AIUI the overvoltage tolerance is only one aspect (though the most publicised one) of the IEC categories.
Indeed - or, at least, there appear to be other specific requirements underlying overvoltage tolerance (e.g. spacing between PCB tracke stc.)
I know there are reports of multimeters blowing up and causing serious injury or even death, for example
https://www.ecmweb.com/arc-flash/case-deadly-arc-flash , some of them may be down to use of the current range by mistake, but this one doesn't seem to be (he had made several measurements before the one that blew up in his face).
Yes, I'm aware of a very small number of anecdotal stories, including that one, which is why I put 'ever' in quotes. However, that one (which is the case I've most often seen mentioned) was clearly a long time ago. The article was written in 2010, and the author talks in the past tense about an investigation he undertook "several years after the accident" - so it could probably easily have been 20 or more years ago. Had there been similar cases since, perhaps we would have heard something about them? The circumstances of that particular case were, of course, pretty complicated. I still think that such incidents are probably incredibly rare, and probably much rarer than other (electrical) causes of injuries and deaths amongst electricians.
As to why measurement equipment is "special" I suspect it's a combination of being right in the user's face and being frequently connected directly to high power circuits, often via questionable connections.
Maybe, but I'm not all that convinced by that reasoning. For a start, I would never put a meter 'right in my face', and would have thought that electricians would be taught not to do that. If we are talking about supply transients causing electronic items to spontaneously explode, then I would have thought that there were countless other 'everyday' situations in which that would be a risk (and possibly 'in the face' of those nearby) - hence I'm still not sure that I understand why these concerns seemingly only exist in relation to measuring equipment.
I have seen a bunch of multimeters for sale that only claim Cat II. Just go look up the cheapest multimeters on Farnell or RS and you'll find them.
Faie enough. As I said, I'd not noticed any - but I suppose I haven't been looking for them.
It's also common to see dual-ratings, where it will have a higher category at a lower voltage and a lower category at a higher voltage.
I'm not quite sure how, in practice, that would work (usefully). In particular, if the voltage rating is <300V, then it presumably would not be suitable for anything above Class II.
The
screwfix meter at £10 for a CAT III 600V seems to be getting into the region of almost too good to be true territory.
Maybe. However, despite what one might first think, I'm not sure that getting a higher 'rating' necessarily has toi have a major impact on production costs. A lot of it is presumably to do with careful design - in terms of arrangement and spacing of components, spacing of PCB tracks etc. - which factors have little or no impact on production costs. There probably will be a need for some higher-speced components, maybe slightly thicker insulation and maybe a few additional 'protective' components, but that might not have a major impact, either. As with so many things, the prices we are used to at the top of the market probably include a major component of "paying just for a name", rather than anything else.
Kind Regards, John
