Consumer Unit Standards: RCBO, SPD, and Amendment 2 Requirements

Summary

BS 7671:2018 Amendment 2, effective from 28 March 2022, significantly tightened surge protection requirements by removing the previous risk assessment opt-out and making SPDs the default for virtually all installations. The amendment deleted the old Clause 443.5 risk assessment method and replaced it with a straightforward requirement: fit SPDs unless the owner explicitly accepts the risk in writing. For consumer unit hardware, the industry has moved decisively toward full RCBO boards, where every circuit has its own combined RCD/MCB device, eliminating the nuisance tripping and half-property blackouts common with split-load dual-RCD arrangements. Metal (non-combustible) enclosures have been mandatory for domestic consumer units since 1 January 2016 under the 17th Edition Amendment 3, carried forward into the 18th Edition. These three requirements together -- SPDs, appropriate RCD protection, and metal enclosures -- define the modern domestic consumer unit installation.

Key Facts

• SPDs are the default under Amendment 2 (Regulation 443.4.1). The previous risk assessment route (old Clause 443.5) has been deleted entirely.
• Safety services make SPDs non-negotiable. If the installation feeds a mains-wired smoke alarm, fire alarm, or emergency lighting, the owner cannot decline SPD protection.
• Type 2 SPDs are appropriate for the vast majority of domestic consumer units. Type 1 devices are for installations with external lightning protection systems or overhead supply lines.
• Metal enclosures have been required for domestic consumer units since 1 January 2016 (Regulation 421.1.201), driven by London Fire Brigade data showing 253 consumer unit fires in 2013/14 alone.
• Full RCBO boards are not mandated by BS 7671 but are now the dominant choice among professional electricians, offering per-circuit fault isolation.
• Split-load boards remain compliant under BS 7671 provided RCD groupings meet the selectivity requirements of Regulation 531.3.
• EV charger circuits require Type A (minimum) RCDs per Regulation 722.531.3.101, each individually protected -- they cannot share an RCD with other circuits.
• The cost premium for a full RCBO board over a dual-RCD split-load is typically 200-400 pounds on material, a marginal uplift against the labour cost of a consumer unit change.

Detailed Guidance

What changed with BS 7671 Amendment 2?

Amendment 2 of BS 7671:2018 (published as BS 7671:2018+A2:2022) came into effect on 28 March 2022. The changes most relevant to consumer unit specification are:

Surge protection (Chapter 443):

• The risk assessment method previously in Clause 443.5 has been completely removed. Under the original 18th Edition, designers could calculate a risk factor and potentially justify omitting SPDs. That route no longer exists.
• Regulation 443.4.1 now requires transient overvoltage protection wherever the consequence could cause serious injury, loss of life, failure of a safety service, or significant financial/data loss.
• For all other cases, SPDs must still be fitted unless the installation owner formally declares the risk of damage to wiring and equipment is tolerable and they accept any consequential loss.
• The practical effect: SPDs are now fitted to almost every new or rewired domestic installation. The paperwork burden of documenting a client refusal, combined with the low cost of a Type 2 SPD module (typically 40-80 pounds), means most electricians simply include them as standard.

Safety services definition (Part 2):

• BS 7671 defines a safety service as any electrical system provided to protect or warn persons in the event of a hazard, or essential to their evacuation. In a domestic context, a mains-wired smoke alarm qualifies. Since almost every domestic installation has one, the SPD requirement is in practice near-universal.

Other Amendment 2 changes affecting consumer units:

• Updated requirements for arc fault detection devices (AFDDs) in certain premises types, though not yet mandatory for standard domestic installations.
• Clarified requirements around prosumer (solar PV/battery) installations and their protection.

When is an SPD required?

If the owner declines SPD protection:

• Obtain the refusal in writing.
• Explain clearly the benefits of SPDs and the consequences of not fitting them, including potential damage to sensitive electronic equipment from transient overvoltages.
• Record the decision on the Electrical Installation Certificate.
• The owner cannot decline if any part of the installation constitutes a safety service (mains-wired smoke alarm, fire alarm, emergency lighting).

