The short answer
A dual-RCD consumer unit — sometimes called a high-integrity board — divides circuits into two groups, each covered by a shared RCD. If any circuit in a group develops an earth leakage fault, the RCD for that group trips, taking all circuits in that group off together. A full RCBO consumer unit replaces each individual MCB with a combined RCBO device that provides both overload and residual-current protection per circuit, so only the single faulted circuit trips. Under the 18th Edition of BS 7671, the RCBO configuration is now the more common default for new domestic installations because it provides finer fault isolation. Dual-RCD boards remain a compliant option but require careful circuit assignment to minimise the impact of a group trip.
Both configurations meet the residual-current protection requirement of BS 7671, but they handle faults differently. The difference is most noticeable when something goes wrong: one trips a group of circuits, the other trips just one.
Dual-RCD vs RCBO
- Dual-RCD (high-integrity)Two shared RCDs; MCBs per circuit
- RCBO boardOne RCBO per circuit; no shared RCDs
- Fault behaviour (dual-RCD)Trips the whole group
- Fault behaviour (RCBO)Trips only the faulted circuit
- Cost differenceRCBO typically £200–£400 more installed
How a dual-RCD board is structured
A dual-RCD (high-integrity) consumer unit has the following arrangement:
- A main switch at the top isolates the whole board.
- Two shared RCDs — sometimes referred to as the 'left bank' and 'right bank' — each cover a group of circuits.
- Each circuit has its own MCB (miniature circuit breaker) for overload and short-circuit protection.
- Residual-current protection for each circuit comes from the shared RCD that covers its group.
The critical design question for a dual-RCD board is: which circuits go on which RCD? A well-designed board puts circuits that are genuinely likely to cause nuisance tripping (old appliances, outdoor sockets) on one RCD, and critical circuits (freezer, alarm panel, boiler) on the other. A poorly designed board may leave important loads sharing an RCD with noisy circuits, meaning a fault elsewhere takes them down too.
The 'high-integrity' name comes from the attempt to ensure that critical circuits are always on a live RCD group. Some boards go further and provide three groups (main switch plus two RCDs, with some circuits having double protection), but the two-group arrangement is the most common.
How a full RCBO board is structured
A full RCBO consumer unit replaces the MCBs and shared RCDs with a single RCBO device per circuit. Each RCBO:
- Provides overload and short-circuit protection (the MCB function).
- Provides residual-current protection (the RCD function) tuned to that circuit.
- Trips only its own circuit — there is no group effect.
The practical result is that a fault on the dishwasher circuit trips the dishwasher's RCBO; everything else stays live. The freezer keeps running, the lights stay on, and the boiler keeps firing. The board is more resilient to the consequences of individual faults.
An RCBO board is also slightly simpler to manage from a user perspective: if a circuit trips, the tripped RCBO is the one that has moved to the 'off' position, so finding and resetting it is straightforward.
| Characteristic | Dual-RCD board | RCBO board |
|---|---|---|
| Protection devices | Main switch + 2 RCDs + MCBs per circuit | Main switch + 1 RCBO per circuit |
| Fault isolation | All circuits on that RCD trip | Only the faulted circuit trips |
| Circuit design freedom | Requires careful group assignment | No group to design |
| Typical installed cost (10-way) | ~£500–£800 | ~£800–£1,200 |
| BS 7671 preference (new installs) | Compliant but less preferred | Preferred / most common new default |
| Identifying a trip | Which RCD tripped; then which MCB | Which RCBO tripped — one device |
Indicative comparison. Sources: IET BS 7671 18th Edition; Electrical Safety First guidance; Checkatrade and MyJobQuote cost guides.
Which does BS 7671 18th Edition prefer?
The 18th Edition of BS 7671 does not prohibit dual-RCD boards, but its guidance and the broader direction of practice has moved toward the RCBO configuration for new domestic installations. The reasoning is that individual protection per circuit provides a more discriminative response to faults — only the affected circuit is interrupted — which is both more resilient for the occupant and easier to diagnose and reset.
In practice, most registered electricians now default to RCBO boards for consumer unit replacements, particularly following the widespread adoption of the 18th Edition from 2018 onwards. Dual-RCD boards remain installed in many UK homes and are not required to be replaced — but when a board is being replaced, the RCBO configuration is typically the recommended choice.
Frequently asked questions
What does 'high-integrity' mean on a consumer unit?
A high-integrity consumer unit is a type of dual-RCD board designed to ensure that genuinely critical circuits (such as a freezer or alarm panel) remain live if one RCD group trips. It does this by dividing the circuits between two RCD groups, with the aim that a fault on one group does not take down the most important loads. The term refers to the design intent, not a different safety standard.
Can I tell from looking at my board whether it's dual-RCD or RCBO?
Yes. On a dual-RCD board, you will see two wider switches (the RCDs) with a row of smaller switches (MCBs) on each side. On an RCBO board, every device in the row is the same width — each one is a combined RCBO with its own test button. If every breaker has a test button, it is an RCBO board.
Is it worth upgrading from dual-RCD to RCBO?
Only if the board needs replacing for another reason, or if the group-trip behaviour is causing a real problem (a persistent fault taking down critical loads). Upgrading a sound, compliant dual-RCD board purely to switch to RCBO configuration is an improvement in resilience, not a safety requirement.
Sources & further reading
- IET — BS 7671 18th Edition wiring regulations
- Electrical Safety First — consumer unit replacement best practice guide
- NICEIC — consumer units explained
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.