RCBOs: The Protective Device That Stops One Fault Taking Out Your Whole House
UK guide to RCBOs: Type A vs Type B, circuit sizing, brand compatibility, Amendment 3 bidirectional rules, and prices from £12-30 per device.
Your electrician installs a new consumer unit with a dual RCD board. Six months later, the fridge freezer defrosts overnight because a faulty LED downlight in the bathroom tripped the RCD, killing every circuit on that half of the board. You didn't notice because you were asleep. You lost £50-£200 of food because the board design meant one circuit's problem became every circuit's problem. An all-RCBO board would have tripped only the bathroom lighting circuit. The fridge would have kept running. The cost difference between the two boards? Around £50 – £200 on a typical installation.
What it is and what it's for
An RCBO (residual current breaker with overload protection) combines two protective functions in a single device. It does the job of an MCB (which trips when a circuit draws too much current, preventing cable fires) and an RCD (which trips when current leaks to earth, preventing electrocution). One device, both protections, dedicated to a single circuit.
In a traditional dual RCD consumer unit, one RCD protects half the board. All the circuits on that side share the same earth-fault protection. If any one of those circuits develops a leak, the RCD cuts power to all of them. With RCBOs, each circuit gets its own independent protection. A fault on the cooker circuit trips the cooker RCBO. The lights stay on. The sockets keep working. You know exactly which circuit has the problem.
That's the practical difference. It's not about more safety in the abstract. The protection level is identical. What changes is discrimination, the ability to isolate a faulty circuit without disrupting everything else.
Types: the naming that catches everyone out
RCBO specifications have two separate type classifications, and they use overlapping letters. This confuses electricians, let alone homeowners.
RCD type (the letter after "Type")
This describes what kind of earth-fault current the device can detect.
| RCD Type | What it detects | UK status (2026) | When you need it |
|---|---|---|---|
| Type AC | Pure sinusoidal AC faults only | Banned for socket circuits since September 2022. Restricted to simple fixed equipment with zero electronics (rare). | Almost never. Don't buy these. |
| Type A | AC faults plus pulsating DC faults from electronic loads | Minimum standard for all domestic circuits under BS 7671:2018+A2:2022 | Every circuit in a modern home. Phone chargers, LED lights, washing machines, induction hobs all produce pulsating DC. |
| Type F | Everything Type A detects, plus high-frequency faults from variable-speed drives | Required for circuits feeding inverter-driven equipment (heat pumps, some washing machines) | Where the appliance manufacturer specifies it. Check the installation manual. |
| Type B | All of the above plus smooth DC faults | Required for EV charger circuits where the charger has no built-in DC fault detection (RDC-DD) | EV charging points without RDC-DD. Your EV installer will specify this. |
Type A is the minimum for essentially every circuit in your house. If your electrician quotes Type AC devices for any circuit with sockets, LED lighting, or electronic appliances, they're quoting to pre-2022 standards. Push back.
MCB trip curve (the letter before the amperage)
This describes how quickly the device trips on overcurrent (too much current flowing, not an earth fault). It has nothing to do with the RCD type above. The overlap in letter naming is unfortunate.
B curve (3-5 times rated current to trip): standard for all domestic circuits. Lighting, sockets, cookers, showers. This is what you want.
C curve (5-10 times rated current): for circuits with high inrush equipment like large motors or commercial air conditioning. Rarely needed in domestic work.
The trap: you see "Type A B-curve RCBO" on a product listing. That means RCD Type A (detects pulsating DC faults) with a B-curve MCB trip characteristic (standard domestic). It does not mean "Type B RCD". A Type B RCD is a completely different, much more expensive device for EV charger circuits. If your electrician says they're fitting "Type B", clarify whether they mean the RCD type or the MCB curve.
Amendment 3 and bidirectional RCBOs
Since 31 July 2024, BS 7671:2018+A3:2024 (Amendment 3) introduced a new requirement. Any circuit connected to a generating source, including solar PV panels, battery storage systems, or vehicle-to-grid (V2G) capable EV chargers, must be protected by a bidirectional RCBO.
Standard RCBOs are unidirectional. They detect faults in current flowing one way: from the grid supply, through your circuits, to earth. Solar panels and batteries push current the other direction. A unidirectional RCBO may not trip on a fault in reverse-current conditions.
If you're having solar panels or a battery system installed alongside your extension work, your electrician must use bidirectional RCBOs on those circuits. The products are now widely available from Wylex and other manufacturers at similar prices to standard devices (Wylex bidirectional Type A from around £17 at Toolstation).
If you're planning solar PV or battery storage within the next few years, ask your electrician to fit a consumer unit that accepts bidirectional RCBOs throughout. Swapping individual devices later is straightforward, but the board itself needs to support them.
