At the risk of digressing and for the folks still interested in all this, here's what the IET (the folks who publish the wiring regs that apply everywhere in the UK) said in explanation when this non flammability thing was introduced.
https://electrical.theiet.org/wirin...ble-enclosure-requirement-for-consumer-units/It nicely demonstrates how the folk writing the regs realised that there could be arguments over interpretation and they tried to anticipate the obvious ones.
The scope was "
consumer units and similar switchgear assemblies ".
"
The phrase ‘similar switchgear assemblies’ in Regulation 421.1.201 means those assemblies used for the same fundamental application as a consumer unit. A consumer unit is defined in Part 2 of BS 7671:
‘Consumer unit(may also be known as a consumer control unit or electricity control unit). A particular type of distribution board comprising a type-tested co-ordinated assembly for the control and distribution of electrical energy, principally in domestic premises, incorporating manual means of double-pole isolation on the incoming circuit(s) and an assembly of one or more fuses, circuit-breakers, residual current operated devices or signalling and other devices proven during the type-test of the assembly as suitable for such use.’
An example of a similar switchgear assembly is a three-phase distribution board that is intended to be operated by ordinary persons. This would have to have isolation that interrupts the three incoming line conductors and the neutral, rather than just double-pole isolation as mentioned in the above definition.
"
This is rather more elaborate than just an MCB in a box to protect one outlet. An MCB doubles as both a functional switch and an isolator if locked off, but it's not a double pole isolator. It doesn't break both the live and neutral connections. One MCB in a box doesn't sound like a consumer unit to me. I think the writers of the regs found the best words they could to cover large through small versions of what we all expect. The big red switch, one or more breakers, plus optional RCD's, and / or combined RCBO devices.
Then, is it still a "consumer unit or similar" if it's in a shed or garage ?
"
Consumer units in outbuildings or on the outside of a building
Regulation 421.1.201 uses the term ‘premises’. The question could therefore arise: do the requirements of the regulation apply to a consumer unit or similar switchgear assembly within an outbuilding such as a garages or shed, or mounted on the outside or a building?
Some dictionary definitions of ‘premises’ are ‘a house or building, together with its land and outbuildings’ and ‘the land and buildings owned by someone’.
However, Regulation 421.1.201 was principally introduced to cover the interior of a household building and any garage or other outbuildings integral, attached, or in close proximity to that building.
Doubt could exist about whether or not a particular outbuilding could reasonably be considered to be in ‘close proximity’ to the household building. A way of resolving this might be to make a judgement of the likelihood that fire originating inside the enclosure of a consumer unit or similar switchgear in the outbuilding might lead to the outbreak of fire in the household building or in any outbuilding integral or attached to it. Relevant factors to consider about such an outbuilding might include whether or not that building or its expected contents are highly combustible.
Regulation 421.1.201 is not intended to apply to a consumer unit or similar switchgear assembly that is not within a building, such as a consumer unit mounted outdoors on the outside of a building.
Now you see why you're paying your sparky the big bucks
If he interprets stuff like this wrong and is unlucky, the cost of your new house could end up on on his business insurance. No wonder some will play safe and charge you for a tin box every time, just in case.
As a side note, Engineers, Lawyers, all sorts of specialists are sometimes, but by no means always, blessed with the skills to write clear, unambiguous English. When they don't achieve clarity, we all get into arguments like this and because of the cost and consequences of litigation, the ordinary tradesman has to play safe and we pay for over specified kit that was never justified by the facts. Regulation is good when it pushes standards up. Like law, it's bad when poorly drafted. Sometimes less is better.