Consumer Unit RegulationsAmendment 4 Guide (A4:2026)
Consumer unit regulations in the UK are set by BS 7671:2018+A4:2026 (the IET Wiring Regulations, 18th Edition). They require metal (non-combustible) enclosures under Regulation 421.1.201, 30 mA RCD protection for almost all domestic circuits (Regulations 411.3.3 and 411.3.4), and transient overvoltage (surge) protection under Regulation 443.4. Amendment 4 (A4:2026) makes arc fault detection devices (AFDDs) a requirement for socket-outlet final circuits up to 32 A in higher-risk residential buildings, HMOs, student accommodation and care homes under Regulation 421.1.7, and requires devices to be selected for the correct current direction under Regulation 530.3.201.
The definitive guide to consumer unit regulations under BS 7671:2018+A4:2026. Metal enclosures, RCD protection, RCBO design, SPD requirements, AFDDs, and the Amendment 4 bidirectional device rule (Regulation 530.3.201).
Current Consumer Unit Regulations
The consumer unit (or distribution board in commercial terminology) is the central point of every electrical installation. It houses the main switch, the protective devices for each circuit (MCBs, RCBOs, or RCDs with MCBs), and increasingly, surge protection devices (SPDs). The regulations governing consumer units have evolved significantly over the past decade, driven primarily by fire safety concerns and the increasing complexity of domestic electrical installations.
The current standard for consumer units in domestic premises is set by BS 7671:2018+A4:2026 — the 18th Edition of the IET Wiring Regulations with Amendment 4, issued in July 2024. This standard, together with the product standard BS EN 61439-3 (which specifies construction and performance requirements for distribution boards), defines what a consumer unit must be made of, what protection it must contain, and how it must be installed.
For electricians, the consumer unit replacement is one of the most common jobs in domestic work. It is also one of the most regulation-intensive: it is notifiable under Part P, requires a full Electrical Installation Certificate (EIC), and involves decisions about RCD architecture, SPD provision, circuit labelling and (since Amendment 4) the suitability of protective devices for bidirectional current flow. Getting it right requires a thorough understanding of the current regulations.
Protective Devices at a Glance
| Device | Protects against | Typical use in a consumer unit |
|---|---|---|
| MCB | Overload and short-circuit (overcurrent) | Per-circuit overcurrent protection, grouped behind a shared RCD on a split-load board. |
| RCD (30 mA) | Earth leakage / electric shock (additional protection) | One or two main RCDs covering groups of circuits on a split-load board. |
| RCBO | Overcurrent + earth leakage in one device | Individual per-circuit protection — the basis of a full RCBO board. |
| AFDD | Series and parallel arc faults (fire risk) | Socket-outlet final circuits up to 32 A; required in higher-risk premises (Reg 421.1.7). |
| SPD | Transient overvoltage (surge / lightning, switching) | Single module at the origin, protecting the whole installation (Section 443). |
Metal Consumer Unit Requirement (Regulation 421.1.201)
Regulation 421.1.201 states that in domestic (household) premises, consumer units and similar switchgear assemblies shall comply with BS EN 61439-3 and shall have their enclosure manufactured from non-combustible material. This regulation was first introduced as part of Amendment 4 to the 17th Edition (BS 7671:2008+A3:2015) and carried forward into the 18th Edition.
The background to this requirement was a series of investigations by the Electrical Safety First charity (formerly the Electrical Safety Council) and the London Fire Brigade, which identified that a significant number of domestic fires originated at or near the consumer unit. In many cases, plastic consumer unit enclosures contributed to the spread of fire by melting and dripping burning plastic onto materials below. A metal enclosure, while it cannot prevent the initial fault, contains the fire within the enclosure and prevents the spread of burning material.
The practical implications are clear. Every new consumer unit installed in a domestic premises must have a metal enclosure. Every consumer unit replacement — whether upgrading from an old rewirable fuse board or replacing a relatively modern plastic board — must use a metal enclosure. The only exception is where the consumer unit is installed within a purpose-built, non-combustible enclosure that provides equivalent fire containment — but in practice, this is rare, and metal consumer units are the universal solution.