Circuit sizing: which RCBO for which circuit
Your electrician specifies the RCBO rating for each circuit based on the cable size and the load it carries. You don't choose this yourself, but understanding the logic helps you read quotes and spot errors.
| Circuit | Typical RCBO rating | Cable size (twin and earth) | Notes |
|---|---|---|---|
| Lighting | 6A | 1.0mm or 1.5mm | Covers most domestic lighting circuits. LED inrush can cause nuisance tripping (see below). |
| Radial sockets (small room) | 16A or 20A | 2.5mm | Bedroom, study, single-room circuits. |
| Ring final (sockets) | 32A | 2.5mm | Standard UK ring circuit for socket outlets. The ring must have confirmed continuity. |
| Cooker | 32A or 40A | 6.0mm | Depends on cooker rating: 3-7kW appliances typically need 32A; larger ovens may need 40A. |
| Electric shower | 40A or 50A | 10.0mm | 9-10.5kW showers draw 38-45A. Your cable and RCBO must both handle continuous load. |
| EV charger | 32A (Type A or B) | 6.0mm minimum | Type A minimum with RDC-DD in the charger. Type B if charger lacks DC detection. |
| Immersion heater | 16A | 2.5mm | 3kW element draws about 13A. |
| Smoke/fire alarm | 6A | 1.5mm | Dedicated circuit for mains-wired alarms. |
Never oversize an RCBO "just to be safe". A 32A RCBO on a circuit wired with 1.5mm cable creates a fire risk. The RCBO won't trip until 32A flows, but the cable overheats at anything above 20A. The RCBO rating must match the cable's current-carrying capacity, not the appliance rating. Your electrician calculates this using BS 7671 Appendix 4 tables.
LED nuisance tripping on lighting circuits
LED drivers produce a brief inrush current at switch-on, typically 50-80 times the lamp's normal running current. One 5W LED draws 20mA in normal use, but 1-4A for a fraction of a second when you flick the switch. Twelve LED downlights switching on simultaneously can produce a combined inrush that trips a 6A RCBO.
This isn't a fault. It's a characteristic of LED drivers interacting with the RCBO's magnetic trip mechanism. Solutions: split lighting onto more circuits so fewer LEDs switch together, or ask your electrician about Type F RCBOs for large LED arrays (they're more tolerant of transient inrush). Most installations work fine, but if you're fitting 10+ LED downlights on a single circuit, mention it during planning.
Brand compatibility: this is non-negotiable
Consumer units are type-tested as complete assemblies. The consumer unit chassis, the busbar, and every protective device are tested together as a system. Fitting an RCBO from a different manufacturer invalidates the type-test certificate, even if it physically clicks onto the DIN rail.
The busbar connection points differ between manufacturers. A device that physically fits may not make proper electrical contact. It may work initially, then fail under load when the contact resistance generates heat.
Always match the RCBO brand to your consumer unit brand. If you have a Hager board, buy Hager RCBOs. A BG board gets BG RCBOs. The only widely accepted exception is within the Schneider Electric group: Schneider, Merlin Gerin, and Square D devices are cross-compatible because they're made by the same parent company.
| Consumer unit brand | Compatible RCBOs | Where to buy | Typical price per RCBO |
|---|---|---|---|
| Hager | Hager ADA series | Electricpoint, CEF, electrical wholesalers | £13-14 (6kA trade) |
| BG (British General) | BG Fortress Type A | Screwfix, Toolstation | £17-18 |
| Wylex | Wylex Mini RCBO Type A | Toolstation, Screwfix | £17-18 |
| MK (Honeywell) | MK Sentry Type A | Screwfix, Electricpoint | £13-20 (varies by trade/retail channel) |
| Schneider Electric | Schneider Easy9, Merlin Gerin, Square D | Trading Depot, electrical wholesalers | £25-30 |
| Contactum | Contactum Defender Type A | Screwfix, Toolstation | £10-16 |
If your electrician is buying from an electrical wholesaler (CEF, Edmundson, Rexel), prices are lower than retail. Don't compare their trade prices against Screwfix retail and assume they're using cheap products. Trade pricing on identical branded RCBOs runs 15-30% below retail.
How much they cost
Individual RCBOs range from about £12 – £30 depending on brand and where you buy.
Budget RCBO (Axiom, Contactum on promotion, MK at trade)
£12 – £17
Mainstream retail RCBO (BG Fortress, Wylex, MK Sentry)
£16 – £20
Premium RCBO (Schneider Easy9)
£25 – £30
For a typical 10-circuit extension board populated entirely with RCBOs, materials run around £120 – £250 depending on brand. The cost of the consumer unit chassis adds another £120 – £250.
All-RCBO vs split-load: the real cost difference
The upgrade premium from a dual RCD split-load board to an all-RCBO board is surprisingly small at the installed level:
10-circuit split-load board, installed with testing and certification
£450 – £600
10-circuit all-RCBO board, installed with testing and certification
£470 – £950
The materials difference is roughly £50 – £130 (RCBOs cost more than MCBs, but you save on the two RCDs you no longer need). Labour is almost identical because the electrician still has to wire, test, and certify the same number of circuits.
The wide range on the all-RCBO figure reflects regional variation. London and the South East sit at the top end. A 12+ circuit installation in a large property with rewiring needs will push into the £900 – £1,200 range.
The installed cost premium for an all-RCBO board over split-load is typically £50 – £200 on a 10-circuit installation. For that price, you get individual fault isolation on every circuit. It's the best-value upgrade in a consumer unit specification.
