Indeed - they generally use the Renard R10 series which, rounded to whole numbers, gives:Products designed to EN/IEC standatds OTOH tend to use ratings of 6A, 10A, 16A, 32A, 40A, 50A, 63A, 100A and 125A.
Indeed - and, as has been said, far too small an 'fiddly' to use, at least for me!Plus , most of those commonly sold "5A Junction boxes" were actually 20A for a long while, true 5A junction boxes tend to be cheap nasty things that cost a farthing less
Lighting circuits.
Historically they were 5 amps.
The ratings of junction boxes is more to do with the size and number of conductors that will fit, rather than the current it can deal with.
A 5A junction box will not fail if used on a 6A circuit.
Curiously wikipedia claims that the "most rounded" ISO renard series contains 12. Despite the fact that 13 is closer to value of 10^0.1 than 12 is.Indeed - they generally use the Renard R10 series which, rounded to whole numbers, gives:
6A, 8A, 10A, 13A, 16A, 20A, 25A, 32A, 40A, 50A, 80A, 100A, 125A ....
Having just looked, yes, you're right - and, as you say, it's not really mathematically logical (aka 'correct'!). However, I think you'll find that it' the "least rounded" Renard series that is used for most purposes - and the figure then is 12.5 - which, at least in my book, definitely rounds to 13!Curiously wikipedia claims that the "most rounded" ISO renard series contains 12. Despite the fact that 13 is closer to value of 10^0.1 than 12 is.
I would have guessed that it was probably based in perceptions of what 'round numbers' would be useful/practical figures to use, rather than any mathematical series. However, if one takes the 'British' figures (including 10A and 20A) which plugwash cited, those figures are (at least, from 15A onwards) very close to a hypothetical "R7" Renard series [ i.e. increments of "x 10^(1/7)" ], although it escapes me as to why anyone would have chosen that!I wonder where the british values came from. Were they also Renard values but just rounded differently.
2 thing come to mind, I was always taught it's multiplied by 1 1/6 but only some values used. I've not looked anything up or done any calculations and never given it a thought until now however just the glimmer of a thought and I think that has to be very incorrect.Having just looked, yes, you're right - and, as you say, it's not really mathematically logical (aka 'correct'!). However, I think you'll find that it' the "least rounded" Renard series that is used for most purposes - and the figure then is 12.5 - which, at least in my book, definitely rounds to 13!
I would have guessed that it was probably based in perceptions of what 'round numbers' would be useful/practical figures to use, rather than any mathematical series. However, if one takes the 'British' figures (including 10A and 20A) which plugwash cited, those figures are (at least, from 15A onwards) very close to a hypothetical "R7" Renard series [ i.e. increments of "x 10^(1/7)" ], although it escapes me as to why anyone would have chosen that!
View attachment 297935
Kind Regards, John
Are you talking about the 'British' figures? If so, a 1 1/6 ratio is 1.1666..... but that is appreciably less than all of the 'differences' in the common 'British' figures, so I don't think that what you suggest would be a credibl;e explanation....., I was always taught it's multiplied by 1 1/6 but only some values used. I've not looked anything up or done any calculations and never given it a thought until now however just the glimmer of a thought and I think that has to be very incorrect.
You were right, and they were plain wrong! What they have done, by repeatedly rounding, is to invoke what is known in the trade as the concept of the 'propagation of errors'.He made a calculation and rounded the answer 78.498... to 2sf as 79.0. I stopped him entering 79.0 but 78.0 was marked as incorrect as was 78
A different set of figures gave another answer marginally les than a 0.5 and same thing happened.
..... I gave up and looked at the tutorial, one of my methods was bang on the nail, right up to the point they rounded 78.4989.. to 79.0 their method showed:
78.4989 rounded to 78.498+0.001 = 78.499, to 78.49+0.01 = 78.50, to 79.0
Quite frankly I don't know which system, it's just something from the mists of time which I hadn't given any thought to until this thread, my initial intent was to correct/augment your information but even without making any calculations I realised it was way out of kilter with both, however seeing your table everything apart from the 80-100 step is based on multiples of 1/6; ie 12/6, 9/6, 8/6. I'm not trying to clutch at straws with this by any stretch of the imagination.Are you talking about the 'British' figures? If so, a 1 1/6 ratio is 1.1666..... but that is appreciably less than all of the 'differences' in the common 'British' figures, so I don't think that what you suggest would be a credibl;e explanation.
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I'm sure that was the original case and not only in UK, I think your list, or something relatively similar, was fairly universal.I still think the most likely explanation is that they (particularly 5, 15, 30) were simply 'very round numbers' which were 'pulled out of the air' as being appropriate for purpose.
I completely agree but that is what is curently being taught in schools.You were right, and they were plain wrong! What they have done, by repeatedly rounding, is to invoke what is known in the trade as the concept of the 'propagation of errors'.
To avoid such errors, one must round 'in one go' on the basis of the entire figure to be rounded - that way, one only gets a 'rounding error' once, rather than multiple times. In other words, ANY number less than 78.50000000000000000..... rounds (to 2 signif figures) to 78 - and that includes 78.4989 and even 78.499999999999999999999999...
Kind Regards, John
That's mathematically true, but probably a 'co-incidence' and/or 'red herring', not the least because, as one moves through the values, the number of multiples of 1/6 goes repeatedly 'up and down'Quite frankly I don't know which system, it's just something from the mists of time which I hadn't given any thought to until this thread, my initial intent was to correct/augment your information but even without making any calculations I realised it was way out of kilter with both, however seeing your table everything apart from the 80-100 step is based on multiples of 1/6; ie 12/6, 9/6, 8/6.
It certainly seems like the most probable original explanation (of the 'British' system). I don't think there is any doubt that what we now have is based on a Renard R10 series.'m sure that was the original case and not only in UK, I think your list, or something relatively similar, was fairly universal.
Hopefully not in all schools! Was this a primary school, where individuals teachers may well not have any 'specialist' further education in maths? In a secondary school, most maths teachers would have maths degrees, and I would hope that not many of them would make such an elementary mathematical mistake!I completely agree but that is what is currently being taught in schools.
Indeed, but that is exactly the same mistake (rounding 'in stages', rather than in one go) that I described before.Stupidly though 78.499 does round to 78.5 to 3sf and then using that and rounding to 2sf does go to 79 rather than 78
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