It is important to note that this regulation does not require the retrospective replacement of existing plastic consumer units. If a plastic consumer unit was installed when it was compliant (before the regulation change), it does not need to be replaced solely because of this regulation. However, if any work is done that requires the consumer unit to be replaced or significantly modified, the new installation must comply with the current regulations, including the metal enclosure requirement.
RCD Protection Requirements (BS 7671 Section 411)
BS 7671 Section 411 sets out where additional protection by a 30 mA RCD is required. An RCD providing additional protection must have a rated residual operating current (I∆n) not exceeding 30 mA and meet the characteristics in Regulation 415.1.1. The requirements have expanded with each edition, and in the current 18th Edition almost every circuit in a domestic consumer unit needs RCD protection.
When 30 mA RCD Protection Is Required
| Circuit / situation | Regulation | Requirement |
|---|---|---|
| Socket-outlets up to 32 A (general locations) | 411.3.3 | 30 mA RCD required. A documented risk assessment may omit protection in certain non-domestic cases, but never for sockets liable to be used by ordinary persons or children. |
| Mobile equipment for use outdoors up to 32 A | 411.3.3 | 30 mA RCD required — external sockets, garden supplies, and outbuilding feeds. |
| AC final circuits supplying luminaires (domestic) | 411.3.4 | 30 mA RCD required — added by Amendment 4 era updates; applies to all household lighting final circuits. |
| Cables concealed in a wall or partition at < 50 mm depth | 522.6.202 (Table 52.1) | 30 mA RCD required unless the cable is in a prescribed zone with an earthed metallic covering or earthed containment (per 522.6.204). |
| Cables in walls containing metallic parts (e.g. metal-stud) | 522.6.202 (Table 52.1) | 30 mA RCD required regardless of depth, unless equivalent protection is provided. |
The practical effect is that almost every circuit in a domestic consumer unit now requires 30 mA RCD protection. The main exceptions are circuits supplying fixed equipment with cables that are clipped to the surface (not concealed in walls), run in earthed metallic containment, or routed at a depth exceeding 50 mm. Note that an RCD is never recognised as a sole means of protection (Regulation 415.1.2) — it supplements basic and fault protection rather than replacing them.
RCBO Board vs Dual RCD Split-Load Design
When designing a domestic consumer unit, the electrician must choose between two main RCD architectures: a dual RCD split-load board (two RCDs each protecting a group of circuits via MCBs) or a full RCBO board (individual RCBOs providing combined RCD and overcurrent protection for each circuit).
Dual RCD Split-Load
- Lower cost — Two RCDs plus MCBs cost less than individual RCBOs for every circuit, making the split-load arrangement the lower-cost option at the point of installation.
- Poor discrimination — A fault on any single circuit trips the RCD for that entire group, disconnecting all circuits on that side. This can mean losing the cooker, lighting, and sockets on one side of the house due to a single fault.
- Cumulative leakage — The standing earth leakage from all circuits on one RCD side adds up. If the cumulative leakage exceeds 30% of the RCD rating (approximately 10 mA), nuisance tripping becomes likely.
Full RCBO Board
- Excellent discrimination — A fault on any circuit trips only that circuit. Every other circuit remains live, minimising disruption to the household.
- No cumulative leakage — Each RCBO monitors only its own circuit, so standing earth leakage from other circuits does not affect it. This virtually eliminates nuisance tripping.
- Easier fault diagnosis — When a single RCBO trips, you know immediately which circuit has the fault. No need to isolate circuits one by one to identify the problematic one.
| Consideration | Dual RCD split-load | Full RCBO board |
|---|---|---|
| Upfront cost | Lower | Higher per circuit |
| Fault on one circuit | Disconnects the whole RCD group | Trips only that circuit |
| Cumulative earth leakage | Shared across the group — nuisance-trip risk | Isolated per circuit |
| Fault diagnosis | Isolate circuits to find the fault | Tripped device pinpoints it |
For most new domestic installations and consumer unit replacements, full RCBO boards are now the preferred choice among professional electricians. The additional material cost is easily justified by the improved discrimination, reduced nuisance tripping, and better fault diagnosis. The time saved in call-backs for nuisance tripping alone often pays for the cost difference. For pricing context, see our consumer unit replacement cost guide.
