RCD board vs RCBO board

The short answer

The key difference is how faults are isolated. An RCD board groups circuits behind one or two shared RCDs — if a fault triggers the RCD, all the circuits on that half of the board lose power. An RCBO board fits a combined breaker (which does the job of both an MCB and an RCD) per circuit, so a fault on any one circuit trips only that circuit. The RCBO board provides better fault isolation and is the setup mandated for most new domestic installations under BS 7671; it costs more — typically £800–£1,200 installed versus £500–£800 for an RCD board. For a new installation or full replacement, an RCBO board is the better-specified choice. An RCD board is not unsafe, and a sound, compliant one does not need replacing purely on grounds of type.

Both board types provide the residual-current protection required by BS 7671, but they do it differently. Understanding that difference is useful if you are choosing between them for a replacement or comparing two quotes.

RCD boardshared RCDs; cheaper; trips a group on fault
RCBO boardone breaker per circuit; trips only that circuit
Typical installed cost (RCD)~£500–£800
Typical installed cost (RCBO)~£800–£1,200
New installation standardRCBO board preferred under BS 7671

How each type protects your circuits

An RCD board — also called a dual-RCD board or high-integrity board — splits the circuits into two groups. Each group is protected by a shared RCD. The individual circuit breakers (MCBs) protect against overload and short circuit; the shared RCD detects earth leakage. If something on a circuit leaks current to earth, the RCD for that group trips, taking all the circuits in that group off.

This has a practical consequence: if your freezer circuit and your kitchen lighting are on the same RCD, a fault on the freezer trips the kitchen lights too. Many dual-RCD boards try to address this by splitting 'critical' circuits (fridges, freezers) to a separate group, but this requires careful design when the board is installed.

An RCBO board replaces each MCB with a combined RCBO device that contains both the overload/short-circuit protection and the residual-current protection in one unit. A fault on any circuit trips only that circuit — the rest of the board keeps running. This is a more resilient arrangement and is the one most consistent with current BS 7671 design practice for domestic installations.

Feature	RCD board	RCBO board
Protection per circuit	Shared RCD + MCB per circuit	RCBO per circuit (combined)
Fault isolation	Half the board trips	Only the faulted circuit trips
Critical circuit separation	Requires careful design	Not required — each isolated
Typical installed cost	~£500–£800	~£800–£1,200
Unit complexity	Simpler; fewer devices	More devices; more to service
Compliance (new installs)	Acceptable but less preferred	Preferred under BS 7671

Indicative comparison for guidance. Source: IET BS 7671 18th Edition; Electrical Safety First best practice guidance; Checkatrade and MyJobQuote cost guides.

Is an RCD board 'unsafe'?

No. A properly installed, compliant RCD board — with the circuits correctly divided between its two RCDs and all circuits tested and certified — provides the residual-current protection required by BS 7671. The RCD trips fast enough to prevent most fatal electric shocks, and the MCBs protect against fire from overload. The drawback is the group-trip behaviour, not a safety failure.

An older RCD board that was installed correctly, has not been damaged, and passes its periodic test is not unsafe and does not need replacing purely because RCBO boards are now more common. The driver to replace is condition and compliance, not board type alone.

Where an RCD board becomes a concern is if the RCDs themselves are old, known to have nuisance-tripped repeatedly, or if the circuit assignment means genuinely critical loads (life-support, medical devices, a sole heating system in winter) are sharing a group with other circuits. In that case the resilience argument for an RCBO board carries real weight.

For a new replacement: if your board is being replaced anyway, most electricians and BS 7671 design guidance now default to the RCBO configuration. The cost difference on a replacement is typically £200–£400 more, and the benefit of per-circuit isolation is genuine. If budget is a hard constraint, a well-designed dual-RCD board is not unsafe.

Which to choose for a replacement

For a new or replacement consumer unit installation, the case for an RCBO board is straightforward:

Per-circuit fault isolation — a tripped circuit does not affect others.
No need to carefully design the split between RCD groups.
More consistent with current BS 7671 design practice and what most registered electricians now recommend as the default.
The cost difference is moderate (roughly £200–£400 more on the installed price for a typical 10-way board) and represents a permanent installation that should last many years.

For a repair or small upgrade to an otherwise sound existing RCD board, there may be no need to change the whole arrangement. The most useful conversation is with the registered electrician quoting the job, who can explain whether the existing board type is adequate for your specific circumstances.

Frequently asked questions

Is an RCBO board worth the extra cost?

For a new installation or a full replacement, the extra cost — typically £200–£400 on the installed total — is modest relative to the per-circuit fault isolation it provides. If budget is tight, a correctly designed dual-RCD board is a compliant alternative. If the board is being replaced anyway, most electricians now default to RCBO.

Will an RCBO board stop my whole house tripping?

Yes, in the sense that a fault on one circuit will only trip that circuit's RCBO, leaving the rest of the board live. On a dual-RCD board, a fault trips the whole group protected by that RCD. This is the main practical argument for the RCBO configuration.

Can I upgrade a dual-RCD board to RCBO without replacing the whole unit?

Sometimes. If the enclosure has spare ways and the busbars are compatible, individual MCBs can be replaced with RCBOs on some board designs. This is a partial upgrade rather than a full replacement and may not be possible on all board types. Ask a registered electrician whether your specific board supports it.

Sources & further reading

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.