What Is an RCBO?

LIVE ELECTRICAL GUIDE  |  Harbro Electrical

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An RCBO (Residual Current Breaker with Overcurrent protection) is a single DIN-rail device that protects one circuit against three fault types at once: overload, short-circuit, and earth leakage.

A thermal-magnetic mechanism handles the first two, the same way an MCB does, while a current transformer inside the same housing watches for current escaping to earth and disconnects the circuit within milliseconds if it finds any.

Because each RCBO covers only its own circuit, a fault on one way never affects any other circuit on the board - which is why RCBOs have become the standard specification for new UK consumer units.

RCBO gets thrown around at the trade counter as if everyone already knows what it stands for, and most people roughly do. What is less often explained is how one small device actually manages to do a job that used to take two separate ones, and what that means when you are choosing which RCBO to fit.

This guide covers what is physically happening inside an RCBO, the different types available, where BS 7671 expects to find one, and the practical questions that come up when selecting and installing them.

Harbro Electrical stocks the Live Electrical RCBO (Type A, single module, 6A B-curve, 30mA) alongside the full FuseBox RCBO range, at ex-VAT trade prices with free UK mainland delivery on orders over £70.

Two Fault Types, One Device

The overcurrent side of an RCBO works exactly like a standard MCB. A bimetallic strip heats up and bends under sustained overload current, tripping the breaker within seconds to minutes depending on how far over the rating the current sits. A separate magnetic coil reacts to a short-circuit almost instantly, within milliseconds, because a short-circuit current is large enough to pull the trip mechanism directly rather than waiting for heat to build up.

The earth leakage side is a different mechanism entirely, built around a small current transformer that the line and neutral conductors both pass through. In normal operation, whatever current flows out on the line comes straight back on the neutral, so the transformer sees no net magnetic field. The moment current escapes the circuit - through a person, a damaged cable, or a faulty appliance casing - the two currents no longer match, the transformer detects the resulting imbalance, and a trip coil fires to open the contacts, typically within 30 to 40 milliseconds for a 30mA device.

Both mechanisms sit inside the same moulded case and share the same set of contacts, which is the entire reason an RCBO exists: rather than routing a circuit through an MCB and then a shared RCD, one device now does both jobs for that circuit alone.

What an RCBO Replaced

Before RCBOs became affordable enough for routine use, the standard way to add earth fault protection to a board was a single RCD sitting upstream of a whole bank of MCBs - a split-load board. It worked, and it is still a perfectly valid way to meet BS 7671, but it has one structural weakness: every circuit behind that RCD shares its fate. An earth fault on any one of them trips the lot.

An RCBO fixes that by moving the earth leakage detection down to the individual circuit, so it sits alongside the MCB function rather than upstream of several MCBs at once. Modern RCBOs are also built to the same physical width as a single-module MCB, so swapping one for the other on a board rarely costs any extra space - the main difference the installer sees is a second, independent test button on the front of the device.

RCBO Types Explained

Not every RCBO detects the same shape of fault current, and getting the type wrong is a compliance issue as much as a technical one.

Type A is the baseline for general use under BS 7671 Amendment 2. Type B is specifically required for certain EV charge points and equipment with electronic power conversion - check the manufacturer's instructions for the specific load before assuming Type A is sufficient.

Where BS 7671 Expects to Find One

BS 7671:2018 (as amended) does not use the word RCBO in its 30mA protection requirements - Regulation 411.3.3 asks for 30mA protection on socket outlets up to 32A, outdoor mobile equipment supplies, and cables concealed less than 50mm in walls, and Amendment 3 extended that to domestic lighting circuits. A shared RCD technically satisfies the same wording an RCBO does.

What tips most new consumer unit specifications toward RCBOs is a different regulation: 314.1 and 531.3.2 require circuits to be divided so that a fault causes the minimum practical disruption. A shared RCD covering half a dozen circuits sits awkwardly against that requirement in a way an RCBO never does, which is why RCBOs have become the default on replacement boards even though the regulations never mandate them by name.