Where to buy
Your electrician normally supplies RCBOs as part of the installation. This is the right approach, because they need to match the devices to the consumer unit brand, verify the specification against BS 7671, and include them in their testing and certification.
If you're buying a consumer unit yourself (not recommended unless your electrician has given you an exact specification), these are the main UK sources:
Screwfix and Toolstation stock BG, Wylex, MK, Contactum, and Lewden. Good for retail pricing and next-day collection. Expect to pay £15 – £20 per RCBO for mainstream brands.
Electrical wholesalers (CEF, Edmundson, Rexel, City Electrical Factors) carry Hager, Schneider, MK, and professional-grade ranges. Trade accounts get better pricing. Some will sell to the public at slightly above trade.
Online specialists like Electricpoint and Trading Depot offer wider ranges including Hager and Schneider models that Screwfix doesn't stock.
Pre-populated consumer units (boards that come with RCBOs already fitted) are now widely available. These are factory-assembled and type-tested as a complete unit. They cost less than buying components separately and eliminate compatibility risk. Ask your electrician about pre-populated boards.
How your electrician works with them
RCBOs are installed during second fix, after plastering. The consumer unit chassis and supply tails go in during first fix, but the protective devices are fitted later to avoid plaster dust contamination.
Installation takes 3-6 hours for a full consumer unit change, including:
- Isolating the supply (your electricity company's fuse or the main switch)
- Mounting the new consumer unit and connecting the meter tails
- Transferring each circuit cable to its corresponding RCBO
- Connecting the neutral and earth bars
- Testing every circuit: insulation resistance, earth fault loop impedance, RCD trip time
- Labelling every circuit on the schedule card
- Issuing an Electrical Installation Certificate (EIC)
The EIC is a legal requirement under Building Regulations Part P. Without it, you don't have proof the work was done safely, and it will cause problems when you sell the property.
The interconnected neutrals trap
In older properties (pre-2000 wiring), electricians sometimes borrowed neutral conductors between circuits. The upstairs lighting neutral might share a cable with the downstairs sockets. On an old fuse board, this doesn't matter. On an RCBO board, it causes multiple RCBOs to trip simultaneously because the return current takes a path through the wrong device.
A competent electrician tests for interconnected neutrals before installing RCBOs. If they find them, the affected circuits need rewiring to separate the neutrals. This adds cost and time, but it's unavoidable. If your electrician doesn't mention this test on an older property, ask about it.
Testing: the button you should press monthly
Every RCBO has a small test button on its face. Pressing it simulates an earth fault and should trip the device instantly. BS 7671 recommends quarterly testing. Monthly is better.
The test button only confirms the mechanical trip mechanism works. It doesn't test the sensitivity or trip time. That requires an electrician with a calibrated RCD tester, which is part of the five-yearly Electrical Installation Condition Report (EICR).
A test button that doesn't trip the RCBO means the device has failed. Don't keep using the circuit. Call an electrician. RCBOs can fail silently, especially on circuits that run near their rated capacity for long periods (electric showers, immersion heaters). The test button is the only way to catch this.
Alternatives
MCBs with shared RCDs (split-load board) provide the same overload and earth-fault protection at lower component cost. The trade-off is discrimination. A fault on one circuit affects every circuit protected by the same RCD. For a small installation with few circuits, split-load can be acceptable. For a new extension with 8-12 circuits, RCBO is the better choice.
AFDDs (arc fault detection devices) are the next evolution. An AFDD detects dangerous arcing in damaged cables, something neither MCBs nor RCDs can sense. BS 7671 Amendment 2 recommends them for certain circuits (bedrooms in houses of multiple occupation). They cost £90 – £105 each and are typically combined with RCBO functionality. Not yet required for standard domestic installations, but likely to become more common.
Common mistakes
Buying the wrong RCD type. Searching for "RCBO" online still returns Type AC devices from marketplace sellers. These are not compliant for socket circuits or any circuit with electronic equipment. Always confirm Type A minimum.
Confusing "B curve" with "Type B". A "B32" marking means B-curve MCB at 32A. That's standard. A "Type B" RCD is a specialist device for EV chargers costing three to five times more. Product listings don't always make the distinction clear.
Mixing RCBO brands with the consumer unit. Even if it physically fits, it's not compliant. Match the brand. No exceptions apart from the Schneider group.
Not specifying bidirectional for solar/battery circuits. Standard unidirectional RCBOs have been sold for decades. If your installation includes solar PV, battery storage, or V2G, you need bidirectional devices on those circuits. Amendment 3 has been mandatory since July 2024 with no grace period.
Fitting a 32A RCBO on a lighting circuit. It sounds like it gives headroom. It creates a fire risk. The RCBO rating protects the cable, not the appliance.
Where you'll need this
- Second fix electrics - RCBOs are installed in the consumer unit during second fix, one per circuit
- Electrical layout planning - the number of circuits you plan determines how many RCBOs your board needs
RCBOs appear in any project that involves new electrical circuits: extensions, loft conversions, garage conversions, rewires, or adding high-power circuits for EV chargers and heat pumps. The principles on this page apply regardless of project type.