RCBORCDMCBConsumer Unit

How an RCBO works

An RCBO (Residual Current Breaker with Overcurrent protection) combines the residual-current protection of an RCCB with the overcurrent protection of an MCB — usually in a single 17.5mm-wide module (some models take 2 modules). It is the practical choice for residential consumer units with limited space.

2 June 2026·9 min read·

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One device, three protections

If you have read the explanations for the MCB and the RCCB, you already understand 90% of the RCBO. It physically integrates both mechanisms: the toroid that detects the residual current and the bimetal/electromagnet that protects against overcurrent, both acting on a common tripping mechanism.

Fig. 3 — The RCBO covers all 3 electrical fault scenarios

Photo of an opened RCBO with the MCB and RCCB components labelled — Fig. 1 — Internal construction of an RCBO: left — the MCB section (thermal bimetal, electromagnetic coil, trip coil); right — the RCCB section (zero-sequence current transformer — both the LINE and NEUTRAL conductors pass through the toroid)

Fig. 1 — RCBO: the RCCB toroid and the MCB mechanism share the same contact assembly

The internal components

Looking at an RCBO in cross-section, inside its body we find:

The toroid — identical to the one in the RCCB, both conductors (L and N) pass through it
The electronic detection relay — an amplifier sensitive to the flux imbalance
The bimetallic strip — identical to the one in the MCB, for thermal protection
The electromagnetic coil — for instantaneous magnetic protection against short circuits
The common tripping mechanism — any signal from any protection opens the same set of contacts
The arc-quenching chamber — the same as in the MCB

Miniaturisation is achievable because manufacturers have managed to reduce the toroid to less than 15mm in diameter while keeping the detection sensitivity at 30mA.

The RCCB section: the detection toroid

The operation of the toroid in an RCBO is identical to that of a standalone RCCB — both conductors (L and N) pass through the same core, and any imbalance generates a trip signal.

The same principle as in the RCCB: the imbalance I_L ≠ I_N induces flux in the toroid and trips the relay

The MCB section: thermal and magnetic protection

The bimetallic strip and the electromagnetic coil are the same components as in a standard MCB. On overload, the bimetal bends slowly; on a short circuit, the coil trips instantly.

The thermal mechanism in the MCB section of the RCBO — the bimetallic strip bends on overload

The magnetic mechanism in the MCB section of the RCBO — instantaneous tripping on a short circuit (<10ms)

RCBO vs. RCCB + MCB: when to choose each

Fig. 2 — An RCBO usually takes 1 module per circuit; RCCB + MCB takes 1 module per circuit plus 2 modules for the RCCB shared by the group

Criterion	RCBO	RCCB + MCB
DIN modules	1 module (17.5mm)	RCCB (2 modules, shared by the group) + 1 module per circuit
Selectivity with upstream RCD	The same: needs a type S RCD upstream, with IΔn ≥ 3× the downstream one	The same: needs a type S RCD upstream, with IΔn ≥ 3× the downstream one
Cost per circuit	Higher (25–40 EUR)	Lower: MCB + share of RCCB
Selective replacement	You replace the whole RCBO	You replace only the faulty MCB
Small board (<12 modules)	Ideal ✓	Fits: N + 2 modules versus N with compact RCBOs
New installation, generous space	Acceptable	Recommended ✓

Selectivity with the upstream RCD

Selectivity against an RCD at the board incomer works the same way for an RCBO as for RCCB + MCB. The residual-current part of a general-type RCBO has the same tripping times as a general-type RCCB, 40...300 ms (Art. 4.1.5.2.7), so everything depends on the upstream device. In its 2011 wording, Art. 4.1.5.2.8 of I7-2011 put it by device type: selectivity between a type S device upstream and a general type downstream “may be considered achieved” if the ratio of their rated residual currents is at least 3. Since 2023 the article is a single sentence referring to Fig. 4.1/4.2 and to the recommendations of SR CEI 61200-53; the schemes in both figures keep the same ratio.

