Why are consumer units now made of metal? (BS 7671 18th Edition explained)

The short answer

Consumer units in UK domestic premises must now have non-combustible enclosures — in practice, a metal case — under Regulation 421.1.201 of BS 7671 18th Edition, which came into effect from January 2016. The change was driven by evidence that internal arcing faults inside a plastic-cased board could ignite the enclosure material and cause or worsen a house fire. A metal enclosure is not combustible; if an arc or fault occurs inside, the metal case contains it rather than contributing to it. The regulation applies to new or replacement consumer units in domestic premises — not retrospectively to older boards already installed. An existing plastic consumer unit is not in breach of the regulations that were in force when it was fitted, but if the board is replaced for any reason, the replacement must be in a non-combustible enclosure.

If you have looked at a modern consumer unit you will have noticed that virtually all of them have a metal case. This is not a design preference — it is a requirement of the current edition of the UK wiring regulations. Understanding why the change was made helps clarify what the rules mean for older plastic boards already in place.

Metal enclosure requirement

RegulationBS 7671 Regulation 421.1.201
In force fromJanuary 2016 (18th Edition requirement)
Applies toNew and replacement boards in domestic premises
Material requiredNon-combustible (in practice, metal)
Retrospective?No — existing plastic boards are not in breach
EICR codingTypically C3 (improvement recommended) for plastic boards

The fire risk that drove the change

A consumer unit is the point in a home where protective devices operate under fault conditions — by definition, the place where fault currents flow and arcing can occur. In the event of an internal arc or connection failure inside the board, the enclosure is exposed to heat and molten material.

Investigations into domestic electrical fires found a pattern where the arc energy from a fault inside a plastic consumer unit ignited the case material. Thermoplastic enclosures (the standard material for older boards) can begin to soften and char at temperatures well below those involved in an arc fault, and once alight they can sustain combustion and spread it to adjacent materials. A metal enclosure will not ignite — an arc inside a metal board may damage the board internally but will not cause the case itself to catch fire and spread flames to a wall or ceiling.

The concern was not that plastic boards were immediately dangerous simply by being plastic — most functioned safely for decades. The issue was the consequence when a fault did occur inside them.

Important distinction: the requirement for a metal enclosure applies to the consumer unit casing. The individual MCBs, RCDs and RCBOs inside the board are still made of thermoplastic — that is how circuit breakers are manufactured. The metal case contains any arc fault that bypasses those devices or occurs at a connection; the plastic bodies of the devices themselves are protected within that steel shell.

What the regulation says and when it applies

Regulation 421.1.201 of BS 7671 18th Edition states that in domestic (household) premises, consumer units and similar switchgear shall be enclosed in a non-combustible material, or be housed in a cabinet of non-combustible material. The requirement came into force when the 18th Edition took effect in January 2016, replacing the 17th Edition.

Critically, the regulation applies only to new or replacement boards. It is not retroactive. A plastic-cased consumer unit installed in 2010 under the then-current 17th Edition was compliant when installed and does not need to be replaced simply because the regulations have since changed.

However, if that board is replaced for any reason — age, capacity, EICR recommendation, or because new circuits are being added — the replacement must comply with current regulations and therefore must be in a non-combustible enclosure.

What happens to a plastic board on an EICR

When a periodic inspection (EICR) is carried out on an installation with a plastic consumer unit, the inspector will note the plastic enclosure. The standard coding for an existing plastic board on an EICR is typically a C3 (improvement recommended) rather than a C2 (potentially dangerous) or C1 (danger present). A C3 does not mean the board must be replaced immediately; it means it does not meet the current standard for new installations and replacement is recommended.

However, the EICR may also reveal other issues — particularly the absence of RCD protection or an inadequate number of circuits — that could attract higher codes independently of the enclosure material. In those cases, the recommendation to replace the board would be driven by the protection deficiencies rather than the material alone.

Scenario	EICR code (typical)	Action required
Plastic board, otherwise sound, with RCDs	C3	Improvement recommended; not urgent
Plastic board with no RCD protection	C2	Should be addressed — significant deficiency
Plastic board with visible scorching	C1 or C2	Urgent investigation and likely replacement
Metal board, modern, properly protected	No code	No action

Typical EICR coding for plastic consumer units. Actual codes depend on the full inspection findings. Source: BS 7671:2018+A2:2022; ECA/NICEIC inspection guidance.

Frequently asked questions

Do I need to replace my plastic consumer unit?

Not immediately and not solely because it is plastic. An existing plastic board that was compliant when installed is not in breach of current regulations by being in use. However, if it lacks RCD protection, fails an EICR, or is being replaced for another reason, the replacement must use a non-combustible (metal) enclosure. An EICR will tell you the condition of your specific installation.

When did the metal enclosure requirement come in?

Regulation 421.1.201 requiring non-combustible enclosures for domestic consumer units came into effect with the 18th Edition of BS 7671, from January 2016. All new and replacement domestic boards installed from that date must use a metal or other non-combustible enclosure.

Can I buy a plastic consumer unit for replacement work?

Not for domestic premises in the UK — consumer units for domestic installation must comply with Regulation 421.1.201, which requires a non-combustible enclosure. Plastic-cased boards are still available in some markets but should not be installed as a replacement board in a domestic property in England, Wales or Scotland.

Sources & further reading

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.