New Reg 421.1.201- Metal Consumer Units

by Editor on October 30, 2014

This post highlights some of the main points and concerns regarding the 17th Edition  Amendment 3 reg 421.1.201 about the use of metal consumer units (non-combustible enclosures).

Electrical fire damage from faulty cut out

Picture of fire damage caused by loose wiring connections.

Fire Brigade Lobbying

The London Fire Brigade consider consumer units in domestic installations to be a high fire risk as the majority of electrical fires are caused by loose electrical connections inside fuseboards. Plastic boards are generally flammable and will melt in a fire so the LFB have lobbied the government and the IET to introduce the changes included in the latest edition of BS7671.

Amendment 3 now requires that domestic Consumer Units must comply with BS61439-3 and IP2XC. This generally means that they will need to be made of steel. Metal consumer units will be more effective in the containment of fire.

The Regulation

Reg 421.1.201 – Consumer Units, Switch Boards and assemblies in domestic electrical installations are now to be made of non-combustible material such as metal, or enclosed in a fire proof boxing or enclosure.

There is a proposed extended implementation period until January 2016 (presumably to allow the manufacturers to sell off all their plastic stock and make lots of metal ones!).

Insulated Is Good?

  • Do RCDs prevent fires in consumer units?

I’ve always thought that having anything electrical enclosed in plastic was a very good idea. It insulates and you can’t get a shock from it. There’s obviously no need to earth or bond plastic, and unlike metal there’s a reduced shock hazard should earths become disconnected.  Plastic is generally not as strong as metal but there are tough plastics and flimsy metals, so the suitability is really down to the construction and application in each case.

It would be interesting to know how many house fires are caused by consumer units. I’ve seen 100s of plastic insulated consumer units in my time as an electrician. A very small number of them have had signs of burning around the wiring terminals due to loose connections, but I’ve never seen anything bad enough to cause the unit to catch fire. I’d be more concerned about the state of the mains intake cables and cut outs in most older properties. Also the lack of regard given to space around electrical intake equipment.

Most circuits in modern domestic electrical installations would be protected by a 30mA RCD and a suitably rated MCB. I’d like to think that these would trip if arcing and was to occur inside a consumer unit.

The Challenges

A fire in a metal enclosure would only be contained if there were no holes in it. So it would be important to ‘fire seal’ any cable entries. The same applies to any fire proof boxing surrounding a plastic consumer unit.

  • In reality, how well would metal consumer units and boxes be fire sealed?
  • All cable entries must be sealed to contain a fire inside the enclosure. This won’t be easy, especially for back entry cables.
  • IP rated enclosures like garage units, plant room CUs…  will need to be metal.
  • TT systems will require 100mA RCDs in a plastic enclosure as they’re not allowed to be metal.
  • The MCBs themselves are flammable and also plastic blanking modules, so will metal covers or doors (many of them currently plastic even on metal CUs) need to be lockable to comply with IP2XC?
  • One area for concern could be the main isolator terminals on a consumer unit. In modern consumer units these are normally a 100 amp DP switch which would only be overload protected by a 60 or 100 amp fuse in the DNO’s cut-out. Main switch terminals are of course at higher risk of overheating because they carry a heavier load, so why not improve them?
  • Cost compared to plastic CUs will be higher. ie.- Cost for replacement plastic CU is £200 t0 £400. Cost for replacement metal CU is £400 to £600. I’ve just purchased a Hagar 6+6 consumer unit- Cost £135 (inc VAT) + mcbs £60. Thats twice the price of a trade deal of a plastic CU. Additional costs would be fireproof glands and gaskets. For TT systems a 100mA RCD and enclosure (plastic?) would also be required at around £80. Maybe the materials cost will come down as suppliers start to do deals including MCBs but the labour cost will probably be around £200-250.

Brainstorming ideas

The manufacturers will be brainstorming for ideas of how to make their products comply. Here are some suggestions for making metal consumer units amendment 3 compliant and to improve terminals (and prevent fires!):


  • Intumescent seals for rear entry cabling which would expand incase of a fire to IP2XC.
  • Could an intumescent ‘bead’ be pre-installed (by the manufacturer) all around the rear inside of the enclosure? This would expand and fill the enclosure in the case of a fire. Similar to those installed in fire rated lights or in collars on extract fan ducting.
  • Crabtree now have some good ideas for cable seal entries which include cable glands for double insulated incoming mains tails and outgoing circuits. They also have a rear entry back entry plate which creates space behind the board and can be fixed to the wall first.
  • Metal blanks- Would these replace plastic flammable ones?
  • Metal lids made lockable in case plastic MCBs ignite and push lid open.


