The short answer
The 18th Edition of BS 7671 (in force from January 2019) introduced a requirement that consumer units in domestic premises must be housed in a non-combustible enclosure — in practice, a metal enclosure (or a plastic enclosure meeting a specific fire resistance classification). The change was driven by evidence that fires starting inside older plastic consumer units could spread more rapidly because the casing itself could ignite and contribute fuel. For new installations and replacements, a metal-enclosure consumer unit is now the standard. However, an existing plastic consumer unit is not automatically required to be replaced — the regulation applies to new work. A plastic board that is otherwise sound and compliant does not fail an EICR solely because it has a plastic enclosure, though a surveyor or assessor may note it.
The switch to metal consumer unit enclosures is one of the most visible changes from the 18th Edition of BS 7671. Here is what the rule actually says, why it was introduced and what it means for existing boards.
Metal vs plastic — the rule
- 18th Edition in force fromJanuary 2019
- New installationsNon-combustible (metal) enclosure required
- Existing plastic boardsNot automatically required to be replaced
- EICR impactSurveyor may note it; C3 code likely
- Fire test standard850°C glow-wire test for non-combustible equivalents
What BS 7671 18th Edition actually says
Regulation 421.1.201 of BS 7671 18th Edition states that consumer units and similar switchgear used in domestic premises must be enclosed in a material that satisfies a specific fire resistance requirement — the 850°C glow-wire test as specified in BS EN 60695-2-11. In plain terms, the enclosure must not ignite or sustain combustion when exposed to a severe internal heat source.
A standard metal enclosure meets this requirement straightforwardly — steel does not burn. A plastic enclosure that passes the glow-wire test at 850°C also meets the requirement technically, but in practice, most compliant boards fitted since 2019 use metal enclosures.
The regulation applies when a new consumer unit is installed — whether as a new installation or as a replacement of an existing board. An electrician fitting a consumer unit from January 2019 onwards must use a compliant enclosure. A plastic enclosure that was fitted before 2019 and has not been replaced is not automatically in breach of anything — the 18th Edition does not apply retroactively to existing installations.
Why the change was made
The rationale behind the change came from analysis of domestic electrical fires. Electrical faults — particularly arcing faults caused by deteriorated connections, overloaded circuits or rodent damage — occur more often in the consumer unit than in any single other location in a house. The evidence from fire incidents suggested that plastic enclosures could contribute to fire spread: once the casing ignited, it added fuel and extended the duration of the fire.
A metal enclosure contains the fault and limits fire spread from the unit itself. An arc flash or overheating event inside a metal enclosure is more likely to burn itself out without setting fire to the wall or the surrounding wiring. This is not a theoretical difference — it reflects documented fire investigation data available to the IET when the 18th Edition was drafted.
The change was also relatively straightforward to implement: metal consumer units were already the standard in commercial and industrial installations, and manufacturing and supply for the domestic market quickly shifted.
Does an existing plastic consumer unit need to be replaced?
No — not automatically. An existing plastic consumer unit that was lawfully installed before the 18th Edition came into force is not required to be replaced by the regulation itself. The 18th Edition applies to new work, not to existing installations as a retroactive obligation.
What the enclosure type can affect is:
- EICR (Electrical Installation Condition Report): an assessor carrying out a periodic inspection may record the plastic enclosure as a C3 code — 'improvement recommended'. C3 codes do not constitute a fail; they flag something that does not meet current recommendations but is not an immediate danger. A plastic board that is otherwise sound and compliant would typically receive a C3 for the enclosure type, not a C2 (potentially dangerous).
- Property sale: a surveyor may note the plastic enclosure in a home buyers' survey, which can prompt buyer questions. It is unlikely to block a sale in isolation, but combined with other age-related observations it can contribute to a negotiation.
- Replacement trigger: if the board needs replacing for another reason — it is full, damaged, or has rewireable fuses — the replacement will naturally use a compliant metal enclosure. The plastic-to-metal upgrade comes as part of the replacement.
| Scenario | Enclosure requirement | Notes |
|---|---|---|
| New installation (from Jan 2019) | Metal or non-combustible required | Regulation 421.1.201 applies |
| Replacement of existing board | Metal or non-combustible required | Treated as new installation |
| Existing plastic board, no other issue | Not required to replace | C3 code on EICR likely |
| Plastic board with damage or faults | Replacement recommended | Fault, not enclosure, is the trigger |
| Pre-2019 plastic board, sound, tested | No immediate action required | Monitor at periodic EICR interval |
Indicative guidance. Sources: IET BS 7671 18th Edition Regulation 421.1.201; Electrical Safety First guidance.
Frequently asked questions
When did the metal consumer unit rule come in?
The requirement for a non-combustible (in practice, metal) enclosure for consumer units in domestic premises came into force with the 18th Edition of BS 7671, which applied to new installations from January 2019.
Will my plastic consumer unit fail an EICR?
A plastic consumer unit will typically receive a C3 code ('improvement recommended') on an EICR rather than a fail. C3 is not a failure — it flags that the installation does not meet current recommendations but does not indicate an immediate danger. However, if the plastic board has other faults (C1 or C2 codes), those are the priority.
Can I still buy a plastic consumer unit?
Compliant plastic enclosures — those that pass the 850°C glow-wire test under BS EN 60695-2-11 — remain technically permissible under the 18th Edition. However, the domestic market has almost entirely shifted to metal enclosures, which meet the requirement straightforwardly and are what most electricians now fit as standard for domestic installations.
Sources & further reading
- IET — BS 7671 18th Edition Regulation 421.1.201
- Electrical Safety First — consumer unit replacement best practice guide
- NICEIC — 18th Edition consumer unit requirements
Figures on this page are typical UK ranges drawn from published sources and depend on your specific property and installation. They are guidance, not a quotation.