MCBs, RCDs and RCBOs — What Do All the Switches Do?

Open your consumer unit and you'll see a row of switches, probably labelled with things like "Lights", "Sockets", "Cooker". But what are those switches actually doing? Why are some of them slightly different sizes? And what does it mean when your electrician tells you that you need "RCBOs throughout"?

There are three types of device in a modern consumer unit, and they each do a completely different job.

The Three Devices

MCB — Miniature Circuit Breaker

Think of an MCB as a bouncer on the door of a nightclub. Its one job is to make sure too many people don't try to get in at once. In electrical terms, if current exceeds the safe limit for the cable — because you've plugged in too many things, or something has short-circuited — it trips and cuts the power. MCBs protect your wiring. They do not protect you from electric shock.

RCD — Residual Current Device

An RCD constantly compares the current going out along the live wire with the current coming back along the neutral. If they're not equal, current is escaping somewhere it shouldn't — through faulty insulation, through water, or through a person. The moment it detects an imbalance, it cuts the power in around 30 milliseconds. Fast enough to prevent a fatal electric shock. An RCD protects you. It doesn't care about overloads — that's the MCB's job.

RCBO — Residual Current Breaker with Overcurrent Protection

An RCBO does both jobs in one device. It monitors overloads (MCB function) and earth faults (RCD function) simultaneously, for a single circuit. If one circuit develops a fault, only that circuit trips. Not half the house — just the one.

Why Does This Matter in Practice?

With an all-RCBO board, if your washing machine trips, only the washing machine circuit goes off. Everything else stays on. You immediately know which circuit has the problem, and you can carry on cooking while you investigate.

When Should You Care About This?

You're getting a new consumer unit. Ask for RCBOs throughout. The difference in cost is small; the difference in performance is meaningful. I fit all-RCBO boards as standard.

Your RCD keeps tripping and taking multiple circuits with it. An RCBO board tells you exactly which circuit has the problem.

What Does Your Board Have?

If you open the cover of your consumer unit (the front panel — don't touch the wiring behind it), you can usually tell what you've got:

MCBs only, no RCDs — your board predates current safety standards. You've got overload protection but no earth fault protection. This is typically flagged during an EICR and is the most common reason for a consumer unit upgrade recommendation.

MCBs plus one or two larger RCD switches — this is a split-load board and was the standard for many years. It gives you earth fault protection, but in groups. A fault on one circuit can trip several. Still meets minimum safety requirements if everything's working properly.

All RCBOs — you've got the current best setup. Every circuit has its own combined overload and earth fault protection. If this is what's in your board, you're in good shape.

Do You Need to Upgrade?

Not necessarily. If your current setup passes an EICR and has RCD protection, it meets the safety standards. Nobody should be telling you that you must upgrade to RCBOs just because they're the newer technology — that's like saying your car needs replacing because a newer model came out.

But if you're already having work done on your board — maybe adding a circuit for an EV charger or a new kitchen — it's worth discussing an upgrade at the same time. The labour is the expensive part, not the devices. And if your board currently has no RCD protection at all, that's a more urgent conversation.

If you're not sure what you've got, the simplest thing is to send me a photo of your consumer unit on WhatsApp. I can usually tell you what's in there and whether anything needs attention, all from a picture.

Frankie Sewell
NICEIC Approved Contractor • YRLA Recognised Service Provider • Bright Sparks of York