  • Whatever material the consumer unit is made of, it won’t prevent overheating of loose connections. Surely it would be better to ensure that connections don’t come loose in the first place?
  • Clamps to hold tight the mains tails inside the enclosure would prevent movement, damage and loosening. Loosening of incoming meter tails can happen over a period of time or by operatives when meter is replaced (they are not trained or authorised to open a consumer unit’s cover and check connections).
  • Rounded terminal clamps. Nearly all electrical cables are round shaped so curved terminals would grip a cable better than a square or flat bottomed one.
  • Two screws instead of one may be a good idea for cable terms above 6mm.

What about PME & TT systems?

  • RCDs are required to protect incoming meter tails on TT systems.
  • The On-Site Guide- For TT installations- insulated enclosure OR further mechanical protection to meter tails.

On TT earthing systems its always been best practice to install an insulated (plastic) consumer unit where there would be no RCD protection of the meter tails. This is because should a live meter tail short out to the earthed metal enclosure, the typically high resistance earth path offered by an earth electrode wouldn’t be sufficient to blow the supply fuse. All metalwork would then remain at mains voltage with the obvious high risk of electric shock.

Also on TN-C-S (PME) systems a risk of the metal casing (or any metalwork) becoming ‘live’ could arise should the neutral of the incoming supply be lost.  The only way that I can see to safeguard against this would be by installing a time delayed 100mA RCD on the incoming supply (before the Consumer unit). Of course, this would need to be in a plastic enclosure!

The problem arises because RCDs only look forward from their output terminals to the load. They don’t monitor the supply side of the circuit. This means that any earth fault to the supply side, ie meter tails and the metal enclosure, could go undetected.

If the consumer unit is of metal construction then an RCD would need to be installed between it and the origin of the supply. A main 100mA ‘S’ type would give discrimination between any downstream 30mA RCDs. Find more about problems with metal CUs on TT systems.

Fire Proof Plastic?

  • Are insulated consumer units too cheap?
  • Could insulated consumer units be made of fire retardant or self-extinguishing insulating materials?

I do wonder why electrical enclosures and consumer units aren’t manufactured using a self-extinguishing or fire rated or retardant type of plastic. I appreciate that the need to cut costs but there must be a wide range of suitable applications for this type of material in electrical enclosures. Maybe the cost of domestic consumer units has been driven down too far and quality has suffered. Has any consideration been given to the fire retardant properties of consumer units?

Cheap Is Not Cheerful

  • Only use quality recognized brands of consumer unit (plastic or metal).
  • Metal doesn’t necessarily mean strong.

I’d always recommend using recognized manufacturers for any electrical fixtures and fittings, especially consumer units. It’s not worth risking (and being embarrassed by) the failure of something you’ve installed. All of the products that we install as electricians should be manufactured to a specified standard. I’ve come across some very flimsy metal enclosures which would not be as good as the best plastic ones in construction.


  • Has the Reg 421.1.201 been influenced by fire safety alone?
  • Do the manufacturers have a vested interest?
  • Are plastic consumer units being blamed for other types of electrical fires?

I’m not suggesting that metal consumer units are at all a bad thing in a domestic installations (I’ve used them myself) but plastic insulated units have become preferred by the majority of installers. Electricians will always comply with the guidance set out by the regs but are generally driven by the common sense approach that they are improving the situation by there actions, but we do often see some other potential electrical hazards that we are not allowed to touch…

Consumer units are only one aspect of a domestic electrical installation and new ones are generally installed as part of an upgrade where improvement is required. Electrical Installation Condition Reports are usually issued by electricians and then acted upon by homeowners. One aspect of an installation that is vary rarely upgraded (except in extreme  cases) is the electricity supply intake equipment on the DNO or electricity company’s side of the meter. Many of the cables, connections and cut-outs have been in place and rarely touched since they were first installed up to 100 years ago. Dare I suggest that these items pose an ongoing higher risk to fire safety than plastic consumer units?

Thanks for reading. Your comments are very welcome…

More info about the 17th Edition updates in Amendment 3.

More info about Torque Screwdrivers.

RCD devices can detect electrical faults that may cause fires.


{ 9 comments… read them below or add one }

Ian Macdonald September 21, 2015 at 09:34

I agree with the fire service concerns, but reckon it would be more relevant to change the design of the terminal clamps on DIN-rail devices. It’s scary how often you tighten one of these as firmly as a reasonably strong person can, only to have a 10-25mm cable fall out a few seconds later. They are basically a very inferior design when compared to the earlier semi-enclosed fuse types which used a screw projecting into a round receptacle.

