MCBConsumer UnitProtectionIEC 60898-1

How an MCB miniature circuit breaker works

An MCB (Miniature Circuit Breaker) is the most common protective device in a domestic consumer unit. It protects the circuit against overloads and short circuits, but not against earth fault currents. Here is what happens inside when it trips.

29 May 2026·10 min read·

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Fig. 1 — The internal components of an MCB (cross-section view)

The internal components

A standard MCB contains five functional systems working together:

The terminals — screws or clamps that secure the conductors
The bimetallic strip — two welded metals (usually copper and steel) with different thermal expansion coefficients, which bends when heated
The electromagnetic coil — a winding carrying the circuit current, with a moving core
The trip mechanism — a spring-loaded lever that holds or releases the contacts
The arc-extinction chamber — a stack of steel plates that fragments and cools the electric arc when the contacts open

1. Thermal protection — overload

When the current exceeds the rated value In marked on the MCB, the bimetallic strip starts to heat up. Because the two metals expand differently, the strip bends gradually. At sufficient deflection it releases the trip lever and the contacts open.

Fig. 2 — The thermal mechanism: the bimetallic strip bends on overload and releases the trip lever

The thermal mechanism has an inverse-time characteristic relative to the overload: at 1.5×In it may take a few minutes, at 5×In a few seconds. IEC 60898-1 requires that the MCB must not trip in less than 1 hour at 1.13×In (the hold current), but must trip in less than 1 hour at 1.45×In.

Remember: An MCB at the edge of tripping (current just above 1.13×In) can run for hours without tripping. The conductor can overheat for that whole time. That is why it is essential that the MCB’s In is matched to the current-carrying capacity of the protected cable (In ≤ Iz).

2. Magnetic protection — short circuit

In a short circuit, the current rises sharply to values tens or hundreds of times In. The electromagnetic coil instantly generates a strong magnetic field that pulls in the moving core. This acts directly on the trip mechanism, opening the contacts in less than 10 milliseconds.

Fig. 3 — The magnetic mechanism: on a short circuit the coil instantly pulls in the moving core and opens the contacts (<10ms)

The magnetic trip current threshold varies with the characteristic (tripping curve) of the MCB:

Curve	Magnetic threshold	Typical applications
B	3–5 × In	Resistive circuits, lighting, long cables
C	5–10 × In	Socket-outlets, general domestic use
D	10–20 × In	Motors, transformers, discharge lamps

Fig. 4 — The I-t characteristic: thermal zone (inverse-time curve) + instantaneous magnetic zone (B/C/D bands)

3. The arc-extinction chamber

When a live contact opens, an electric arc forms between the two contacts — a column of ionised plasma at thousands of degrees Celsius. If the arc is not quenched quickly, the contacts melt and the MCB becomes unusable or even dangerous.

The extinction chamber contains a stack of thin steel plates that:

fragment the long arc into a series of short arcs
rapidly cool the plasma, reducing its conductivity
force de-ionisation of the gas between the contacts

All this happens within a few milliseconds. The hot gas is vented through ventilation openings in the MCB housing — which is why consumer units need ventilation and are not fully sealed.

Fig. 4 — The electric arc formed when the contacts open is fragmented and quenched by the plates in the de-ionisation chamber

4. Breaking capacity (Icu)

The breaking capacity (Icu) is the maximum short-circuit current the MCB can interrupt without destroying itself. It is expressed in kA and must be greater than the prospective short-circuit current at the point of installation.

Typical values:

6 kA — standard for domestic installations (a long distance from the transformer)
10 kA — needed at the main board of an apartment block or when the board is close to the transformer
15–25 kA — industrial installations or at the origin of the installation

Caution: An MCB with insufficient breaking capacity can explode under a heavy short circuit, causing fire or burns. The breaking capacity must be verified in the design by calculating or estimating the Icc.

What an MCB does NOT do

The MCB does not detect earth fault currents (residual currents). If a person touches the line conductor and the current passes through the human body to earth, the MCB will not trip (the current through the circuit is almost the same — the difference is too small to activate the thermal or magnetic mechanism at the lethal residual values of a few milliamps). That is the job of an RCCB or an RCBO.

Reference standard

The reference standard for domestic MCBs is IEC 60898-1 (SR EN 60898-1 in Romania), which defines the B/C/D characteristics, the minimum breaking capacity (1.5 kA standard, 6 kA enhanced) and the tripping times for overloads and short circuits.

The I7-2011 code refers to MCBs in the context of overcurrent protection (Ch. 4.3) and of protection selectivity (Art. 4.3.7.1): cascaded MCBs must be coordinated so that, on a fault, the one nearest the fault trips, not the one at the incomer.

ElectroSchema

In the visual consumer unit in ElectroSchema, the MCB is placed on the DIN rails with the chosen rated current and characteristic. The validation rules V29 and V44 automatically check the selectivity between the MCB on the sub-main and those on the circuits, warning you if the magnetic band of the upstream MCB overlaps that of the downstream one.

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