The distribution board is the heart of every electrical installation. This guide covers split load vs dual RCD vs RCBO board configurations, circuit arrangement and allocation, BS 7671 labelling requirements, type testing under BS EN 61439, SPD installation, wiring best practice, and the common mistakes found during EICR inspections.
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Written and reviewed by Andrew Moore, founder of Elec-Mate, against BS 7671:2018+A4:2026, IET Guidance Note 3 and the IET On-Site Guide.
The incoming supply enters a double-pole main switch, then feeds the protective devices — MCBs, RCBOs or RCDs — that protect each final circuit. A surge protective device (SPD) sits on the supply side where a risk assessment requires it. Earths land on the earth bar, neutrals on the neutral bar (or the RCBO for RCBO boards), and a circuit chart identifies every way.
Domestic consumer units must be type-tested to BS EN 61439-3 and built from non-combustible material (Regulation 421.1.201). Replacing a board is notifiable under Part P and needs an Electrical Installation Certificate.
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A distribution board (commonly called a consumer unit in domestic installations) is the central point where the incoming electrical supply is split into individual circuits that serve different areas and appliances throughout the building. It houses the main switch, the protective devices (MCBs, RCBOs, or RCDs), and in modern installations, the surge protective device (SPD).
The distribution board is arguably the most important component in any electrical installation. It provides overcurrent protection for every circuit cable, earth fault protection via RCDs or RCBOs, a means of isolation for the entire installation and for individual circuits, and a central point for circuit identification and labelling.
Under BS 7671:2018+A4:2026, domestic consumer units must be constructed from non-combustible material (metal), must be type-tested assemblies complying with BS EN 61439-3, and must be installed by a competent person. Replacing or upgrading a consumer unit is notifiable work under Part P of the Building Regulations and requires an Electrical Installation Certificate (EIC).
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The three main configurations for domestic consumer units are split-load, dual-RCD, and full RCBO boards. Each has advantages and drawbacks.
| Feature | Split-Load | Dual-RCD | Full RCBO |
|---|---|---|---|
| Earth-fault protection | One RCD covers a bank of MCBs | Two RCDs, one per bank of MCBs | Individual 30 mA RCD per circuit |
| Effect of one earth fault | All circuits on that RCD lost | All circuits on that RCD lost | Only the faulty circuit trips |
| Fault discrimination | Poor | Moderate | Best — circuit-level |
| Relative cost | Lowest | Moderate | Highest |
| Typical use today | Rare in new work | Legacy / budget upgrades | Standard choice for new boards |
Main switch + one RCD protecting a group of MCBs + a group of MCBs without RCD protection (typically for the non-RCD-required circuits like the cooker). The simplest and cheapest option, but offers limited protection — if the RCD trips, all circuits on that side lose power. Less common in new installations.
Main switch + two RCDs, each protecting a separate group of MCBs. Circuits are divided between the two RCDs so that if one trips, essential services remain on the other side. Better protection than split-load, but a fault on one circuit still trips all circuits on the same RCD. The most common domestic configuration before RCBO boards became affordable.
Main switch + individual RCBOs for every circuit. Each RCBO provides both overcurrent and 30mA RCD protection for its circuit independently. A fault on one circuit trips only that RCBO — all other circuits remain live. The best protection and discrimination. Higher cost but increasingly the standard choice for new installations and consumer unit upgrades.
Regulation 421.1.7 was redrafted in BS 7671:2018+A4:2026. It is now a requirement to protect final circuits supplying socket-outlets rated up to 32 A with AFDDs in Higher Risk Residential Buildings, Houses in Multiple Occupation, purpose-built student accommodation and care homes. For all other premises — including ordinary dwellings — the regulation recommends AFDDs on single-phase AC final circuits supplying socket-outlets up to 32 A. Using an AFDD does not remove the need for the other protective measures in the standard.
AFDDs fit within the consumer unit alongside RCBOs, and several manufacturers now offer combined RCBO+AFDD devices providing overcurrent, RCD and arc-fault protection in a single module. Confirmation that any installed AFDDs are operational is an inspection item under BS 7671 (Regulation 421.1.7).
The trend in the UK is strongly towards full RCBO boards. The cost premium over dual-RCD boards has reduced significantly as RCBO prices have fallen. For new installations and consumer unit replacements, an RCBO board provides the best combination of safety, reliability, and fault discrimination.