SPD type selection:

For the vast majority of domestic work, a Type 2 SPD fitted in the consumer unit is sufficient. Type 1 or combined Type 1+2 devices are needed where the supply is via an overhead line or where a lightning protection system is installed on the building.

RCBO board vs split-load MCB+RCD -- which should I specify?

This is the single most common specification question on domestic consumer unit work. Here is a direct comparison:

When to specify a full RCBO board:

• New installations and full rewires (the default choice in 2024+).
• Any installation where the customer values reliability (home offices, medical equipment, aquariums, security systems).
• Properties with EV charger circuits or planned future EV provision.
• Rental properties where callback costs for nuisance tripping eat into margin.

When a split-load board may still be acceptable:

• Budget-constrained domestic work where the customer understands and accepts the nuisance tripping risk.
• Like-for-like replacements where the existing wiring configuration strongly favours a split-load arrangement.
• Small installations with few circuits (4-5 ways) where cumulative leakage is unlikely to cause issues.

The industry direction is clear. The majority of UK wholesalers now stock RCBO boards as the primary product line. Several major manufacturers have reduced or discontinued their split-load ranges. The 200-400 pound material premium is marginal against the total installed cost and the reduction in callbacks.

What about metal consumer units -- is that still required?

Yes. Metal (non-combustible) enclosures remain mandatory for all domestic consumer units.

History:

• 17th Edition Amendment 3 (BS 7671:2008+A3:2015) introduced Regulation 421.1.201, effective from 1 January 2016.
• The regulation was a direct response to London Fire Brigade data: 253 fires involving consumer units in 2013/14, with plastic enclosures contributing to fire spread.
• Regulation 421.1.201 requires that consumer units and similar switchgear assemblies in domestic premises shall comply with BS EN 61439-3 and have enclosures manufactured from non-combustible material.
• Steel (ferrous metal) is the accepted non-combustible material. Some manufacturers offer stainless steel or coated steel options.

Current position under 18th Edition:

• The requirement is carried forward unchanged into BS 7671:2018 and its amendments.
• It applies to all new consumer units installed in domestic (household) premises.
• It does not require retrospective replacement of existing plastic consumer units, but any consumer unit that is replaced or newly installed must be metal.
• The requirement applies to the main consumer unit and to any sub-distribution boards in domestic premises.

Practical notes:

• All major UK manufacturers (Hager, Schneider/Acti 9, Wylex, Lewden, BG) now produce only metal-enclosed domestic consumer units.
• Plastic consumer units can still be used in commercial and industrial premises where the fire risk assessment supports it.

How do I select the right consumer unit?

1. Number of ways

Count the circuits you need, then add spare capacity:

Always allow a minimum of 2 spare ways for future expansion (EV charger, home office circuit, garden supply). BS 7671 Regulation 132.8 requires the designer to consider future requirements.

2. Main switch rating

• 100 A is standard for most domestic installations fed by a 100 A DNO service fuse.
• 80 A main switch boards exist but are becoming less common.
• Ensure the main switch is a double-pole isolator disconnecting both line and neutral.

3. RCD/RCBO selection

• For full RCBO boards: specify 30 mA Type AC or Type A RCBOs as default. Use Type A for circuits feeding electronic equipment with DC components (EV chargers, variable speed drives, some modern appliances).
• For split-load boards: specify two 63 A 30 mA RCDs. Distribute circuits evenly, keeping lighting and power on separate RCDs where possible.
• Regulation 722.531.3.101 requires EV charger circuits to have individual Type A (minimum) RCD protection.

4. Type A RCD considerations

Type A RCDs detect both AC and pulsating DC fault currents. They are required for:

• EV charger circuits (where the charger incorporates an internal RDC-DD for DC fault currents above 6 mA; otherwise Type B is required).
• Circuits feeding equipment with electronic power conversion (inverter-driven motors, some modern washing machines).
• Any circuit where the designer's risk assessment identifies the possibility of DC fault current components.

Type A RCBOs cost marginally more than Type AC but are increasingly specified as the default for all circuits.

