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 — in a single 17.5mm-wide module. 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 — The RCBO takes up 1 module versus 3 modules for RCCB+MCB, but has more limited selectivity

Criterion	RCBO	RCCB + MCB
DIN modules	1 module (17.5mm)	3 modules (52.5mm)
Selectivity with upstream RCD	Limited (depends on manufacturer)	Excellent (type S RCCB)
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 ✓	Does not fit
New installation, generous space	Acceptable	Recommended ✓

The selectivity limitation — why it matters

The most important limitation of the RCBO is selectivity against an RCCB at the board incomer. According to Art. 4.1.5.2.8 of I7-2011, for cascaded RCCB selectivity, the sensitivity ratio must be at least 3:1.

The problem: RCBOs have their rated residual current fixed (usually 30mA). If the incoming RCCB is 100mA, the ratio is 100/30 ≈ 3.3 — only just within the limit. If the incoming RCCB is also 30mA (a frequent mistake), selectivity is impossible.

Rule of thumb: If you use 30mA RCBOs on the circuits, the main RCCB at the board incomer must be type S (selective) with a sensitivity of 100mA or 300mA — not 30mA as well. 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 feeder protection
→ 1× RCBO 30mA / 16A C — living-room socket circuit
→ 1× RCBO 30mA / 16A C — bedroom 1 socket circuit
→ 1× RCBO 30mA / 16A C — bedroom 2 socket circuit
→ 1× RCBO 30mA / 10A B — lighting circuit
→ 1× RCBO 30mA / 20A C — washing-machine circuit (dedicated)
→ 1× RCBO 30mA / 20A C — boiler circuit (dedicated)

Total: ~8 modules = a 12-module board with free space for extension

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 — it is usually marked with the corresponding symbol from IEC 62423.

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 greater than the one downstream (per Art. 4.1.5.2.8 of I7-2011). Rule V02 checks that all socket circuits have residual-current protection ≤ 30mA.

Discussion

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