A general-type 100mA RCD at the incomer is not selective against a 30mA RCBO, even though the ratio is 100/30 ≈ 3.3: having no time delay, on a bolted fault it trips together with the RCBO. If the incoming RCCB is also 30mA (a frequent mistake), selectivity is impossible. The sum is exactly the same when the downstream device is a 30mA RCCB feeding a group of circuits. Where the two solutions really differ is between circuits: with RCBOs a fault takes out only its own circuit; with RCCB + MCB it takes out the whole group behind the RCCB. On 11 September 2026 we corrected this section: the first version claimed that the RCBO has poorer selectivity than RCCB + MCB, although the condition is the same for both.

Rule of thumb: If you use 30mA RCBOs on the circuits, the main RCCB at the board incomer must be type S (selective), preferably 300mA, as in Fig. 4.2 referred to by Art. 4.1.5.2.8 — not 30mA as well. A 100mA type S still meets the ratio of at least 3 from the 2011 wording of that article, but ElectroSchema flags it with a warning (V29), because it additionally applies a practice margin of two steps on the sensitivity scale. Otherwise, a fault on any circuit will also open the main RCCB, leaving the whole flat without power.

The RCBO in the residential board — practical layout

A typical RCBO board layout:

Example 3-room flat board:

→ 1× RCCB 300mA type S — main protection, board incomer
→ 1× MCB 40A — main switch of the board; it can protect the feeder against overload (Art. 4.3.2.1.2), but the feeder’s short-circuit protection stays at its origin, in the meter cabinet
→ 1× RCBO 30mA / 16A C — living-room socket circuit
→ 1× AFDD+RCBO 30mA / 16A C — bedroom 1 socket circuit
→ 1× AFDD+RCBO 30mA / 16A C — bedroom 2 socket circuit
→ 1× AFDD+RCBO 30mA / 10A B — lighting circuit (it also feeds the bedrooms)
→ 1× RCBO 30mA / 20A C — washing-machine circuit (dedicated)
→ 1× RCBO 30mA / 20A C — boiler circuit (dedicated)

Total: around 12 modules (13 with a 2P MCB), because the AFDD usually adds one module per circuit; widths differ between manufacturers, so go for an 18-module board.

Since 2023 the AFDD has been mandatory on all AC circuits of no more than 32 A in the rooms fitted out for sleeping in residential buildings (Art. 4.2.2.10) and is installed at the origin of the circuit it protects (Art. 4.2.2.12) — either a combined AFDD+RCBO or an AFDD module assembled with the RCBO, as the manufacturer specifies. The example was corrected on 12 September 2026: in the first version the three circuits serving the bedrooms appeared without an AFDD.

Type A mandatory for electronics: Computers, televisions and switch-mode power supplies can generate fault currents with pulsating DC components. The standard requires a type A RCBO (not AC) for circuits with such equipment. Check the type marked on the housing: the type A symbol is defined in IEC 61009-1 (IEC 61008-1 for RCCBs), while IEC 62423 covers only types F and B.

Normative reference

The reference standard for the RCBO is IEC 61009-1 (SR EN 61009-1 in Romania). The difference from IEC 61008 (RCCB) lies in adding overload and short-circuit protection per IEC 60898-1. The I7-2011 standard accepts the RCBO as a functional equivalent of the RCCB+MCB combination.

ElectroSchema

In the ElectroSchema distribution board, the RCBO is configured with its rated current, characteristic (B/C/D), residual sensitivity and type (AC/A). At validation, rule V29 checks cascaded RCD selectivity: the residual current of the upstream RCD must be at least 3 times the downstream one and, as a practice margin of the app itself (not a requirement written in I7), at least two steps higher on the IEC scale — which is why 30mA → 100mA gets a warning while 30mA → 300mA, the combination of Fig. 4.2 referred to by Art. 4.1.5.2.8 of I7-2011, passes. It does not check whether the upstream RCD is type S; that choice is yours. Rule V05 checks that all sockets have residual-current protection ≤ 30mA (directly or through an upstream RCD).

Discussion

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