5. SPD provision

• Ensure the chosen board has a dedicated SPD slot or DIN rail space for a Type 2 SPD.
• Some boards come pre-fitted with SPDs; others require a separate module.
• The SPD must be connected upstream of the RCDs/RCBOs, typically directly after the main switch, with its own dedicated MCB (usually a dedicated SPD disconnector or a short-circuit protection device as specified by the SPD manufacturer).

6. Labelling and documentation

• BS 7671 Regulation 514.8 requires circuit charts at or near the consumer unit.
• All circuits must be labelled clearly and durably.
• Record the SPD type and manufacturer on the Electrical Installation Certificate.

Frequently Asked Questions

Do I need to replace an existing plastic consumer unit?

No. There is no retrospective requirement to replace a functioning plastic consumer unit. The metal enclosure requirement (Regulation 421.1.201) applies only when a consumer unit is newly installed or replaced. However, if any work triggers a consumer unit change (e.g., rewire, board upgrade, additional circuits requiring a larger board), the new unit must be metal.

Can the customer refuse an SPD and still get a certificate?

It depends. If the installation includes any safety service (mains-wired smoke alarm, fire alarm, emergency lighting), SPD protection is mandatory under Regulation 443.4.1 and cannot be declined. If no safety services are present, the owner can formally decline SPD protection under Regulation 443.4.2, but this must be documented in writing on or alongside the Electrical Installation Certificate. In practice, almost every domestic installation has a mains-wired smoke alarm, making SPDs effectively non-optional.

Is an AFDD (Arc Fault Detection Device) required in domestic consumer units?

Under the current BS 7671:2018+A2:2022, AFDDs are recommended but not mandatory for standard domestic installations. Regulation 421.1.7 recommends their use in premises with sleeping accommodation, locations with risk of fire due to stored materials, locations with combustible construction, and fire-propagating structures. AFDDs are likely to become more widely required in future amendments. They are already mandatory in some other European countries and in specific UK premises types (e.g., Houses in Multiple Occupation under some local authority requirements).

What SPD do I need for an overhead supply?

Installations supplied via an overhead line are more exposed to transient overvoltages from lightning strikes to the network. A Type 1 or combined Type 1+2 SPD should be installed at the origin of the installation. Check the SPD manufacturer's guidance for the correct impulse current (Iimp) rating -- typically 12.5 kA per pole for a Type 1 device in a domestic TN-S or TN-C-S installation.

Do I need a separate SPD for a three-phase supply?

Yes. Three-phase SPDs protect all three phases and neutral. A single-phase Type 2 SPD will not adequately protect a three-phase installation. Ensure the SPD is rated for the system voltage and earthing arrangement (TN-S, TN-C-S, or TT). TT installations require additional consideration as the SPD discharge path is via the earth electrode, which has a higher impedance than TN arrangements.

Regulations & Standards

• BS 7671:2018+A2:2022 -- Requirements for Electrical Installations (IET Wiring Regulations, 18th Edition with Amendment 2). Effective 28 March 2022.

• BS EN 61439-3 -- Low-voltage switchgear and controlgear assemblies: Distribution boards intended to be operated by ordinary persons (DBO).

• BS EN 61643-11 -- Low-voltage surge protective devices: Requirements and test methods for SPDs connected to low-voltage systems.

• Building Regulations Approved Document P -- Electrical safety in dwellings. Requires notification of consumer unit replacements to Building Control (or self-certification via a registered competent person scheme).

• Electricity at Work Regulations 1989 -- General duty to ensure electrical systems are safe.

• IET Wiring Matters: The Impact of Amendment 2 of the 18th Edition

• NAPIT / Professional Electrician: BS 7671:2018+A2:2022 Surge Protection Changes

• Professional Electrician: Amendment 2 Key Changes Explained

• NICEIC: When Is It Necessary to Install Surge Protection Devices?

• Lewden: Amendment 2 and the Design and Installation of Residential Consumer Units

• Electrical Safety First: Consumer Unit Mythbuster

• IET Wiring Matters: RCDs for Electric Vehicle Supply Equipment

• IET Wiring Matters: Surge Protective Devices (SPDs)

• Electrician Courses 4U: 17th Edition Amendment 3 Metal Clad Consumer Unit

• Hager: RCD Requirements for EV Charging

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