Ring Continuity Test
- Thread starter Jupiter01
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You say that you are "using "the physics" - but they are very simple and the crucial thing you actually are 'using' is that number (1.68x10^-8 Ωm at 20C) which has come from somewhere' and, as EFLI has suggested, the figures quoted/asserted/whatever by different sources tend to vary slightly.
I agree that reactance (and 'skin effect') will not be significant at 50Hz, but one thing you might be missing (even though you mention it) is that you are talking about 'pure copper' - it's certainly not going to be literally ("100%") pure (what, in the real world, is?!), and it may actually be appreciably 'impure' (even possibly 'deliberately'?).
However, also, as above, even for pure copper different sources appear to give slightly different resistivities, so I don't know what one should regard as 'the truth'!
Kind Regards, John
If you look on the IET Wiring and the Regulations Forum then MAPJ1 mentions "the rule of 16" which is some useful approximations of copper resistance amongst other things
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Thanks. However, I don't think his/her "rule of 16" is either anything different from what we've been doing, or anything like the optimum "rule of thumb". He/she wrote:
For a start, for comparison with BS 7671 VD figures, it really becomes a "rule of 19" (since BS 7671 figures relate to 70°C, which I presume corresponds with 'running hot'). Also, for comparison with BS7671, the figures need to be doubled (because the BS 7671 fig relate to VD in two conductors) - so it becomes a "rule of 38".
If one works pro-rata from that for other CSA's we get the following, which shows that a "rule of 38" results in figures which are all about 13% - 16% lower than the BS7671 figures.
On the other hand, one could do as EFLI did, and use the BS 7671 figure for 1mm² cable as the starting point - so effectively a "rule of 44". That then produces pro-rata results for other CSAs which are all within about 2% of the BS 7671 figures, with three of them (1mm², 4mm² and 10mm²) being identical to the BS7671 figures.
I therefore know which one I would prefer as my "rule of thumb" (which is just a way of describing extrapolation from BS7671's figure for 1mm² cable)!
Kind Regards, John
My understanding is that although we reffer to conductors by CSA, this is only nominal, the actual standard IEC 60228 defines them in terms of maximum diameter and maximum resistance.
But this is second-hand info, I haven't read IEC 60228 personally.
But this is second-hand info, I haven't read IEC 60228 personally.
I haven't read it, either, but I find what you suggest rather hard to believe (particularly 'maximum diameter').
In any event, no matter what the Standard says, there's no doubt that they are sold, marked and specified in terms of their nominal CSA (around which there will obviously be some manufacturing tolerance) - and it would be very strange if the specifications gave resistances at other than that nominal CSA.
Provided one knows for certain what the material actually is (and hence it's resistivity) it is the (actual) CSA (and nothing else) which will determine the resistance of any given length of the conductor.
Kind Regards, John
Yes MAPJ1 does state that the 16 is often around 19 but I think he keeps it within his "the Rule of 16" as an easy to remember group of things 16ish along with 1.6 ,1/16" and 16 SWG too. An aid for a ready reckoner if you like.
He is a very clever fellow, extremely knowledgeable and a very helpful chap and a much respected contributor to that forum. In fact he once sent me a few of the old type belt buckles for cables free of charge
He is a very clever fellow, extremely knowledgeable and a very helpful chap and a much respected contributor to that forum. In fact he once sent me a few of the old type belt buckles for cables free of charge
Oh, fair enough, I didn't find any of the other "!6"s, only the one I quoted - so didn't realise that there was 'more' to his rules of thumb..
In terms of the one context we were discussing, not only does he say himself that The "Rule of 16" should really be "a Rule of 19" (since we, and certainly BS 7671,usually consider the worst-case {VD-wise} of 'running hot') but, as I wrote a "Rule of 22" would produce figures very much closer to the BS 7671 (and everyone else's) figures - and 22 is quite a long way from 16, even for a "rule of thumb"
Again, fair enough. A search for him by name on that forum found only 165 posts from him but, from what you're saying I might suspect that the truth would be one or two orders of magnitude greater than that? Anyway, I'll have a look!
Kind Regards, John
So Part drum of 2.5mm² T&E. It predates living in present house - 1994.
Took 5 resistance readings each of red and black average 615mΩ / 7.41 or 82.996m according to my 2004 brown OSG
From that I can predict the resistance of the earth wire, OSG shows 18.10 using 83m = 1502.3mΩ or 615x2.5 = 1537.5mΩ
So the first reading of 1.05Ω came as a surprise. straight away I assumed it's 1.5mm² earth and I'd made a mistake identifying the cable.
New figures; 1004.3mΩ and 1025.0mΩ
next8 readings averaged, (yes I continued just to make sure) 1.50366Ω so pretty close to OSG, closer than I should have hoped for.
Similar tests on a length of 1.5mm² T&E just a single test each 0.33Ω brown & blue = 27.27m. for 1mm²; OSG 493.64mΩ, calculate 0.33x1.5 = 0.495 my measurement of 0.49 is non conclusive.
Do I think there is cause for concern with any of the above? apart from the erroneous reading; no.
I'm not sure anyone is being slapdash, as I see it we are using different sets of published figures and I assume that where things get rounded the errors creep in, in my last post my meter only reads to 2 dp so the error at a reading of 0.33Ω being 0.325 to 0.334 is considerable, then calculations based on those get rounded and suddenly adding say 3% error at each of several steps becomes unmanageable.
Did you not work it out to verify? Both can't be right; 7.41 x 2.5 = 18.525. 18.10 / 2.5 = 7.24 so not just a rounding problem.
83 x 18.525 = 1537.575. Spot on.
18.10 clearly not correct.
That's what I would call slap-dash.
18.10 clearly correct in this particular test
So where was the error then?
The error existed in expecting the resistance of 1.0mm² to be 250% of the resistance of 2.5mm² as opposed to the almost perfect match to the OSG quoted figures in this particular case, the test was only made due to the contents of this thread.
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