BS 7671 does not require an all-RCBO board, but Regulation 314.1 requires the installation to be divided so that a fault does not cause danger or unnecessary inconvenience, and a dual-RCD split-load board only just manages that when a single earth fault takes out half the house. On a 2026 consumer unit change the price gap between a ten-way dual-RCD board and a ten-way RCBO board is smaller than it was, typically the cost of the RCBOs themselves less the two RCDs they replace, and the fault-finding and nuisance-tripping benefits are large. This guide sets out the trade-offs, the regulations that drive the decision, and what has to go on the EIC either way.
- Both layouts can comply. RCBOs make Regulation 314 division easy; dual-RCD boards need careful circuit grouping.
- An RCBO board isolates a fault to one circuit and halves the time spent finding it.
- Type A is the practical minimum RCD type for domestic circuits now; type AC is only for pure sinusoidal residual current.
- Metal enclosures (Regulation 421.1.201), SPDs (443) and AFDD consideration (421.1.7) apply regardless of layout.
- The schedule of test results must show device type, RCD type, rating and trip times for every circuit.
What does BS 7671 actually require?
Three requirements shape the choice:
Regulation 314.1: every installation shall be divided into circuits as necessary to avoid danger and minimise inconvenience in the event of a fault, to facilitate safe inspection, testing and maintenance, and to reduce the possibility of unwanted tripping of RCDs due to excessive protective conductor currents. Regulation 314.2 adds that separate circuits shall be provided for parts of the installation which need to be separately controlled.
Regulation 411.3.3 and 411.3.4: additional protection by 30 mA RCD for socket outlets rated up to 32 A and for final circuits supplying luminaires in domestic premises. Together with 522.6.202 (cables in walls) and Section 701 (bathrooms), this means virtually every domestic final circuit needs 30 mA protection.
Regulation 531.3.2: the RCD arrangement shall be such that unwanted tripping is avoided, and protective conductor currents shall not exceed 30% of the rated residual current on any one device. That is 9 mA of standing leakage per 30 mA RCD. A modern house full of switch-mode power supplies, LED drivers, a heat pump and an EV charger can exceed that on one half of a dual-RCD board.
Put those together and the regulations push towards more, smaller groupings of RCD protection. A dual-RCD board with five circuits on each RCD is the minimum that meets 314.1 in most domestic cases. An RCBO board meets it by default.
How do the two layouts compare?
| Feature | Dual-RCD split-load | All-RCBO |
|---|---|---|
| Effect of an earth fault | Every circuit on that RCD trips (typically half the house) | Only the faulty circuit trips |
| Fault finding | Reset and re-trip to identify the circuit; may need every load unplugged | The tripped device names the circuit |
| Standing leakage per device | Sum of all circuits on that RCD; easily approaches 9 mA | One circuit's leakage; rarely an issue |
| Circuit grouping design | Needs thought: lights and sockets on different RCDs, freezer and boiler separated, upstairs/downstairs split | Not needed |
| Regulation 314 compliance | Achievable with good grouping | Straightforward |
| Board cost (materials, 2026, ten ways) | Lower; two RCDs plus ten MCBs | Higher; ten RCBOs, difference has narrowed with compact single-module RCBOs |
| Board width | Compact | Single-module RCBOs now fit the same enclosure |
| Neutral wiring | Circuit neutrals to the correct RCD's neutral bar; wrong-bar errors cause tripping | Each neutral to its own RCBO; no shared neutral bar errors |
| Borrowed neutral discovery | Hidden if both circuits are on the same RCD | Exposed immediately |
| Testing time | Two RCD tests plus per-circuit MCB checks | An RCD test on every circuit; longer |
The last row is the main cost of RCBOs to the electrician rather than the client. Testing a ten-circuit RCBO board means ten sets of RCD tests (×1, ×5, and the 0 ° and 180 ° phase checks, plus the test button) rather than two. With a modern MFT's auto-RCD sequence this is a few minutes per circuit. See RCD Testing Explained: Types, Test Currents and Trip Times for the sequence and the pass times.
When does a dual-RCD board still make sense?
There are honest cases for the split-load layout:
- Budget-limited replacements where the alternative is no RCD protection at all. A dual-RCD board that replaces a rewirable fuse board is a large safety improvement, and Regulation 314 is satisfied by grouping.
