MCB vs RCD vs RCBO vs AFDD: Differences Explained (2026)

Open your consumer unit and you see a row of switches, probably labelled lights, sockets, cooker. Those switches do four different jobs depending on which device you are looking at. This guide walks through the four, what each one protects against, how to tell them apart on your board, and where BS 7671:2018+A4:2026 now expects each one in a modern installation.

For the separate question of which RCD type you have, see RCD types explained: AC, A, F and B.

MCB, Miniature Circuit Breaker

Think of an MCB as a bouncer on the door of a nightclub. Its one job is to stop too many people getting in at once. In electrical terms, if the current going through a circuit exceeds the rating of the cable (because you have plugged in too much, or because something has short-circuited), the MCB trips and disconnects the circuit before the cable overheats.

MCBs protect the wiring. They do not protect people from electric shock. A person touching a live wire typically draws far less current than an MCB needs to trip, so the MCB sits there unaware while the person goes into cardiac arrest. That is the job of the RCD.

RCD, Residual Current Device

An RCD continuously compares the current leaving the consumer unit on the live wire with the current coming back on the neutral. The two numbers should match. If they do not, current is leaking somewhere it should not, through damaged insulation, through water, or through a person. The moment the RCD sees an imbalance of 30 mA or more, it opens the circuit fast enough to prevent a fatal electric shock ^[2].

An RCD protects people. It does not care about overloads, that is the MCB's job. On an older board, the RCD is usually the largest device on the row, with a prominent T test button you are supposed to press every three months.

RCBO, Residual Current Breaker with Overcurrent

An RCBO does the MCB and RCD job in one device, for one circuit. The MCB side handles overload and short-circuit. The RCD side handles earth leakage. The two mechanisms are packaged into a single module the same width as an MCB, with a small T button on the front.

When something goes wrong on a circuit protected by an RCBO, only that circuit trips. Not half the house. Not a group of circuits. That single fact is why I fit an RCBO on every circuit, and RCD vs RCBO goes into it in depth ^[1].

AFDD, Arc Fault Detection Device

An AFDD looks a lot like an RCBO but does a different job. It watches the current waveform on a circuit for the signature of an electrical arc, a small sparking connection. Arcing happens at loose terminals, at damaged cables nipped by furniture, and at old sockets with worn contacts. Because arcing produces heat and is a leading cause of electrical fires, an AFDD disconnects the circuit as soon as the signature appears, before the arc can start a fire.

AFDDs are not a retrofit for every circuit in every property. A4:2026 requires them on final circuits supplying socket outlets rated up to 32 A in specific higher-risk dwellings: Houses in Multiple Occupation (HMOs), purpose-built student accommodation, and care homes. In other domestic premises A4:2026 recommends them but does not mandate them ^[1].

4-way comparison table

Type AC vs Type A vs Type B, and why it matters for EVs and solar

Not all RCDs detect the same kind of leakage. The type letter on the device tells you which waveforms it can see. Getting this wrong on an EV charger circuit is the most common expensive mistake I am asked to fix.

If you are planning a home EV charger, the installer needs to either fit a Type B RCD on the dedicated circuit, or use an EV charger with built-in 6 mA DC residual current monitoring. Skipping this is a C2 on an EICR and a recall-level issue on a new install ^[3].

How to read your board, five steps

1. Open the hinged coverPull open the front flap of the consumer unit. No tools. Do not touch anything behind the rocker switches.
2. Read the scheduleMost boards have a printed or handwritten schedule inside the cover listing every circuit by number, function, and device rating (e.g. 6 A lighting, 32 A sockets).
3. Find the main switch and any RCDsThe largest device on the left is usually the main switch. Larger switches with a T test button are RCDs. Count how many, and note which circuits each covers.
4. Identify MCBs and RCBOsSingle-module devices without a T button are MCBs. Single-module devices with a small T button are RCBOs. If every circuit has a T button, you have an RCBO-per-circuit board.
5. Look for AFDDsAFDDs are typically wider than MCBs and often have a status LED. They sit on specific final circuits, not every circuit. In an HMO or student property, expect to see them under A4:2026.

What BS 7671:2018+A4:2026 now requires

The 4th amendment to the 18th Edition came into force on 15 April 2026. For a domestic dwelling it is worth knowing the following headlines:

• 30 mA RCD protection on all socket outlets rated up to 32 A, and all lighting circuits
• 30 mA RCD protection on cables concealed in walls at less than 50 mm depth where not otherwise protected
• Type A as the minimum RCD type for most circuits
• Type B RCD or 6 mA DC-sensing integrated into EV charger circuits
• AFDDs on socket circuits in HMOs, purpose-built student accommodation and care homes
• Type 2 Surge Protection Device as a default expectation in most domestic premises

The cleanest way to deliver all of this on a new or replacement board is one RCBO per final circuit, the correct RCD type for that circuit, AFDDs where the property type requires them, and an SPD at the origin.

Do you need to upgrade?

Not every older board needs ripping out. A board with RCD protection that passes an EICR meets safety expectations. Four situations where an RCBO-per-circuit upgrade is worth the money:

• You have a split-load board and one side has no RCD protection. This is almost always a C2 on an EICR and is the most common single trigger for an upgrade.
• Your RCD trips and takes multiple circuits with it. An RCBO-per-circuit board isolates faults and tells you exactly where the problem is.
• You are having a new circuit installed. EV charger, heat pump, electric shower, new kitchen. The new work has to meet A4:2026, which in practice means at least RCBO protection on that circuit, and ideally the whole board brought up at the same time.
• The enclosure is plastic and the property is a flat, HMO or student house. A4:2026 wants steel and it wants AFDDs. Those two together are a full board replacement.

A note on cost

Every consumer unit job is scoped on a site visit. Circuit count, enclosure type, the state of the tails and main bonding, whether cabling needs making good, and whether Type B RCDs or AFDDs are needed all change the figure. I give you a clear written estimate before any work starts. Book a no-obligation survey.