Obvious question? What’s the use for 5A 4 pole junction boxes?

[robinbanks]

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

I hope you called the school to complain; teaching kids that every digit to the right of the SF cutoff point must be rounded in turn is frankly apalling

 

[ebee]

Hah right. Reminds me of an instance a few years back. Luckily he was a pal of mine. He had some odds and sods and suggested £24 for them. I offered £12. "Ok" he says "Let`s split the difference". The I retorted "OK that`s agreed then, I will give you a fiver for the lot!" . Good job he knew me well.

 

[robinbanks]

Did you go to the same school of rounding as SUNRAY's grandson?

 

[Ragnar_AT]

Do not take a look at the strip connectors of old, Our old imperial 2Amp connectors now take 5 or 6 Metric amps easily anyway. Our old 240 Imperial Volts is more than a match for those new fangled 230V metric Volts too. Don`t worry.

I know I should not have said "Metric" I should have said SI , sorry!

Yes, and the old 15A plugs have pins almost the same size as heavy-duty industrial 32A plugs (commando plugs). Even more impressive, the old 5A plug is reasonably close to the modern 16A plug, over three times the original current. The pre-WWII British ratings remind me of some pre-WWI electrical regs that simply stated „1 amp per mm2“ for conductor sizing.

 

[JohnW2]

I hope you called the school to complain; teaching kids that every digit to the right of the SF cutoff point must be rounded in turn is frankly apalling

Quite - but as I wrote ...

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!

One of my neighbours is a primary schoolteacher, with a degree in English and a PGCE (both obtained when she was 'very mature'). I don't know to what extent, if any, the latter includes any 'maths education' but it's quite possible that everything she knows about rounding derives from her GCSE (or maybe O-Level!) in maths a good few decades ago. Whilst that is still no excuse for teaching kids incorrectly, that might make it more 'understandable' in a primary school environment - but, as I wrote, I would certainly be shocked if the same happened in a secondary school.

Having said all that, I think that the 'error' we are talking about makes little material difference. Provided one is doing 'conventional rounding'(rounding up or down according to whether the least significant digit is, or is not ≥5), I think that with either approach the range of possible 'rounding errors' (in comparison with the unrounded figure) is essentially ±0.5 least significant digits and the average error essentially zero.

Kind Regards, John

 

[ebee]

Why is it when people hear "Split the difference" they auto assume an equal split even if not stated?
In the case I mentioned I deliberately split the 12 into 5 and 7 rather than into 6 and 6.
In the particular example he assumed I meant splitting 12 equally than adding it to my original 12 making 18 ie the average (OK as the word average is so very dangerous I will re-state it as the Arithmetical Mean).
My confoundment was No, now I would give him £5 just £5 nothing else.

Who says I am awkward?

 

[DetlefSchmitz]

Yes, and the old 15A plugs have pins almost the same size as heavy-duty industrial 32A plugs (commando plugs). Even more impressive, the old 5A plug is reasonably close to the modern 16A plug, over three times the original current. The pre-WWII British ratings remind me of some pre-WWI electrical regs that simply stated „1 amp per mm2“ for conductor sizing.

I think that it is worth considering that a Type-C USB plug which has 20 pins (so you can picture how tiny they are) uses just 2 of those pins to supply 5A. The physical size of pins on a plug seems to be wildly unrelated to the current capability.

 

[JohnW2]

Why is it when people hear "Split the difference" they auto assume an equal split even if not stated?

Valid ploint, but I think the answer to your "Why?" is that 'an equal/even split' is very commonly (perhaps almost always) what people have in mind when they use the phrase.

For that reason, if it were me and I wanted to propose some 'unequal split', I would probably say "can we compromise (between those figures)", or something like that, rather than talking in terms of "splitting the difference", since I know that most people would take the latter to be referring to an equal split.

Kind Regards, John

 

[ebee]

Agreed John, it is just my warped sense of humour.

 

[sparkwright]

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 ....

[ the mathematical basis being that each number is approximately 1.259 (the "tenth root" of 10) times the one before ]

Most of those numbers are familiar in relation to electrical items - whether as the In of MCBs/RCBOs/fuses and/or as 'maximum current ratings' of other things (RCDs, switches, JBs etc.), the two main exceptions (in those contexts) being 8A and 13A (although 8A and 13A MCBs do exist, although seemingly rarely used). The latter is, of course, very familiar in relation to BS1362/BS1363,and I can but suspect that 8A was considered to usually be too close to 6A and 10A to be 'useful'. Some manufactures do, of course, produce 45A MCBs, which does not exist in the R10 series (but does in the R20 and 'higher' Renard series), I guess because there was a 'perceived need'.

Kind Regards, John

It would be handy if more manufacturers did produce 45A, as well as 25A - which isn't always available.

 

[JohnW2]

It would be handy if more manufacturers did produce 45A, as well as 25A - which isn't always available.

Agreed, for both.

Kind Regards, John

 

[SUNRAY]

Yes, and the old 15A plugs have pins almost the same size as heavy-duty industrial 32A plugs (commando plugs). Even more impressive, the old 5A plug is reasonably close to the modern 16A plug, over three times the original current. The pre-WWII British ratings remind me of some pre-WWI electrical regs that simply stated „1 amp per mm2“ for conductor sizing.

You are partly correct, in my little book I used to have dated just after WW1 it suggested if no figures were available for a particular cable to use 1000A per inch²

 

[robinbanks]

Who says I am awkward?

Me

 

[FrodoOne]

I think that it is worth considering that a Type-C USB plug which has 20 pins (so you can picture how tiny they are) uses just 2 of those pins to supply 5A. The physical size of pins on a plug seems to be wildly unrelated to the current capability.

The "current capacity" of any plug/socket combination is more closely related to the thickness/tension of the socket connections.

 

[JohnW2]

The "current capacity" of any plug/socket combination is more closely related to the thickness/tension of the socket connections.

Yes, it's certainly the quality of connection between the pins and receptacles that is the primary determinant of the safe 'current capacity' of a plug/socket connection.

It's certainly nothing to do with the size of the pins. A "13A" BS1363 plug has L & N pins of about 6.3mm x 4mm (i.e. about 25mm²) and if it were a conductor with that CSA in a cable, then it would be considered capable of safely carrying well over 100A.

Kind Regards, John