What is the difference between an RCD, an RCBO and an MCB?

The three protective devices in a UK consumer unit, the different fault each one is watching for, and how to tell which you have from the front.

Difficulty: beginner
Time: 10 min
Read: 4 min
Safety: caution

Short answer

An MCB watches current and trips on too much of it — overload or short circuit. An RCD watches the balance between live and neutral and trips when current is leaking somewhere it should not go, usually through a person or damp. An RCBO is both in one module, protecting a single circuit, which is why a modern board with RCBOs only drops the one circuit that faulted.

People use "the trip switch" for all of these, which is why a tripping problem is so confusing to describe. They are three different devices watching for two completely different kinds of fault, and knowing which one operated tells you most of what you need to know before you touch anything.

Safety

Identifying the devices in your consumer unit is a look-and-read job done with the cover closed. Never remove the front cover of a consumer unit — live parts sit directly behind it and there is no owner-level reason to be in there.

What each device in a UK consumer unit does

Step by step

Find the main switch first.Usually the largest switch, often at one end or in the middle, marked 100A or 80A. It kills everything downstream of it. It is not a protective device — it is an isolator.
Identify MCBs by their amp rating.A row of small single-width switches marked 6, 10, 16, 20, 32 or 40 with a letter such as B or C. That number is the current at which the device starts to say no.
Identify an RCD by its test button.An RCD is wider, usually two modules, has a T or TEST button on it, and is marked with a milliamp figure — 30mA for the sockets and circuits people can touch. It sits upstream of a group of MCBs.
Identify an RCBO by both marks on one module.A single-circuit device carrying both an amp rating and a 30mA figure, with its own test button. One per circuit. Modern boards from roughly 2019 onward are usually all-RCBO.
Understand what an MCB is protecting.The cable. An MCB trips to stop the wiring in the walls overheating, either from a slow overload or from a dead short. It does nothing at all about someone touching a live wire.
Understand what an RCD is protecting.You. It compares the current going out on the live with the current coming back on the neutral. If more than about 30mA of the difference is going to earth — through wet insulation, a damaged flex, or a hand — it disconnects in well under a tenth of a second.
Note which device operated when something trips.This is the single most useful thing you can record. An MCB alone means a current problem on that circuit. An RCD with a group of circuits behind it means leakage somewhere in that whole group. An RCBO tells you both the circuit and, from which way the switch sits and any flag, often which fault.
Photograph and label the board once, while everything works.A labelled photo on your phone turns every future trip from an archaeology exercise into a two-minute job.

Tips

"RCD tripped" and "MCB tripped" are different problems with different causes. Say which one when you describe it to anyone.
A board where one RCD covers half the house is not faulty, just old-style. Splitting circuits onto RCBOs is a common and worthwhile upgrade at the point a board is replaced anyway.
The 30mA figure is not a shock-free promise. It is the level chosen to disconnect fast enough that a shock is survivable. Treat it as a last line, not a licence.

Common mistakes

Assuming a bigger MCB will stop the nuisance tripping — The MCB is sized to the cable behind it, not to the appliance in front of it. Uprating it removes the protection from a cable that then has nothing stopping it overheating inside a wall.
Removing the consumer unit cover to look more closely — The incoming tails and the main switch terminals are live even with the main switch off, and they are directly behind that cover. Everything an owner needs is readable from the front.

If it doesn't work

One small switch is down, everything else is on

Cause: An MCB or RCBO has tripped on that one circuit — Fix: Unplug everything on that circuit, reset once, and reintroduce appliances one at a time. If it will not reset with nothing connected, stop and call an electrician.

A wide switch with a test button is down and several circuits are dead

Cause: An RCD has tripped on earth leakage somewhere in the group behind it — Fix: Turn off all the MCBs behind it, reset the RCD, then bring the MCBs back one at a time to find which circuit is leaking.

Nothing in the house works and the main switch is down

Cause: Someone has switched the main switch, or it has been tripped by a serious fault — Fix: Check for an obvious cause, switch it back once, and if it will not hold, call an electrician rather than trying again.

Questions people ask

What is a borrowed neutral?

An old wiring practice where a circuit takes its live from one MCB and its neutral from another — commonly upstairs and downstairs lighting sharing a neutral. It is invisible until the board is split onto separate RCDs or RCBOs, at which point the imbalance trips them. It is a rewiring job for an electrician, not something to work around.

Is an RCD the same as a fuse?

No, and this is the important distinction. A fuse or MCB protects the cable from too much current. An RCD protects a person from current going through them to earth. A circuit can have a perfectly healthy 32A MCB and still kill someone; that is the gap the RCD fills.

How often should I press the test button?

Every three months on every RCD and RCBO. It should trip instantly. If it does not, the device is not protecting anyone and needs replacing by an electrician.