Amendment 4 (A4:2026) — Key Changes
BS 7671:2018+A4:2026 is an amendment document that modifies and adds to the existing 18th Edition rather than being a new book. The most significant changes for consumer unit design are the bidirectional device rule in Regulation 530.3.201, the redrafted AFDD requirement in Regulation 421.1.7, and the redrafted surge protection criteria in Regulation 443.4.
Regulation 530.3.201 — Bidirectional and Unidirectional Devices
This new regulation addresses a growing concern in modern electrical installations: the presence of power sources that can feed energy in both directions through the consumer unit. Traditional installations have a single source of supply (the DNO mains), and all fault current flows in one direction — from the supply, through the fault, and back to the source. Standard MCBs, RCDs, and RCBOs are designed for this unidirectional fault current.
However, with the rapid growth of domestic battery storage systems, solar PV with battery backup, and vehicle-to-grid (V2G) EV chargers, many installations now have local generation or storage that can supply fault current in the reverse direction — from the battery or generator, through the consumer unit, into the fault. If the protective devices are only rated for unidirectional fault current, they may not operate correctly under a reverse fault condition, potentially failing to clear the fault and allowing it to persist.
Regulation 530.3.201 (Amendment 4) requires that the selection and erection of equipment for protection takes account of the appropriate use of either a unidirectional or a bidirectional protective device. The accompanying note records that product standards for devices such as RCCBs, RCBOs, circuit-breakers and AFDDs require them to be marked to show direction — for example “in”/“out”, “line”/“load”, or arrows. For consumer unit design, this means checking the device markings and the manufacturer's data so that a device installed where reverse current can flow is actually rated for it, or implementing measures so reverse fault current is cleared correctly.
Regulation 421.1.7 — Arc Fault Detection Devices (AFDDs)
Regulation 421.1.7 sits in Part 4 — Protection for Safety, Chapter 42. Amendment 4 redrafted it so that arc fault detection devices (AFDDs) are now a firm requirement for some premises and a recommendation for others. AFDDs mitigate the risk of fire caused by arc fault currents in AC final circuits.
| Premises type | AFDD status under 421.1.7 |
|---|---|
| Higher-risk residential buildings, HMOs, purpose-built student accommodation, care homes | Required (shall) for socket-outlet final circuits up to 32 A |
| All other premises (incl. typical dwellings) | Recommended for single-phase AC final circuits supplying socket-outlets up to 32 A |
Using AFDDs does not remove the need to apply the other protective measures in the standard. Designers should consider AFDD provision at the consumer unit design stage — particularly for circuits in older wiring systems where degraded insulation may be present.
AFDDs detect the characteristic high-frequency signatures of arc faults in electrical wiring and disconnect the circuit before a sustained arc can ignite surrounding materials. They are installed in the consumer unit in place of, or in addition to, standard MCBs or RCBOs. The combination AFDD+RCBO provides overcurrent, earth fault, and arc fault protection in a single device.
Beyond the Section 530 bidirectional device requirements and the new AFDD recommendation (Reg 421.1.7), Amendment 4 includes various corrections, clarifications, and editorial amendments to the standard. It is important to note that Amendment 4 does not change the fundamental requirements for consumer unit design — metal enclosures, RCD protection, circuit protection, and labelling requirements all remain as per the main 18th Edition text. The amendment adds to the requirements rather than replacing them.
Surge Protection Device (SPD) Requirements
BS 7671 Section 443 covers protection against transient overvoltages — voltage spikes caused by lightning strikes (direct or nearby) and switching events on the supply network. Amendment 4 redrafted Regulation 443.4. Protection against transient overvoltages must now be provided wherever an overvoltage could result in either of the two consequences below; for all other cases protection is still required unless the owner of the installation declares it is not required because any loss or damage is tolerable and they accept the risk. The old risk-assessment method of the previous amendment (Regulation 443.5) has been deleted.
When SPD Protection Must Be Provided (Regulation 443.4.1)
- (a) serious injury to, or loss of, human life.
- (c) significant financial or data loss.
Limb (b) was deleted by the BS 7671:2018+A2:2022 Corrigendum (May 2023), leaving the two consequence conditions above.