Selecting the Right One

• Current rating - match the MCB portion to the cable size and the load, the same as choosing a standalone MCB (6A lighting, 32A ring final, and so on)

• Curve type - B-curve for typical domestic resistive and lighting loads; C-curve where inrush current is higher, such as some motor or transformer loads

• Sensitivity - 30mA for standard shock protection; higher ratings such as 100mA are used only on distribution circuits where fire protection, not personal protection, is the goal

• RCD type - Type A as the general-purpose minimum, Type B or Type F where the load specifically calls for it

• Poles - single-pole devices switch line only; double-pole disconnects the neutral too, which matters on TT systems and certain three-phase distribution arrangements

• Module width - modern ranges match a standard single-module MCB, but always check against the specific consumer unit before ordering, since older or budget enclosures can vary

Living With One: Testing and Common Faults

BS 7671 recommends operating the integral test button quarterly to confirm the trip mechanism still functions - it simulates an imbalance electronically rather than testing the wiring itself, so it does not replace a proper insulation and RCD trip-time test at the next periodic inspection.

An RCBO that will not reset, or trips again the instant load is applied, almost always points to a genuine fault on that specific circuit rather than a nuisance trip - unlike a shared RCD, there is no other circuit's leakage current to blame. The most common causes are a damaged cable, a faulty appliance left plugged in, or - on a newly wired circuit - line and neutral conductors that have been crossed with another way on the board, which an RCBO will detect immediately because the current it sees flowing out no longer matches what comes back on its own neutral.

Frequently Asked Questions

What does RCBO stand for?

Residual Current Breaker with Overcurrent protection. “Residual current” refers to the earth leakage current the device monitors for; “overcurrent” covers the overload and short-circuit protection it also provides.

Do RCBOs wear out?

The mechanical trip components can wear with repeated operation, which is part of why BS 7671 recommends quarterly testing - it exercises the mechanism and reveals a unit that has become sluggish or stopped tripping before it matters. A properly rated RCBO that is not being tripped constantly should last for many years of normal service.

Can I fit an RCBO myself?

In England and Wales, most consumer unit work falls under Part P of the Building Regulations and needs to be notified to building control or carried out by a registered competent person; requirements differ in Scotland and Northern Ireland. Even where the legal position allows DIY work, working inside a live consumer unit carries a genuine safety risk and is generally not recommended without the right training and test equipment.

Does every circuit in a consumer unit need its own RCBO?

No - a shared RCD protecting several MCBs is still a fully compliant way to provide the same 30mA protection. RCBOs are simply the more common choice on new work because a fault on one circuit does not affect any other.

Why did my RCBO trip the instant I reset it?

This usually means the fault is still present - a genuine earth leakage or a short-circuit rather than a one-off event. Isolate and disconnect anything plugged into that circuit before resetting again, and if it still trips immediately, the fault is most likely in the fixed wiring or an accessory rather than a plugged-in appliance.

How much does an RCBO cost?

A single-module domestic RCBO typically runs from around £15 to £35 ex-VAT depending on current rating and RCD type, with Type B versions costing noticeably more than Type A.

Is an RCBO just an MCB with extra features?

Functionally it includes everything a standard MCB does, but the earth leakage detection is a genuinely separate mechanism working alongside the thermal-magnetic trip, not an add-on to it - the two systems operate independently and either one can trip the device on its own.

RCBOs at Harbro Electrical

Harbro Electrical stocks the Live Electrical RCBO range and the FuseBox RCBO range at ex-VAT trade prices. Free UK mainland delivery on all orders over £70.

• Live Electrical RCBO (AMR106B-030, 6A Type A, 30mA)  -  https://harbroelectrical.co.uk/collections/live/products/live-electrical-amr106b-030-type-a-single-module-6-amps-b-curve-30ma-rcbo

• FuseBox RCBO range  -  https://harbroelectrical.co.uk/collections/fusebox-rcbo

• Consumer Units and Live Electrical Range  -  https://harbroelectrical.co.uk/collections/live

• Test Equipment  -  https://harbroelectrical.co.uk/collections/test-equipment

For trade orders and account enquiries, call us on 0191 518 1998 or visit our trade counter at 5 and 6 Whitworth Road, Peterlee, County Durham, SR8 2LY.

All prices shown exclude VAT. Free delivery applies to UK mainland addresses on orders over £70.