- Small installations of six circuits or fewer, where the "half the house" problem is two or three circuits.
- Boards where most circuits have low leakage and the grouping keeps critical circuits apart.
The grouping rules of thumb: upstairs lights with downstairs sockets and vice versa, so a fault never leaves a floor dark and without power at once; freezer, boiler and smoke alarm circuits on different RCDs from the kitchen sockets; and nothing that must not trip (medical equipment, a server, a fish tank heater) sharing an RCD with an outdoor socket.
A middle path is the high-integrity board: two RCD-protected groups plus a bank of RCBO ways for circuits that need to stay up. It is often the best value for a larger house.
Which RCD type should the RCBOs be?
Regulation 531.3.3 requires the RCD to be selected for the type of residual current the connected equipment can produce:
| RCD type | Detects | Typical use |
|---|---|---|
| AC | Sinusoidal AC residual current only | Legacy; not suitable where DC components are possible |
| A | AC plus pulsating DC | General domestic circuits; the practical minimum now |
| F | As A plus mixed-frequency residual currents | Circuits with single-phase inverter drives (some heat pumps, washing machines) |
| B | As F plus smooth DC | EV chargers without built-in DC detection, PV inverters, three-phase drives |
A type AC RCBO on a circuit feeding an induction hob or an EV charger may be blinded by DC and fail to trip. Most UK manufacturers now supply type A RCBOs as standard and the price difference from AC has closed. Specify type A throughout unless a load calls for F or B, and check EV charger and inverter documentation for the RCD type they require, covered in EV Charger Installation Certificate: Section 722 and the EIC and Solar PV Installation Certificate: Section 712 and the EIC.
What else has to be in the board?
Layout aside, a new domestic consumer unit in 2026 needs to meet:
- Regulation 421.1.201: the enclosure must be non-combustible (in practice steel) or be in a non-combustible cabinet, to BS EN 61439-3.
- Regulation 443: surge protection. Since Amendment 2, an SPD is required unless the owner declines for a single dwelling after a documented risk assessment; most electricians now fit a type 2 SPD as standard. See Surge Protection (SPD) Requirements: Section 443 Explained.
- Regulation 421.1.7: AFDDs are recommended for domestic circuits up to 32 A and required in HMOs, care homes, purpose-built student accommodation and high-rise residential. Where AFDDs are fitted they combine with the RCBO function in one device on most ranges. See AFDD Requirements: Regulation 421.1.7 Explained.
- Regulation 514: labelling, including the RCD test notice, circuit chart and, where applicable, the mixed cable colours notice.
- Main switch rated for the supply, double-pole, and a means of isolation that the DNO's cut-out fuse does not provide.
An RCBO board makes SPD and AFDD integration simpler, because there is no shared RCD upstream whose neutral arrangement the SPD has to fit around.
What goes on the EIC and schedule of test results?
Whatever the layout, the consumer unit replacement is notifiable under Part P in England and Wales and needs an Electrical Installation Certificate. The schedule of test results needs, per circuit:
- Overcurrent device: BS EN number (60898 for MCBs, 61009 for RCBOs), type (B, C, D), rating in A, and breaking capacity.
- RCD: rated residual operating current in mA, RCD type (A, AC, F, B), and measured operating time at IΔn.
- The usual dead and live results: continuity, insulation resistance, polarity, Zs.
At the origin: the earthing arrangement, Ze, PFC, main switch rating, and SPD details. If any circuit's Zs cannot meet the MCB's maximum and the RCBO is providing fault protection, note it. How to fill in a schedule of test results, column by column goes through the columns and Consumer Unit Replacement: Which Certificate and What to Check covers the pre-work tests.
On an EICR of an existing dual-RCD board, the layout on its own is not a defect. Excessive leakage causing tripping, a critical circuit grouped badly, or a type AC RCD on a circuit with DC loads are the things to observe, generally at C3 unless there is a specific danger.
How Certio helps
Certio produces the EICR, EIC, Minor Works certificate and PAT records on the BS 7671 model forms and turns them into branded PDFs. Photograph the consumer unit and the circuit schedule drafts itself; point the phone camera at a Megger MFT-X1 and the reading goes straight into the schedule of test results, or dictate it while your hands stay on the probes. Suggested observation wording with the regulation reference is there for you to check and confirm. The judgement stays yours; the typing goes. Starter is free for 7 days, then £5 a month.