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How circuits are arranged within the distribution board affects fault discrimination, maintenance, and safety. The key principles are:
For a typical 3-bedroom house, a standard circuit schedule on a full RCBO board might look like the example below. Ratings are indicative — always size every circuit against the actual load, cable and installation method using the cable sizing calculator.
| Circuit | Typical device | Cable |
|---|---|---|
| Downstairs lighting | 6 A Type B RCBO | 1.0 / 1.5 mm² |
| Upstairs lighting | 6 A Type B RCBO | 1.0 / 1.5 mm² |
| Downstairs ring final (sockets) | 32 A Type B RCBO | 2.5 mm² |
| Upstairs ring final (sockets) | 32 A Type B RCBO | 2.5 mm² |
| Cooker | 32 / 40 A Type B RCBO | 6 mm² |
| Electric shower | 40 / 45 A Type B RCBO | 10 mm² |
| Smoke alarms | 6 A Type B RCBO | 1.5 mm² |
| Spare ways | 1–2 ways left free | — |
A modern RCBO board with 12 to 16 ways accommodates this comfortably and leaves room for future additions such as an EV charger or solar PV.
Correct labelling is not optional — it is a regulation requirement and a practical necessity for safe operation and maintenance of the installation.
| Notice / label | Regulation | Where it goes |
|---|---|---|
| Circuit chart / schedule | 514.9.1 | At or near each distribution board |
| Next inspection recommendation | 514.12.1 | At or near the origin |
| RCD six-monthly test notice | 514.12.2 | At or near the consumer unit |
| "Safety electrical connection — do not remove" | 514.13.1 | At earthing / bonding connections |
| Alternative / dual supply warning | 514.15 | Where a second supply exists |
| SPD presence label | 514.16.1 | At or near the SPD |
A4:2026 introduced exceptions to Regulations 514.9.1, 514.12.1 and 514.12.2 for domestic (household) premises — check the current amended text before recording an observation on a domestic EICR.
During an EICR inspection, the inspector will check that all labelling is present, accurate, and legible. Missing or incorrect labelling is a C3 (Improvement Recommended) observation. Illegible or faded labels should be replaced — Elec-Mate can generate professional printed circuit schedules from the circuit data entered during an inspection.
BS EN 61439-3 is the product standard for distribution boards (low-voltage switchgear and controlgear assemblies used by ordinary persons). Consumer units installed in domestic premises must comply with this standard, which requires the assembly to be type-tested by the manufacturer.
The major UK consumer unit manufacturers (Hager, Wylex, MK, Schneider Electric, BG) all offer type-tested ranges. When ordering, confirm that the MCBs or RCBOs you specify are from the same range as the enclosure.
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Start free trialThe main switch provides a means of isolating the entire installation from the supply. BS 7671 Regulation 462.1.201 requires a main linked switch or linked circuit-breaker as near as practicable to the origin of every installation, serving both as a means of switching the supply on load and as a means of isolation. Where it is intended for operation by ordinary persons — a household or similar installation — it must interrupt both live conductors of a single-phase supply (i.e. a double-pole switch breaking line and neutral), rated for the maximum demand of the installation.
In a dual-RCD or RCBO board, the main switch is upstream of all other devices. In some configurations, the main switch also serves as the RCD (a main switch RCD or RCCB), but this is less common in modern boards because a fault on any circuit would trip the main switch and disconnect the entire installation.
Surge protective devices (SPDs) are addressed under Regulation 443 of BS 7671:2018+A4:2026, which determines whether protection against transient overvoltages is needed. Regulation 443.4.1 requires protection to be provided where the consequence of the overvoltage could result in serious injury to, or loss of, human life (limb a) or significant financial or data loss (limb c) — limb (b) was deleted by the BS 7671:2018+A2:2022 Corrigendum (May 2023). For all other cases, protection must be provided unless the owner of the installation declares it is not required because any loss or damage is tolerable and they accept the risk — though in practice many modern domestic installations will warrant an SPD because of the sensitive electronic equipment present. See the dedicated SPD guide for the full risk-assessment methodology. The SPD is installed at the distribution board to protect against transient overvoltages caused by lightning strikes, switching surges, and other disturbances on the supply network.
Some consumer unit manufacturers offer boards with integrated SPD mounting positions, which simplify installation and help keep cable lengths short. If using a separate SPD module, position it as close to the supply terminals as possible.
The quality of the wiring inside and around the distribution board reflects the quality of the entire installation. Neat, well-organised wiring makes maintenance easier, fault finding faster, and inspections smoother.
Take pride in your board wiring. A well-wired consumer unit is a calling card for your workmanship. A poorly wired board — crossed cables, exposed copper, loose terminals — will be photographed and shared by the next electrician who opens it.
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