Brett Hewitt June 13, 2015 at 05:56

With all these consumer units which have caught fire, I would like to know how many were fitted by a professional person and how many of them were from a DIY store fitted by an enthusiast. (I know that we will never know)
While all these items are sold to any member of the public who does not understand the regulations that electricians adhere to, then in my opinion we just wait for the next amendment after a possible DIY disaster as sponsored by Wickes/BQ

Derek Farmer May 2, 2015 at 19:56

A sledgehammer to crack a nut as usual from the IET.
As I understand it the punitive cost of compliance is solely a burden on the UK as no other European Country has bothered to implement this.
Having worked in France, all supplies are TT and the Distributor provides a 500ma isolator at the head of supply as the norm then its lovely cheap plastic consumer units throughout.
Regarding this latest costly and achieving nothing legislation, have the London Fire Brigade done a proper statistical survey? The photo at the head of your superb article seems to show that the origin of the fire was in the service fuse below the meter. You indeed mention this. The Distributors do not appear to accept any blame in all this and have no coherent programme of inspections to prevent this happening to their equipment up to the meter tails.
This bag of worms 421.201.1 should not have been considered until a proper statistical breakdown of fire cause had been fully appraised.
For TT installations it has made things less safe and more costly to implement. It is early days yet and once manufacturers are geared up costs will come down but for now the consumer unit and distribution arrangements are 4 x more costly to achieve for my current TT installation rewire.
Ok I still have the option at this time to save cost by using an insulated consumer unit under the opt out clause but then, post Jan 2016, my 7 month old installation would result in a non compliance with current 17th Edition Regulations comment if any additional circuits were added after this date.

Colin0908 April 23, 2015 at 08:44

Great article, I agree with the sentiment that this change may not of fully been thought out.
If the changes safe lives then who can argue, but I am concerned that making Domestic CCU’s metal is the answer.
Metal although certainly makes them more robust and contained, it does not stop poorly fitted cables and defective components that create the fire, those are the elements that need to be clamped down on. The poor installation and defective (probably cheap) components has not been addressed.

Secondly, If the CCU cannot be plastic, what about the need and other components i.e. Henley Blocks. and even the distributors components cut-out (Main incoming fuse).

The answer to this second point is possibly is that the plastic used should not be the “cheap as chips” type used by some manufactures and made if the same material used by the distributors components.

Joe Bellis April 23, 2015 at 08:34

I agree with Martin.
Especially his comment regarding the extension of the implementation to January 2016, Health and Safety is put back until the manufacturers clear their shelves.
DIY is a multi million pound market and there are no regulations regarding the sale of electrical goods or who install them.
Elsewhere around the world Electrical installation is part of the building regs. and is needed to be installed by a qualified person.
Maybe Electrical Inspections should become part of the household insurance application.

Les Macaulay April 23, 2015 at 07:02

This new Regulation has been ill-thought and needs to be reconsidered.
I was about to upgrade my personal CU in my home, bought a plastic CU for £54, and read this new Reg effective from Jan and decided I couldn’t afford the upgrade, especially since the DNO quoted me £130 to fit an isolator. The rewireable CU has been in situ for nearly 40 years so it ain’t gonna be fixed cause it ain’t broke. Thankfully, I hadn’t opened the CU from its packing and so my supplier refunded me.
How about some manufacturer offering a retro-fit “over the top” metal enclosure?

Dave Waters April 22, 2015 at 20:25

Nice article. This regulation seems to miss the point. The problem is poor connections – and the majority of the connections to 100A main switches leave a lot to be desired. A double screw or cage system would be brilliant. I’m very much against metal CU’s after getting a belt off one which wasn’t earthed at source – but one of the outgoing cables had a L-E fault on it. If they are sold in ‘sheds’ anyone could fit them – sounds like a recipe for disaster.

Martin Franks April 21, 2015 at 21:01

As most domestic boards are underneath the stairs, by using metal, it could save life’s as not all plastic boards use a non toxic material. 5 mins could make all the difference.

Malcolm Briggs November 28, 2014 at 13:22

Having read your comments regarding the new amendment on Fire proof consumer units, I agree with a lot of your comments, I myself have been discussing the subject with the Electrical wholesalers, I have been specifying to them the same points. In my 50 years as an Electrical/Electronics Engineer, I have come across many electrical problems, with many circuits incorrectly protected, more than one circuit protected by the same fuse/MCB. Showers are the biggest problem, in terms of overload and burning, I have had to de-rate the showers, since clients, are reluctant to increase the cable size.

Working in the Domestic industry causes a lot of problems, with DIY, but Part P should have stopped this, and since we as a country are being over whelmed by immigration, who don’t have the same safety concerns as people trained for a 5 years electrical apprenticeship, I have seen the workmanship on many occasions.

I am in the process of trying to implement the 2015/16 regs for new clients, using Consumer Units with Fire Retardant materials, which are on the market, I’m not happy with metal consumer units for Domestic use, since we live in a plastic world, most things today are made from some form of plastic, after all MCB’s use plastic surround’s.

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