For all other cases, protection against transient overvoltages shall be provided unless the owner of the installation declares it is not required due to any loss or damage being tolerable and they accept the risk of damage to equipment and any consequential loss. Designers should therefore evaluate the specific installation rather than assume SPDs are always mandatory or never needed.
Where protection is required — or where the owner does not declare it unnecessary — SPDs should be installed. The average UK home now contains thousands of pounds worth of electronic equipment — smart TVs, computers, broadband routers, smart home devices, LED lighting drivers, heating controls, and appliances with electronic controllers. A single transient overvoltage event can damage multiple items simultaneously, making SPD provision a sound protective measure.
SPDs are installed at the origin of the installation, either within the consumer unit (many modern consumer units have dedicated SPD modules) or in a separate enclosure adjacent to the consumer unit. The SPD requires a dedicated overcurrent protective device (MCB or fuse) for its supply, sized in accordance with the manufacturer's instructions. Regulation 534.4.10 sets the minimum cross-sectional area of SPD connecting conductors:
| Connecting conductor (Reg 534.4.10) | Type 2 SPD at origin | Type 1 SPD at origin |
|---|---|---|
| To main earthing terminal / protective conductor | ≥ 6 mm² copper | ≥ 16 mm² copper |
| Connecting SPDs and OCPDs to live conductors | ≥ 2.5 mm² copper | Per manufacturer / short-circuit rating |
SPD connecting leads should also be kept as short as practicable to preserve the level of protection. A status indicator on the SPD confirms it is operational — when the indicator changes state to show it has operated, the SPD has expended its protection capacity and must be replaced.
For installations on overhead supply lines, the risk of lightning-induced transient overvoltages is significantly higher than for underground supplies, and SPDs are almost always required. The DNO supply arrangement should be considered during the design stage and documented on the Electrical Installation Certificate.
Labelling, Certification, and Documentation
BS 7671 requires comprehensive labelling and documentation of the consumer unit and its circuits. Regulation 514.9.1 requires a diagram, chart, table or equivalent that indicates the type and composition of each circuit, the method used for compliance with automatic disconnection (Regulation 410.3.2), and the information needed to identify each protective, isolation and switching device and its location. For simple installations this may be a schedule, a durable copy of which must be kept within or adjacent to the distribution board. Amendment 4 added an exception so this need not be applied for domestic (household) premises where an initial verification certificate or an Electrical Installation Condition Report — complete with the guidance for recipients in Appendix 6 — has been issued to the person ordering the work.
Other notices required at or near the consumer unit include: an instruction notice on periodic inspection and testing recording the date of last and recommended next inspection (Regulation 514.12.1, with its own domestic-premises exception under Amendment 4 where appropriate certification is issued); an isolation warning notice where live parts cannot be isolated by a single device (Regulation 514.11.1); the earthing arrangement and any RCD test advice; and, where the installation has more than one source of supply, an appropriate warning notice. Devices must be arranged and identified so that the circuit they protect can be easily recognised (Regulation 514.8.1).
A consumer unit replacement requires a full Electrical Installation Certificate (EIC) — not a Minor Works Certificate. The EIC must include the design, construction, and inspection and testing sections completed in full, together with a schedule of test results for every circuit. The schedule must include continuity of protective conductors (R1+R2), insulation resistance, polarity, earth fault loop impedance (Zs), prospective fault current (Ipf), and RCD test results. Elec-Mate generates all of these forms digitally, validates test results against BS 7671 limits, and produces professional PDF output ready to hand to the client. For a full understanding of Amendment 4 requirements, see the 18th Edition training course.
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Frequently Asked Questions
Do all consumer units need to be metal under BS 7671?
BS 7671 Regulation 421.1.201 requires that consumer units and similar switchgear assemblies in domestic premises shall comply with BS EN 61439-3 and shall have their enclosure manufactured from non-combustible material. In practice, this means metal (steel) consumer units. This regulation was introduced in Amendment 4 to the 17th Edition (which carried forward into the 18th Edition) following concerns about the fire risk associated with plastic consumer units. The requirement applies to new installations and to consumer unit replacements in existing installations. It does not require the retrospective replacement of existing plastic consumer units that were compliant when installed — but if the consumer unit is being replaced for any reason, the new unit must be metal. Some manufacturers produce consumer units with a metal enclosure and a non-combustible plastic interior, which also comply with the regulation.
