What is the difference between an MCB and an RCBO?

An MCB (miniature circuit breaker) only protects against overloads and short circuits. An RCBO adds RCD protection on the same device, so a fault to earth on that circuit trips the RCBO without affecting any other circuit. This is better than a shared RCD, where one faulty appliance can black out half the board.

Do I need to turn off the whole board to fit an RCBO?

Yes. You must isolate at the main switch and confirm dead before touching any terminals. Older boards may not have a main switch that isolates the incoming tails — in that case the work must be done by an electrician who can arrange for the supply to be isolated by the network operator.

Does fitting an RCBO require Building Regulations notification?

Replacing like-for-like protective devices in an existing consumer unit is generally minor works and does not require Part P notification on its own. However, if the consumer unit itself needs replacing, or if the work involves a new circuit, that is notifiable work.

Which RCBO type do I need?

Most UK consumer units use Type B RCBOs for general ring finals and lighting, and Type C for motor loads. Match the current rating to the original MCB. The RCD part should be 30 mA Type A for sockets, and 30 mA Type AC is acceptable for pure resistive lighting circuits — but Type A is the safer default.

How to Fit an RCBO in a Consumer Unit (UK)

Video reference. John Ward (jwflame), UK electrician and the creator of Flameport.com, walks through fitting an RCBO in a consumer unit in his video "Fitting an RCBO in a Consumer Unit". John gets the UK-specific detail right: correct neutral bar practice, bus bar clip engagement, and the test button check at the end. Worth watching in full before you pick up a screwdriver.

Before you start. Turn off the main isolator switch on the consumer unit and use an approved voltage tester to confirm dead on several circuits before touching any terminals. If your board has no main switch, or the main switch is downstream of a meter tail that remains live, do not proceed — that is a job for an electrician working with a DNO isolation. Never assume the board is dead because the main switch is off; always test first.

1. Understand why you are fitting an RCBO

A standard MCB trips on overload or short circuit only. It does not detect a leakage current to earth. An RCBO adds a 30 mA RCD element to the same device, so any earth fault on that individual circuit trips only that circuit. On a traditional split-load board, one shared RCD covers six or eight circuits — one faulty kettle or washing machine trips the whole left-hand bank. Fitting RCBOs circuit by circuit is the incremental way to improve protection without replacing the whole board.

Check your board manufacturer before you buy. Hager, Wylex, Schneider and others all make proprietary RCBOs that clip into their own DIN rail profiles. A Hager RCBO will not fit a Wylex board properly. Match the brand, check the current rating and the tripping characteristic (B or C) against the original MCB, and make sure you buy a 30 mA Type A unit for socket circuits.

2. Isolate the mains at the main switch

Turn off the main isolator switch. On most modern boards this is a 100A double-pole switch at the far left. On older rewireable fuse boxes there may not be one. If there is no main switch, stop — you cannot safely do this job without one.

Use a proving unit to confirm your voltage tester is working, then test at the top terminals of several circuit breakers. All should show dead. Lock the main switch open if you have a hasp. Tell other people in the house not to touch the board while you are working.

3. Identify the neutral for your circuit

Open the consumer unit cover (two screws on most metal units). Find the MCB for the circuit you are upgrading. Now trace its neutral conductor back to the neutral bar — the grey or blue cable from the back box of that MCB slot. Note exactly which screw on the neutral bar it connects to, and which circuits are either side. You will need to leave that position vacant once you remove the neutral, because the RCBO has its own terminal for it.

If the neutral conductors are not individually labelled, do this carefully. Pulling the wrong neutral off the bar could de-energise the wrong circuit when you restore power — or worse, leave a live terminal unconnected.

4. Remove the old MCB and its conductors

With the board dead and confirmed, loosen the live terminal on the MCB and withdraw the conductor. Then loosen the neutral bar screw for that circuit and withdraw the neutral. Keep both conductors loose but controlled — tuck them back into the cable bundle so they do not stray.

Prise the old MCB off the DIN rail. Most clip off with a flat-blade screwdriver against the spring catch at the bottom.

5. Connect the RCBO

The RCBO has two terminals on the load side: one marked L (or Line) and one marked N. Connect the circuit live to the L terminal and the circuit neutral to the N terminal. Tighten both to the torque stated on the device — most specify 2.5 Nm for 6 mm and smaller conductors. Do not connect the neutral back to the main neutral bar; the RCBO's N terminal is now the only connection for that neutral.

On the supply side, the RCBO has a bus bar clip that connects to the live bus bar in the board. It has no separate supply neutral connection — the main neutral bar supplies the neutral reference for the RCD measurement internally.

6. Clip the RCBO into the board

Locate the vacant slot where the old MCB was. Angle the RCBO so the bus bar clip engages first, then press the body down firmly until the DIN rail spring catch clicks. Give it a gentle tug to confirm it is properly seated. The face of the RCBO should sit flush with the other devices.

Tidy the neutral conductor back through the loom so it runs cleanly to the RCBO's N terminal. There should be no sharp bends or tension on the connection.

7. Restore power and test

Replace the consumer unit cover. Turn the main switch back on. Switch the new RCBO to the ON position. Now press the TEST button on its face — the RCBO must trip immediately. If it does not trip, it is faulty: switch it off, replace it, and do not leave a non-tripping RCBO installed.

Reset the RCBO and confirm the circuit works normally. Go to the room it supplies, plug in a lamp or check the lights. If the RCBO trips under normal load, switch it off and check that the neutral is connected only to the RCBO terminal and not also to the main bar — a double connection causes problems.

Stop and call an electrician if: the main switch does not isolate the meter tails (they remain live even with the switch off); the board has no main switch; you cannot identify which neutral belongs to the circuit; the RCBO will not reset after fitting; any terminal shows signs of overheating or scorch marks; or the board is so old that no compatible RCBO is available for the manufacturer and you suspect the wiring is degraded. Replacing the whole consumer unit is Part P notifiable work — it cannot be self-certified by a homeowner.

When to call us

If your board is a 1960s or 1970s metal split-load unit and you want to upgrade protection circuit by circuit, Richard can assess whether the existing board is worth upgrading or whether a full consumer unit replacement makes more sense. Board upgrades in east Kent start from an honest quote — no call-out fee for local Sandwich jobs.

Consumer unit work in Sandwich?

Richard is NICEIC-registered and can replace MCBs, fit RCBOs, or quote for a full consumer unit upgrade. Part P certification included.

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How to fit an RCBO in a consumer unit