Consumer Unit Upgrade: RCBO Boards, Metal Enclosures & Amendment 3 Requirements

Summary

Consumer unit replacement is one of the most common electrical jobs in the UK — driven by age (rubber-insulated cables, rewirable fuses), insurance requirements, landlord upgrade obligations, and EV charger or solar PV additions requiring new circuits. Understanding the current regulatory requirements is essential: installing a non-compliant consumer unit (plastic enclosure, insufficient RCD coverage) is a Building Control failure and can affect property sale.

The trajectory of consumer unit regulation has been consistently toward more protection per circuit. Split-load boards (half with one RCD, half with another) were the industry standard for many years and represented an improvement over no-RCD boards. The limitation is that one RCD failure or trip affects multiple circuits simultaneously. RCBO boards (one RCBO per circuit, metal enclosure, with optional main switch) resolve this: each circuit has its own MCB and RCD in a single device. A nuisance trip on the bathroom circuit, for example, does not affect the kitchen or any other circuit.

The addition of SPD (surge protection device) requirements under Amendment 2 of BS 7671:2018 is now well-established. SPDs protect connected equipment from transient overvoltages (from lightning, grid switching events). While risk assessment can justify omission, in practice most installations in domestic premises now include an SPD.

Key Facts

• Metal enclosure mandatory — since Amendment 2 (2016); all new domestic consumer units must be in metal (non-combustible) housing
• RCBO — Residual Current Circuit Breaker with Overcurrent protection; one device per circuit; 30mA sensitivity for domestic circuits
• Split-load board — legacy design; half-board under one RCD, half under another; still compliant but inferior fault isolation
• SPD (Surge Protection Device) — Type 2 SPD required in new installations per BS 7671:2018 unless risk assessment shows low risk
• BS 7671:2018 (Amendment 2:2022) — current wiring regulations; applies to all new work from 28 September 2022
• Amendment 3 (2024) — further updates to BS 7671; check IET for latest changes [verify]
• RCD types: Type AC (detects sinusoidal earth fault currents), Type A (AC + DC pulsating), Type F (AC + DC pulsating + composite frequency), Type B (all including smooth DC)
• EV charger circuits — require Type A or Type F RCD minimum; check EV charger manufacturer requirement
• Solar PV/battery storage — may require Type B RCD on supply circuits; check DNO and inverter guidance
• Part P notifiable — consumer unit replacement is always notifiable under Part P
• Building Control options — NICEIC/NAPIT/ELECSA self-certification or direct Building Control application
• Isolation — main switch must isolate all poles; typically 100A double-pole main switch
• Minimum RCBO sensitivity — 30mA for protection against indirect contact (electric shock); BS 7671 Regulation 411.3.3
• AFDD (Arc Fault Detection Device) — emerging requirement; not yet mandatory in domestic BS 7671 but recommended for cable routes in old buildings; may become mandatory under future amendments
• Consumer unit size — specify number of ways based on current circuits + 20% spare for future
• Earthing and bonding — must be verified as part of any consumer unit upgrade; main bonding to gas and water services

Quick Reference Table

Detailed Guidance

Metal Enclosure: Why It Matters

Prior to Amendment 2 (2016), consumer units were commonly supplied in white plastic enclosures. An arc fault or sustained short circuit inside a plastic consumer unit can ignite the plastic itself and spread fire into the consumer unit compartment and adjacent structure. The metal enclosure amendment was introduced following a significant number of domestic fires originating from plastic consumer units.

Metal consumer units (Hager Invicta, Wylex Amendment 2 Series, Schneider iKQ, British General Fortress) have a steel or aluminium enclosure that contains any arc fault and prevents fire spread. The internal components (MCBs, RCBOs, busbars) are the same as plastic units — only the housing differs.

Key rule: any domestic consumer unit installed from 2016 onwards must be metal. This applies to replacements, not only new installations. A split-load plastic consumer unit installed in 2018 is non-compliant — a subsequent electrical inspection (EICR) will note this as a Code C2 (potentially dangerous) or C3 (improvement recommended) depending on the age and condition.

RCBO Boards: The Current Best Practice

An RCBO (Residual Current Circuit Breaker with Overcurrent) combines the function of an MCB (overcurrent protection) and an RCD (earth fault protection) in a single 18mm module. Each circuit has its own RCBO, meaning:

• Earth fault on kitchen circuit → only kitchen circuit trips
• No effect on boiler, freezer, alarm, or other circuits
• Quicker fault identification — the tripped RCBO tells you exactly which circuit faulted

For comparison, a split-load board with two 30mA RCDs: an earth fault in any bathroom socket trips the entire left half of the board — potentially losing the boiler, bedroom circuits, and hall lighting simultaneously. Finding the fault requires isolating each circuit on that half individually.

RCBO selection:

• Type AC RCBO: standard; for purely sinusoidal earth fault currents
• Type A RCBO: includes detection of pulsating DC; required where devices contain electronic power supplies (most modern equipment)
• Type F RCBO: composite frequency; required for some variable frequency drives
• Type A is now the recommended minimum for all domestic circuits (most loads contain electronics)

RCBO MCB ratings: Select based on circuit design current: 6A (lighting), 16A (sockets small ring or radial), 20A (large kitchen appliance radials), 32A (ring final circuits, showers up to 8kW), 40A (larger showers, cookers), 50A or 63A (large cooker/EV circuits).

