RCBO vs MCB: When Do You Need a Full RCBO Consumer Unit?

Since the 18th Edition Amendment 2 came into force in September 2022, the question of whether to fit a dual-RCD board or a full RCBO board has become one of the most common specification decisions on domestic installs. The rules changed. Many electricians are still working out exactly when each is required. Here's the definitive practical answer.

A single RCBO provides both overcurrent protection (like an MCB) and 30mA RCD protection in one device — one circuit, one module.

What's the Difference Between an MCB, RCD, and RCBO?

These three devices do different jobs — and confusing them is easy if you haven't worked through the theory recently.

• MCB (Miniature Circuit Breaker) — protects against overcurrent and short circuit. Trips when the current exceeds the rated value (e.g. 32A on a ring circuit). No protection against earth faults or shock hazard.
• RCD (Residual Current Device) — detects earth fault current and trips when an imbalance is detected between live and neutral (typically at 30mA). Protects against shock and some fire risks. No overcurrent protection.
• RCBO (Residual Current Breaker with Overcurrent) — combines both functions in a single device. Provides MCB-level overcurrent protection AND 30mA RCD protection for one circuit. One module, one circuit.

In a traditional dual-RCD consumer unit, two RCDs each cover half the circuits. In a full RCBO board, every circuit has its own RCBO — individual protection, no shared RCDs.

What Did the 18th Edition Amendment 2 Change?

The key change in Amendment 2 (effective September 2022) was to Regulation 411.3.4, which now requires 30mA RCD protection for all final circuits in domestic premises — not just "certain" circuits as before. This means:

• Every socket outlet circuit — protected (was already required)
• Every lighting circuit — now required (previously common to omit)
• Cooker circuits, immersion heaters, EV chargers — all require 30mA RCD protection
• Every circuit concealed in a wall at less than 50mm depth — protected

In practice, in a typical domestic install, every circuit in the consumer unit now needs 30mA RCD protection. The dual-RCD approach still satisfies this — provided both RCDs are 30mA Type A and all circuits sit behind one of them. But there's a catch.

The Problem with Dual-RCD Boards

Dual-RCD consumer units split circuits across two RCDs. When a fault trips one RCD, all circuits behind that RCD go dead simultaneously. In a house, that typically means half the sockets, half the lights, and whatever fixed appliances are on that split go off at once.

For most domestic installs, this is a nuisance issue. But there are scenarios where it becomes a specification problem:

• Critical circuits — if a fridge-freezer, medical equipment, or alarm system shares an RCD with a circuit that's likely to trip (e.g. a bathroom circuit), a fault on one takes out the other
• Discrimination — in theory, a fault on one circuit should only trip that circuit's protection. Shared RCDs can't discriminate between circuits behind them
• Nuisance tripping investigations — when an RCD trips, you lose all circuits on that half while you investigate. RCBO boards let you isolate the fault circuit while everything else stays live

A full RCBO consumer unit — each circuit has its own overcurrent and RCD protection. A fault on one circuit trips only that RCBO.

When a Full RCBO Board Is the Right Specification

There's no blanket Amendment 2 requirement to use RCBOs — dual-RCD boards are still code-compliant for most domestic work. But an RCBO board is the better specification in these situations:

• New builds and full rewires — the cost difference between a dual-RCD board and an RCBO board is modest at fit-out stage. The fault-finding and discrimination benefits are worth it on a new install.
• Premises with critical circuits — medical equipment, alarms, freezers. Isolating these on their own RCBO means a fault elsewhere can't knock them out.
• HMOs and rental properties — landlords value individual circuit discrimination. Easier for tenants to reset a tripped RCBO than to work out which half of the board to reset.
• Where the client specifies it — many electricians now specify RCBO boards as standard on all domestic work. The conversation is simpler: every circuit has individual protection, end of discussion.

Consumer Unit Options at APM

We stock both approaches. For dual-RCD installations with SPD (surge protection — now required on new consumer unit installs under Amendment 2), the Axiom Metal Type A Dual RCD 10-Way Consumer Unit with SPD covers most domestic rewire and new-build specs. The Axiom 12-Way Consumer Unit 100A is the standard domestic unit for bigger homes with more circuits.

For RCBO builds, the FuseBox 11-Way RCBO Consumer Unit and FuseBox 15-Way RCBO Consumer Unit come pre-populated with Type A RCBOs — you're buying a complete unit, not building it MCB by MCB.

For top-up MCBs on existing boards, the Axiom 32A Type B MCB covers ring circuits. We stock 10A through 63A in Type B curve.

Don't Forget SPD — It's Now Required

Amendment 2 also added a requirement for Surge Protection Devices (SPDs) on all new consumer unit installations where the installation is in a building protected against lightning, or where the consequences of overvoltage could be significant. In practice, SPD is now standard specification on all new domestic consumer units. Check that whatever unit you're specifying includes an SPD or has provision to fit one.

In Stock — Acton W3 Trade Counter

Consumer units, MCBs, RCDs, and SPDs in stock for same-day collection. Browse online or come to the counter:

Consumer Units | MCBs & Circuit Breakers

APM Electricals & Plumbing Supplies
24 Western Avenue, Acton, London W3 7TZ | 020 8702 8080 | www.apmi.uk