RCBO vs RCD: What Is the Difference in a Consumer Unit?

The difference is scope. An RCD protects a group of circuits against current leaking to earth, which is what happens in an electric shock, while an RCBO does the same job for one circuit only and adds the overload and short-circuit protection of a normal circuit breaker in the same device. In practice, that decides how much of the house goes dark when something trips.

MCB, RCD and RCBO in one line each

Three devices do most of the work in a modern consumer unit, and the names get mixed up constantly.

• MCB (miniature circuit breaker): trips on overload or short circuit. It protects the cable, not you.

• RCD (residual current device): trips when current leaks to earth, set at 30 mA for most home circuits. It protects people.

• RCBO: an MCB and an RCD in one device, protecting a single circuit against both.

The IET's guidance makes one point worth remembering: an RCD on its own (strictly an RCCB) gives no overcurrent protection at all. Its amp rating tells you how much current it can carry, not when it will trip, so every circuit behind it still needs its own breaker. In a typical home, each of your circuits has either an MCB behind a shared RCD, or its own RCBO. For the detail of how the leakage detection works, see our guide to what an RCD is.

MCB vs RCBO: when does the difference matter?

An MCB alone gives no protection against electric shock from a leakage fault. That was normal in boards fitted before RCDs became standard, and it is why so many older boards have a row of MCBs and nothing else. BS 7671 now requires 30 mA RCD protection on almost every circuit in a home, including socket circuits, lighting and cables buried in walls, so a new board built from plain MCBs would have to put them behind an RCD. An RCBO avoids that by building the RCD into each circuit, which is why RCBOs on some or all circuits are now a common choice in a consumer unit replacement.

Why one fault can take out half the house

On an RCD board, one leaking appliance on one circuit trips the RCD, and every circuit behind that RCD goes off with it. The IET's consumer guidance explains why boards are usually split across two RCDs: so that if one trips, the other half of the house stays on, and you are not left in complete darkness if the upstairs and downstairs lighting sit on different sides.

There is a second, quieter problem. Many appliances leak a small amount of current to earth in normal use. On an RCD board that leakage adds up across every circuit on the same RCD. The IET notes that a 30 mA RCD will typically trip at between 18 and 23 mA, and BS 7671 limits the standing leakage on circuits protected by an RCD to 30% of its rating, which is 9 mA on a 30 mA device. Put enough modern electronics behind one RCD and it can trip for no obvious reason. With RCBOs, each circuit carries only its own leakage.

The four consumer unit layouts you will be offered

Board names are used loosely, so ask which of these you are actually getting.

Split load: the older layout. Some circuits sit behind one RCD and the rest have MCBs only, with no RCD protection. It was common before the 18th Edition rules extended RCD protection to lighting and most other circuits, and it is rarely a sensible choice for new work.

Dual RCD: two RCDs, each protecting roughly half the circuits. It is the cheapest layout that gives RCD protection throughout, and the one where a single fault takes out the most.

High integrity: two RCDs covering most circuits, plus spare ways for RCBOs on circuits you do not want to lose to someone else's fault, such as a freezer or a fire alarm supply.

Full RCBO: every circuit has its own RCBO. A fault trips one circuit and nothing else, which also makes it faster to find.

Electrical Safety First's best practice guide for replacing a consumer unit recommends that all circuits are protected either by Type A RCBOs or by enough Type A RCDs, so a full RCBO board or a well-divided RCD board can both meet that.

RCD type matters as much as layout

RCBOs and RCDs also come in types, marked AC, A, F and B. Type AC detects alternating leakage only. Type A also handles pulsating DC, and Type B also detects smooth DC, which is why an EV charger circuit needs a Type B device unless the charger has its own DC fault protection. The IET's FAQ is clear that Type AC devices are only suitable for sinusoidal residual current, and that the DC content produced by modern electronics can "blind" them so they fail to operate. Electrical Safety First's EICR coding guide lists a Type AC RCD where a Type A is required as a C3 (improvement recommended). When comparing quotes, check the devices are Type A or better.

RCBO consumer unit or dual RCD: which suits your home?

For most homes we recommend a full RCBO board or a high integrity board, because the extra cost is small against the inconvenience it saves. Checkatrade's June 2026 cost guide puts a 10-way board supplied with MCBs at around £120 and one supplied with RCBOs at around £250, a difference that is modest against the total price of a board change.

A dual RCD board can still make sense in a small flat with few circuits and little electronic load. It makes less sense in a family home with a home office, a freezer in the garage, an alarm system or anything else you do not want switched off by a fault elsewhere.

Changing the board does not always mean changing the wiring. If your circuits test well, the new board connects to the existing cables, which we cover in can you replace a fuse box without rewiring.

If your board trips more than it should, or you want to know what a change would involve, SM Electrical covers Shifnal, Telford, Wolverhampton and the surrounding area, including anyone looking for an electrician in Bridgnorth, and we are happy to look at what you have.

RCBO and RCD questions, answered

Do RCBOs need testing like an RCD?

Yes. Each RCBO has its own test button, and BS 7671 (regulation 514.12.2) calls for a notice on the board asking you to press it every six months. On a full RCBO board that means working along the row, which takes a couple of minutes.

Can I swap the RCDs in my board for RCBOs?

Sometimes, if the board has room and the manufacturer makes compatible RCBOs for it, but it is work inside the consumer unit and should be done and tested by an electrician. Mixing devices from different manufacturers in one board is not something to guess at.

Is a split load board dangerous?

Not automatically. An older installation built to earlier rules is not unsafe just because it does not meet the current edition. But circuits with no RCD protection will usually be noted on an EICR, with sockets likely to power equipment outdoors coded C2 (potentially dangerous).