What is an RCBO? And do you have one in your consumer unit?

An RCBO is a single breaker that does two jobs at once: it protects the circuit against overload and short circuits (like an MCB), and it also detects earth fault currents that could indicate someone is getting a shock (like an RCD). One device, one circuit, both types of protection. This article explains what that means in practice, how to tell whether you already have RCBOs, and what the difference feels like when something goes wrong.

RCBO: the two-minute explanation

To understand what an RCBO does, it helps to know the two devices it replaces.

An MCB (Miniature Circuit Breaker) monitors the current flowing through a circuit. If that current exceeds the rating of the breaker — because of an overloaded cable or a short circuit — the MCB trips and disconnects the circuit. This protects the wiring from overheating.

An RCD (Residual Current Device) does something different. It compares the current leaving a circuit along the live conductor with the current returning along the neutral. In normal operation, they should be equal. If there is a difference, it means current is leaking somewhere — potentially through a person. An RCD detects that imbalance and cuts the power in milliseconds, fast enough to prevent a fatal shock in most cases.

An RCBO combines both functions in a single unit. Each circuit gets its own MCB protection and its own RCD protection, all in one breaker. There is no shared RCD covering a group of circuits. Every circuit stands alone.

How this differs from a standard split-load board

Many homes have what is known as a split-load consumer unit. On a split-load board, circuits are divided into two groups. Each group sits behind a shared RCD block. The individual breakers on each side are plain MCBs — they provide overload protection, but no earth fault protection of their own. The RCD block does that job for everything on its side.

That arrangement has one significant drawback. If any circuit on a shared RCD side develops an earth fault — a hairdryer dropped in a sink, moisture in a light fitting, a damaged cable — the whole RCD trips. Every circuit on that side of the board goes off at once: lights, sockets, appliances, all of it. You lose half the house.

With RCBOs, only the circuit where the fault actually occurred trips. Your kitchen sockets may go off, but the lights stay on. The fridge keeps running. Everything else in the house is unaffected. This is the key practical difference a homeowner notices — and it makes fault-finding considerably easier, because you already know which circuit the problem is on.

How to tell if you have RCBOs in your consumer unit

Open the cover of your consumer unit and look at the breakers. There are two things to check.

Width

RCBOs are noticeably wider than standard MCBs — roughly double the width. A row of RCBOs looks more spread out and substantial than a row of plain MCBs. If all the breakers in your board are the same, wider size, that is a good sign you have an RCBO board.

The test button

Each RCBO has a small test button on the front face of the unit, usually labelled T or TEST. This is separate from the main rocker switch that you push to reset a tripped breaker. Pressing it trips that individual RCBO — useful for testing that the RCD function is working.

Standard MCBs do not have a test button. On a split-load board, the test buttons live on the larger RCD blocks at the top of the board, not on the individual MCBs. If every single breaker in your board has its own small test button, you have RCBOs.

What happens when an RCBO trips

When an RCBO trips — for either an overload or an earth fault — only that one circuit goes off. Every other circuit in the house stays on. This is the most immediately noticeable difference from a split-load board, where a single fault can take out half the property.

A tripped RCBO will sit in the OFF position or in a middle position between ON and OFF. To reset it, push it firmly to OFF first to reset the mechanism, then push to ON. If it trips again immediately, a fault is still present on that circuit. In that case, do not keep trying to force it on.

Instead, unplug everything on that circuit and try again. If it holds, one of your appliances has a fault and you can identify it by plugging them back in one at a time. If it still trips with everything unplugged, the fault is in the fixed wiring — at which point, call an electrician. See our guides on an RCD that keeps tripping and a tripped MCB for a step-by-step fault isolation process.

Does having RCBOs mean better protection?

Yes, in two distinct ways.

First, every circuit gets its own earth fault protection. On a split-load board, some circuits — often lighting — sit on the unprotected side of the board with no RCD coverage at all. On an RCBO board, every circuit has both overload protection and residual current protection, without exception.

Second, a fault on one circuit does not affect anything else. On a split-load board, a shared RCD tripping takes out multiple circuits at once, including circuits that have no fault. With RCBOs, the fault stays contained. The rest of the house is unaffected.

The 18th Edition of BS 7671 (the current UK wiring regulations) reflects this: it favours full RCBO boards for new domestic installations precisely because they provide more complete protection per circuit without the group-trip drawback of a shared RCD.

If your home currently has a split-load board and you are considering upgrading, our consumer unit upgrade guide covers what is involved.

How to reset a tripped RCBO

The reset process is straightforward, but the order matters.

1. Locate the tripped RCBO. It will be in the OFF position or sitting in a middle position between ON and OFF.
2. Push the rocker firmly to OFF. This resets the internal mechanism and is required before you can restore power.
3. Push the rocker firmly to ON.
4. If it holds, the fault has cleared — likely a transient overload or a brief earth fault from an appliance.
5. If it trips again immediately, a fault is still present. Unplug everything on that circuit and try once more. If it still trips with all appliances removed, there is a fault in the fixed wiring and you should call an electrician.

What an RCBO board looks like vs. a split-load board

The two types of consumer unit look quite different when you open the cover.

RCBO board

At the top of the board, you will see a main switch — typically a large double-pole isolator. Below it sits a single row of identically-sized RCBO units. There are no separate RCD blocks. All the breakers are the same width, and each one has a small test button. The layout is clean and uniform.

Split-load board

A split-load board has one or two large RCD switches (wider and taller than the circuit breakers) plus rows of smaller, narrower MCBs beneath each one. The MCBs have no test buttons of their own. If you see that combination — a large RCD block plus smaller MCBs — you have a split-load board.

For a visual comparison of the different types of consumer unit and what each one means for your installation, see our guide to types of consumer unit.