The short answer
An RCBO (residual current breaker with overload) is a single device that combines the functions of a miniature circuit breaker (MCB) and a residual current device (RCD) in one module. It sits in a single slot in a consumer unit, protects a single circuit, and will trip on three distinct events: an overload (too much current for the cable), a short circuit (a direct fault between live and neutral), or an earth-leakage fault (current taking an unintended path to earth, which can cause electric shock). The key advantage over a shared-RCD arrangement is isolation: when a fault occurs, only the affected circuit trips — the rest of the installation stays on. Under BS 7671 18th Edition the RCBO arrangement is the preferred approach for new or replacement domestic consumer units, and a full RCBO board typically costs more than a dual-RCD board because each of the individual devices is more expensive than a plain MCB.
A full RCBO board is now the arrangement most electricians recommend for new and replacement domestic consumer units. Understanding what an RCBO does — and how it differs from a shared-RCD arrangement — makes it easier to decide whether the extra cost is justified for your installation.
RCBO key facts
- Full nameResidual current breaker with overload
- ContainsMCB function + RCD function in one device
- Sensitivity30 mA (domestic)
- Protects againstOverload, short circuit, and earth leakage
- Trip impactSingle circuit only
- Preferred byBS 7671 18th Edition for new domestic boards
RCBO versus MCB and RCD separately
On a dual-RCD board, the circuits are split between two RCDs. Each RCD covers a group of circuits; each circuit has its own MCB for overload protection. If an earth fault occurs on any of the circuits behind one RCD, that whole group loses power — typically half the house — because the RCD cannot tell which circuit caused the trip. You then need to go through the MCBs one by one to narrow down the fault.
On a full RCBO board, every circuit has its own RCBO. An earth fault on the shower circuit trips only the shower circuit; every other circuit stays on. The lights, fridge and router all keep running while you investigate. This is the key practical benefit — particularly in larger homes or those with many circuits, where a half-board trip can be disruptive.
| Board type | Earth-fault trip impact | Overload trip impact | Relative cost |
|---|---|---|---|
| Single RCD (older) | Whole board trips | Individual MCB trips | Lowest |
| Dual-RCD / split-load | Half the board trips | Individual MCB trips | Mid-range |
| Full RCBO board | Single circuit trips | Single circuit trips | Higher |
Comparison of consumer unit arrangements. Costs are relative indicators; see the cost pages for typical installed price ranges. Source: BS 7671:2018+A2:2022.
How an RCBO works inside
An RCBO has two independent trip mechanisms sharing a single case and a common contact pair:
- Overload / short-circuit trip (MCB element): a bimetallic strip deflects slowly under sustained overload, releasing a latch; a solenoid responds rapidly to a short circuit. Either event opens the contacts.
- Earth-leakage trip (RCD element): a toroidal transformer monitors the balance between live and neutral currents. Any imbalance exceeding the device's rated sensitivity (30 mA for domestic use) energises a relay that opens the same contacts.
Because both mechanisms operate on the same contacts, the RCBO snaps open regardless of which event triggers it. The test button on an RCBO checks only the RCD element; the MCB element is tested indirectly by the periodic inspection regime (EICR).
When a full RCBO board is worth the extra cost
A full RCBO board costs more than a dual-RCD board — the individual RCBO devices are more expensive than plain MCBs, and a 10- or 12-circuit installation multiplies that difference. Whether the extra is worthwhile depends on your situation:
- If you have a large household where a half-board trip causes significant disruption, finer isolation has a real practical value.
- If you have circuits where continuity matters — a fish tank, a sump pump, medical equipment, a freezer — losing half the board overnight could cause real harm. An RCBO board limits any trip to one circuit.
- For a rental property or a home you are selling, a full RCBO board is increasingly what buyers and tenants expect to see on a modern installation.
For a smaller property with simpler circuits, a dual-RCD board to current standards provides perfectly adequate protection, and the cost saving may be meaningful. The right answer depends on your installation and priorities — both arrangements are compliant with BS 7671 when correctly designed and installed.
Frequently asked questions
What does RCBO stand for?
RCBO stands for residual current breaker with overload. It combines the function of an MCB (miniature circuit breaker, which protects against overload and short circuit) and an RCD (residual current device, which protects against earth leakage) in a single module.
Is a full RCBO board better than a dual-RCD board?
In terms of fault isolation, yes — a fault trips only the affected circuit rather than half the board. Both arrangements can be installed to current BS 7671 standards. The RCBO board is more expensive; whether that is worth it depends on how disruptive a half-board trip would be for your household.
Can I replace an MCB with an RCBO in my existing consumer unit?
Potentially, but only if the RCBO is compatible with your consumer unit's busbar system. Each manufacturer's busbar uses a specific clip-in design, and not all RCBOs fit all boards. A registered electrician should check compatibility before any device is swapped, and the work must be carried out safely with the circuit de-energised.
Sources & further reading
- IET — BS 7671:2018+A2:2022 (18th Edition Wiring Regulations)
- Electrical Safety First — replacing a consumer unit best practice guide
- NICEIC — electrical safety guidance
Figures on this page are typical UK ranges drawn from published sources and depend on your specific property and installation. They are guidance, not a quotation.