so on. It says nothing about breaking capacity. A C20 on 2.5 mm² can be fully table-compliant and still be undersized on breaking capacity. The full cross-section ↔ rating mapping is set out in Cable cross-section and breaker rating.

The curve, in turn, concerns operating time. The AREI only requires that the current be interrupted after a dead short circuit before the conductor temperature exceeds the permissible limit, and gives the adiabatic formula for that (sub-section 4.4.2.1, item 2). Which letter fits follows from the load and the circuit length, not from an AREI provision.

How do you record breaking capacity in PlanElec?

Two entries work together. The short-circuit current (Icc) at the supply is set in the project properties under the network configuration; the choices are 3, 4.5, 6 and 10 kA, with kA or A as the display unit. The value appears on the single-line diagram beneath the network connection label.

The breaking capacity Icn is set per breaker: select the MCB on the single-line diagram, then open the “Breaking capacity Icn” field in the inspector. The options are “Not documented” plus 3, 4.5, 6, 10, 15, 20 and 25 kA. Since the update of 28 August 2026, a value that has been set appears as a boxed ampere figure at the breaker — 3 kA as 3000, without a unit, as 5.3.5.5 e prescribes for the device marking — and stays identical in editor, preview and PDF. Above 25 kA no rectangle is drawn, because that is where IEC 60898-1 ends.

“Not documented” is deliberately a state of its own rather than a silent 3 kA: the diagram shows only what you actually read on the device.

What the AREI self-check verifies for breaking capacity

The rule “Breaking capacity versus prospective short-circuit current” works with the two inputs from the previous section:

• It compares each breaker’s Icn with the threshold min(supply Icc, 3 kA) and reports an error if the device falls below it.
• It only assesses with an explicitly set Icc. Without that value you see “Breaking capacity not assessable” – the check counts as incomplete, not as passed.
• If a breaker’s Icn is missing, it reports “not documented” as an open data gap.

The roles are clearly split. Icc is an input: the value comes from the distribution network operator or a calculation according to good practice. Back-up protection under 4.4.1.3 is proven with the manufacturer’s combination tables. Energy-limitation class 3 and the I²t withstand of RCDs (22.5 kA²s at 3,000 A, 5.3.5.5 e) are read on the real device. The assessment of the completed installation is carried out by the approved inspection body.

Further reading

• Cable cross-section and breaker rating
• Distribution board planning
• RCD type A versus type B
• Earthing systems TT and TN-S in Belgium
• Electrical planning across several floors

Document breaking capacity in PlanElec →

Regulatory basis: AREI/RGIE Book 1 V06 — sections 2.6.4 and 2.11.2, sub-sections 3.1.2.2, 3.1.3.3, 4.4.1.3, 4.4.1.4 with table 4.11, 4.4.2.1 and 5.3.5.5. Icn, Icu and Ics come from IEC 60898-1 and IEC 60947-2 respectively, not from the AREI. The official text published by the FPS Economy prevails.