What did Amendment 4 (A4:2026) change for consumer units?
Amendment 4 to BS 7671:2018 introduced several changes relevant to consumer units. Regulation 530.3.201 requires that protective devices be selected and erected with the appropriate use of either a unidirectional or a bidirectional device, and notes that products such as RCBOs, RCCBs, circuit-breakers and AFDDs must be marked to show direction — important where battery storage, solar PV with battery backup or vehicle-to-grid charging can drive current in reverse. Regulation 421.1.7 was redrafted to make AFDDs a requirement for socket-outlet final circuits up to 32 A in higher-risk residential buildings, HMOs, purpose-built student accommodation and care homes (and a recommendation elsewhere). Regulation 443.4 on transient overvoltage protection was also redrafted, with the previous risk-assessment method (Regulation 443.5) deleted. Amendment 4 adds to the requirements rather than replacing the fundamentals of consumer unit design.
Should I use RCBOs or a split-load board with dual RCDs?
Both approaches are acceptable under BS 7671, but each has advantages and disadvantages. A split-load board uses two RCDs (typically 63A or 80A, 30mA) to protect groups of circuits, with non-RCD-protected circuits (such as fire alarm and emergency lighting supplies) on the main switch side. The advantage is lower cost — two RCDs plus MCBs are cheaper than individual RCBOs for every circuit. The disadvantage is that a fault on any circuit protected by a particular RCD will trip that RCD and disconnect all circuits sharing it, causing partial loss of supply. An RCBO board provides individual RCD and overcurrent protection for each circuit. A fault on one circuit trips only that circuit. This provides much better discrimination and avoids nuisance tripping affecting multiple circuits. The cost per circuit is higher, but the practical benefits are significant — particularly in larger installations where losing half the circuits to a single RCD trip would cause major inconvenience. For most new domestic installations, full RCBO boards are now the preferred approach.
When is a surge protection device (SPD) required in a consumer unit?
Under BS 7671:2018+A4:2026, Regulation 443.4.1 requires transient overvoltage protection where an overvoltage could result in (a) serious injury to, or loss of, human life, or (c) significant financial or data loss — limb (b) having been deleted by the A2:2022 Corrigendum (May 2023). For all other cases protection must still 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 of damage to equipment and any consequential loss. The old risk-assessment method (Regulation 443.5) has been deleted. Where protection is required, SPDs are installed at the origin — within or adjacent to the consumer unit — with a dedicated overcurrent protective device. Regulation 534.4.10 sets the minimum connecting conductor as 6 mm² copper for Type 2 SPDs and 16 mm² copper for Type 1 SPDs at the origin. A status indicator confirms the SPD is still operational.
Is replacing a consumer unit notifiable under Part P?
Yes. Replacing a consumer unit is always notifiable under Part P of the Building Regulations, regardless of whether it is a like-for-like replacement or an upgrade. This is because the consumer unit is the central point of the installation that houses all the main protective devices, and its correct design and installation are critical to the safety of the entire installation. The replacement must be either self-certified by an electrician registered with a competent person scheme (NICEIC, NAPIT, ELECSA, or BRE), or notified to the local authority building control department before the work begins. A full Electrical Installation Certificate (EIC) is required for a consumer unit replacement — a Minor Works Certificate is not appropriate because the work involves the installation or replacement of a distribution board.
What RCD protection is required in a domestic consumer unit?
BS 7671 requires additional protection by a 30 mA RCD for several categories of circuit. Regulation 411.3.3 covers socket-outlets rated up to 32 A and mobile equipment for use outdoors up to 32 A. Regulation 411.3.4 requires 30 mA RCD protection for AC final circuits supplying luminaires in domestic premises. Table 52.1 (Regulation 522.6.202) requires it for cables concealed in walls at less than 50 mm depth unless they have an earthed metallic covering or are in a prescribed zone with equivalent protection. In practice this covers the vast majority of circuits in a domestic installation. The circuits that may not need 30 mA RCD protection are typically those supplying fixed equipment with surface-clipped or metallic-contained cabling at greater depth — though many designers provide protection anyway as good practice. Note that an RCD is never recognised as a sole means of protection (Regulation 415.1.2).
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