13A MCB
- Thread starter Oxia
- Start date
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I've certainly never heard of, or come across, any such animals, but I'm not so sure that they are necessarily a particularly 'odd' size (merely very uncommon). Indeed, I might say that 16A is more odd, since most of the situations in which a 16A MCB is used (e.g. immersion circuits), a 13A one might be appropriate, and perhaps even more logical!
Kind Regards, John
I've yet to see one in a CU or dis board, in fact the only place I've actually seen them in the flesh is fitted to 2.5kw lighting dimmers such as these... http://www.anytronics.co.uk/contractor/
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Theres quite a range of seemingly odd sizes between 1 and 16 mainly found as stated earlier in panels,etc, i presume they design them to give some discrimination between the other mcbs and fuses and to help limit the fault currents to reduce panel component damage.
6 16 20 and 32 seem most common in uk, possibly due to rounding off the old 5 15 20 and 30 fuses which most uk circuits were designed for
6 16 20 and 32 seem most common in uk, possibly due to rounding off the old 5 15 20 and 30 fuses which most uk circuits were designed for
I've often wondered (and asked) about how the common MCB ratings came about. It seems difficult to explain it in terms of 'rounding', per se, since 5/15/20/30 are clearly more 'round' than 6/16/20/32. The 20A is particularly odd (although I don't think it was a very common fuse size, was it?), since it's the only fuse size which was not fractionally increased when they decided upon MCB ratings - for 'consistency' one might have expected the MCB to be 21A or 22A.
However, more surprising is that, in cable protection terms, the 'changes' were all seemingly in the wrong direction, weren't they? AFAICC, if one assumes that, at the time MCBs came into being, most circuits were protected by BS3036 fuses of 5A/15A/20A/32A, would not the required 'corresponding' MCB ratings to guarantee adequate protection of a cable (which was possibly 'only just' protected by the fuse), rounded down to a whole number, have been 3A, 10A, 14A and 21A (rather than 6/16/20/32)? It almost sounds as if they were thinking just about design currents of circuits, and forgetting about the need to protect cables (maybe that wasn't even a major consideration at the time?!) ...or have I got this wrong somewhere?
Kind Regards, John
No, because MCBs are more sensitive than fuses. For a 30A BS 3036 semi enclosed fuse the prospective fault current would be 450A to 210A. For a 32A type B BS 60898 it would be 160A and for a 30A BS 1361 280A to 200A.
Cables have to be derated by 0.725 (Cd) when protected by rewirable fuses.
Oddly, Merlin Gerin don't do a 13amp to my knowledge, but do have some odd sizes below 10amp.
1, 2, 3, 4, 6, 10, 16, 20, 25, 32, 40, 50, 63.
1, 2, 3, 4, 6, 10, 16, 20, 25, 32, 40, 50, 63.
I have seen 13A Moeller's in pre-wired caravans for socket circuits, I guess it's either because of the size of cable chosen or as suggested, to help with discrimination. I also have an odd Hager C3 on the burglar alarm, and I can think of a few others, like the smokes, where this would make sense.
John, I have a feeling that the transition to "not divisible by 5*" ratings is more to do with European alignment. I know for certain that my former early 90's MK board had type 2 MCBs (sorry I forget the BS) which were a mixture of 5, 6, 15, 16, 30 and 32 amp ratings. Those to 60898, which has always been based on a Euronorm, have always been built with the "not divisible by 5" ratings. So it's not a fuse-to-MCB thing.
*exceptions of 10, 20, 25, 45, and 50 amps of course, but they didn't change 'from' anything.
John, I have a feeling that the transition to "not divisible by 5*" ratings is more to do with European alignment. I know for certain that my former early 90's MK board had type 2 MCBs (sorry I forget the BS) which were a mixture of 5, 6, 15, 16, 30 and 32 amp ratings. Those to 60898, which has always been based on a Euronorm, have always been built with the "not divisible by 5" ratings. So it's not a fuse-to-MCB thing.
