I had an email conversation with a representative of the JPEL-64 committee over this some time ago. This was in the context that the (then) new regulation had suddenly made a very large number of (in some cases) very newly installed CUs somehow "dangerous". Bear in mind that a lot of organisations won't allow any C3s to be left - they insist on having a "clean" EICR as that's the only way to avoid "but there was a known and documented risk and you did nothing about it, and no it's caused me loss/injury/whatever" type claims. Even before the introduction of mandatory EICRs, I would get periodic ones for my rental properties - and I would not leave any C3 un-fixed for just that reason (yes, there are some people who would look for any opportunity to exploit it).
Anyway, the response I got was that it was expected that it would be C3 if (for example) the CU was under wooden stairs or in a fire escape route, and not mentioned otherwise. But I couldn't get that as anything more than "a personal opinion" even though it was clear that it was in fact guidance from the committee.
Don't forget that most of the plastic cases previously used will have been various self extinguishing and/or fire retardant plastics. So in the absence of a sustained source of heat, they would not continue burning. As it is, if you have the same sustained source of heat, a steel case can a) allow flames to escape (the standard for CUs doesn't mandate it present a fire barrier) and/or b) set fire to the the wood it's screwed to when it gets hot enough. Of course, if we hadn't mandated such a mass meter change program (which now doesn't actually carry a positive cost-benefit after various supposed benefits failed to be possible), and been forced to use so many "poorly trained" fitters, then we'd not have the problem anyway.
Regardless, it's a stain on the reputation of the JPEL-64 committee until they fix this - which could easily be done by changing the regulation to "non-combustible as per <some existing standard>" as that would allow other materials since it would be possible to test to a known specification. As it is, without the "ferrous metals are deemed" note, then steel would not be allowed since that is combustible if you apply enough heat and oxygen (note, the reg doesn't specify what conditions it has to be non-combustible in) - which makes a complete mockery of the reg.