Surge Protection Devices (SPDs)

BS 7671:2018 (the 18th Edition wiring regulations) introduced requirements for surge protection devices in new installations. Amendment 2 (effective September 2022) further clarified this. The regulations require risk assessment to determine whether SPD is needed; in practice, the risk assessment almost always results in SPD installation unless the installation has a separate lightning protection system.

SPD purpose: Transient overvoltages from lightning strikes (direct and indirect) and utility grid switching can reach thousands of volts for microseconds — enough to damage sensitive electronics. A Type 2 SPD clamps transient voltages to a safe level before they damage connected equipment.

SPD types:

• Type 1: Installed at service head; not required in most domestic
• Type 2: Installed at consumer unit; required per 18th edition risk assessment
• Type 3: Point-of-use (plug-in surge protectors at individual devices); supplementary only

SPD installation:

• Installed at the origin of the installation (or as close as possible)
• Requires dedicated earth connection from the SPD to the earthing terminal bar
• Takes 1–2 module spaces in the consumer unit (or external enclosure)
• SPD has a status indicator (green window); turns red/orange when SPD has operated and needs replacement

Consumer Unit Sizing

Select the consumer unit based on:

1. Number of existing circuits (each needs one slot)
2. Additional circuits planned (EV charger, outbuilding, solar, etc.)
3. Spare ways (minimum 20% spare; aim for 4–6 spare ways in a 3-bed house)

Typical circuit count for a 3-bedroom semi-detached:

• 2–3 lighting circuits (ground, first, loft)
• 2 ring final socket circuits (ground and first floor)
• 1 kitchen radial (additional to ring, for fridge, dishwasher, microwave)
• 1 cooker circuit
• 1 shower circuit
• 1 boiler circuit
• 1 bathroom shaver/heated towel rail
• 1 external circuit
• 1 garage/outbuilding = 11–12 circuits + 4–6 spare = 16–18 way consumer unit minimum

Many electricians fit a 20-way or 24-way to provide adequate spare capacity for an EV charger, solar PV, or future extension.

Earthing and Bonding: Part of Every CU Upgrade

A consumer unit upgrade should always include:

• Verification of the earthing arrangement (TN-C-S earthing via PME or TN-S via earth wire in service cable)
• Check of main equipotential bonding conductors to gas and water services (10mm² minimum for TN-C-S)
• Check of supplementary bonding where required (older bathrooms without RCD protection on existing circuits)
• Earth electrode resistance test if TT earthing arrangement

If earthing is inadequate, the consumer unit upgrade cannot be completed safely. This is a common hidden cost that should be factored into survey-stage quotes.

Frequently Asked Questions

Do I need to replace a 2018 split-load board with an RCBO board?

Not immediately — a 2018 split-load board in a metal enclosure is compliant with the regulations at the time of installation. However, if you are carrying out notifiable electrical work (adding circuits, significant alterations), the inspection should include an assessment of the consumer unit. A split-load board is not a Code C2 failure on an EICR unless there is a specific deficiency. It may be noted as a C3 (improvement recommended) observation. Upgrade is best practice but not an immediate requirement.

Is SPD mandatory or can I omit it?

BS 7671:2018 Regulation 443 requires a risk assessment for overvoltage protection. The standard BEIS/IET guidance is that SPDs are required unless the installation is served by an underground cable supply (not overhead) AND the structure has a conventional lightning protection system. Most domestic properties in the UK do not have lightning protection and may have overhead or mixed supply routes — so SPD is required in practice. Omitting SPD without documenting a formal risk assessment is non-compliant.

Can I fit an RCBO board myself if I'm a competent DIYer?

Consumer unit replacement is always notifiable under Part P and is classified as a "complex" operation requiring either a registered competent person or Building Control approval. This means it must be carried out by a qualified electrician registered with NICEIC, NAPIT, ELECSA, or similar scheme, or with a Building Control application. Self-installation by an unregistered person is technically unlawful — it will cause problems on property sale (solicitors now routinely request electrical certificates for notifiable works).

What's an AFDD and do I need to fit one?

An Arc Fault Detection Device detects the signature of an arcing electrical fault — a leading cause of domestic fires from damaged cables (in walls, under floors, inside old equipment). AFDDs are not currently mandatory in UK domestic installations under BS 7671:2018, but are recommended by the regulations and have been mandatory in domestic new builds in some European countries. They are specified by some high-end electricians and may become mandatory in future amendments. In premises with old wiring, extensive hidden cable runs, or following arson risk assessments, AFDDs add meaningful protection.

Regulations & Standards

• BS 7671:2018 (18th Edition, Amendment 2:2022) — IET Wiring Regulations; consumer unit and SPD requirements

• Approved Document P — Part P Building Regulations; consumer unit replacement as notifiable work

• BS EN 61009 — Residual current circuit breakers with integral overcurrent protection (RCBOs)

• BS EN 61643-11 — Surge protective devices connected to low-voltage power distribution systems

• IET On-Site Guide (18th Edition) — Practical guidance on 18th Edition requirements

• IET — 18th Edition Wiring Regulations — Official wiring regulations and amendments

• NICEIC — Domestic Consumer Unit Guide — Guidance on compliant installations for electricians

• Hager UK — Consumer Unit Range — Metal consumer unit technical specifications

• Schneider Electric — SPD Guidance — SPD selection and risk assessment guidance

• consumer units — Consumer unit types, RCD theory, and circuit protection overview

• cable sizing — Circuit design and cable selection

• smoke alarm wiring — Smoke alarm wiring as part of notifiable works

• part p notifications — What electrical work is and is not notifiable