*exceptions of 10, 20, 25, 45, and 50 amps of course, but they didn't change 'from' anything.
That will teach me to try to think/write very late at night, particularly after 'a glass or two'! ... I was actually thinking of overload, rather than fault, protection, but in my mid-night daze, I multiplied by 0.725, rather than dividing by it
... so, if I had done my sums the right way around, I suppose that, if it had been based on 'equivalence of overload protection', I might have expected 5A, 15A and 30A BS3036 fuses to be 'replaced' by 6A (nearly 7A), 20A and 41A MCBs - which may be part of the answer. Maybe 6A and 20A MCBS were intended as 'replacements' for 5A and 16A fuses, leaving the 32A MCB (which one might have expected to be 41A) as the only 'anomaly' - and that might have been to accommodate the CCC of 7/029 cable? That 'explanation' would beg the question as to what the 16A MCB was meant to be 'replacing', that being 'equivalent' (in terms of overload protection) to a 12A BS3036 fuse (if such an animal existed - certainly not common!). Of course, in practice, we came to use the 16A, rather than the 20A, MCB as a replacement for the 15A fuse - but that's probably simply because it was adequate for most of the circuits concerned.
Kind Regards, John
That all sounds credible, but it begs the question as to why Europe (which, in general, has tended to have 'round numbers' approach for a long time) came up with 6, 16 & 32. I must say that I've never seen 5A, 15A or 30A MCBs.
Maybe, but as I've said in what I've just posted, in terms of overload protection (when rounded down) a 6A MCB is equivalent to a 5A BS3036 and a 20A MCB is equivalent to a 15A BS3036 - but that still wouldn't explain where 16A MCBs 'came from', and would leave one guessing that 32A MCBs were dictated by the CCC of commonly-used cables.
Kind Regards, John
Ah - that would appear to be the answer. Thanks.
Mind you, although the officially-preferred sizes of MCBs were presumably based on a Renard series, the 'preferred practices' in the UK need a bit more explaining. As I've said, AFAICC, a 41A MCB would afford the same cable protection as a 30A BS3036, but we decided to use 32A MCBs as the 'replacement', even though a 40A one is available in the same Renard series. 32A (which 'corresponds' to a BS3036 of around 23A) does seem to make sense in terms of standard cable sizes, but that then begs the question as to how well cables were being protected by the fuses!
Kind Regards, John
I suspect that that is merely down to lack of thought in that 30A fuses were just replaced with the nearest MCB.
It would have been more a case of using 4mm² cable for replacement circuits but this is still not done - 32A MCBs and 6mm² cable still being what is done for cookers.
Indeed, other forum reading would indicate that there is a lot of ignorance among 'electricians' who seem to favour 10mm² for cookers, ignoring diversity because 'it doesn't seem right'
Or, how greatly overprotected they are now.
Indeed, as was probably apparent, that was sort-of what I was implying. Indeed, even today, and even in this forum, one often sees references to "a circuit protected by a 30A fuse or 32A MCB", which seems to be overlooking the fact that the protective abilities are appreciably different.
I essentially agree. However, even forgetting our recent semantic discussion, I'm not sure that 'ignorance' is necessarily the right word. Diversity, being a statistical concept, is necessarily a 'gamble' - and, as we've discussed, people vary dramatically in their feelings/attitiudes about 'risks' (including gambles). Unless a cooker has built-in functionality to limit its total current consumption, it is theoretically possible (although very unlikely) to get it drawing something close to its theoretical maximum current for an appreciable time.
Sure, that's another way of looking at it - but there seems to be, at least regs-wise a 'double standard' currently now in operation. For example, in the current regs allows a ring final circuit in which the cable (of ≥20A CCC) is protected by either a 30A BS3036 or a 32A MCB. Given that, overload-protection-wise, a 30A BS3036 corresponds to an MCB of about 41A, the regs appear to be 'authorising' two situations in which the degrees of cable protection are very different. Logically, one might have expected that, if they are happy with protection by a 30A BS3036, they would also have allowed protection by a 40A MCB.
Kind Regards, John
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