help please 2393/2395

I am currently doing the above course and need a by of guidance please just to see if I'm doing this right or wrong any help and calculations / formula would be greatly appreciated the question (doing a continuity of a ring final circuit the small r1 rn is 0.43ohms and the cable size is 2.5/1.5.which is19.51mohms per metre resistance Work out the length, the r1 rn and the r2,and the zs ) I got 22metre cable length ,r1=0.1. r2=0.71 rn=0.1. And struggling wit zs also.

You might want to think about what other bit of information you might require in order to arrive at a zs

Adam_151 said:

You might want to think about what other bit of information you might require in order to arrive at a zs

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that would be the ze Adam ? I will not rest untill I have got a grip of this you have got us thinking which is a good thing thanks

robertseymour said:

... that would be the ze Adam ? I will not rest untill I have got a grip of this you have got us thinking which is a good thing thanks

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Yep, you're doing well. I would suggest that, rather than thinking about formulae and calculations, per se, you should try to think about concepts ...

.... for example, think about what Ze, Zs and (R1+R2) actually mean/represent. Once you understand that, the relationship between the three of them (hence an understanding of how you could calculate one of them, if you were given the other two) should become more-or-less obvious.

Kind Regards, John

robertseymour said:

r1 rn is 0.43ohms and the cable size is 2.5/1.5.which is19.51mohms per metre resistance

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Yes, but r1 and r2 (edit - I mean rn) are 2.5mm² so the 19.51 is not the correct value.

,r1=0.1. r2=0.71 rn=0.1.

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Where did the 0.1 come from?

EFLImpudence said:

robertseymour said:

r1 rn is 0.43ohms and the cable size is 2.5/1.5.which is19.51mohms per metre resistance

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Yes, but r1 and r2 are 2.5mm² so the 19.51 is not the correct value.

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Indeed, that figure appears to be part of the question, but goodness know exactly where it came from ... using 70°C figures (answers would be a bit less at 20°C), one metre of 2.5/1.5mm² cable ought to have an (R1+Rn) of 18 mΩ and an (R1+R2) of 23.5 mΩ (9 + 14.5) - so I'm not sure what 19.51 mΩ is all about! It sounds like an (R1+Rn) at a temperature around 100°C (its about 19.0 mΩ at 90°C) !

Kind Regards, John

JohnW2 said:

Indeed, that figure appears to be part of the question, but goodness know exactly where it came from

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It is the standard figure sum of r1 and r2 @ 20° for 2.5/1.5. (7.41 & 12.10)

My point was that Robert is dividing just r1 by the joint resistance.
Should be -
0.43x1000/7.41 = 58 metres or
0.43+0.71=1.13 x1000/19.51 = 58

EFLImpudence said:

JohnW2 said:

Indeed, that figure appears to be part of the question, but goodness know exactly where it came from

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It is the standard figure sum of r1 and r2 @ 20° for 2.5/1.5. (7.41 & 12.10)

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Ah! I did say that the figure would be a bit lower (than 23.5 m&#937 at 20°C, but I was not expecting it to be that much lower - I should have done the sums!

You say it is the 'standard' figure - but, although I realise that the OSG gives 20°C figures, do electricians actually use them routinely? All of the figures (in their VD tables) in the regs themselves relate to 'operating temperature' (usually 70°C or 90°C), and it would seem reasonable and appropriate to use those figures for 'conservative'/'worst case' calculations, whether one's interest is in VD or Zs, wouldn't it?

EFLImpudence said:

My point was that Robert is dividing just r1 by the joint resistance. ...

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Yes, I realised that, and don't disagree, which is why I didn't comment on it. It was just the quoted resistivity figure which was confusing me.

Kind Regards, John

JohnW2 said:

You say it is the 'standard' figure - but, although I realise that the OSG gives 20°C figures, do electricians actually use them routinely? All of the figures (in their VD tables) in the regs themselves relate to 'operating temperature' (usually 70°C or 90°C), and it would seem reasonable and appropriate to use those figures for 'conservative'/'worst case' calculations, whether one's interest is in VD or Zs, wouldn't it?

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In the case of the matter in hand, yes; that's what the values will be.

The cables are 'cold'.
Using the 70° values will give inaccurate results.

In my mind, that is why the maximum allowed Zs is reduced to allow for temperature difference rather than adjusting the measured figure.

Of course, there is also the point that cables, because of the (huge) margin of error, rarely get to 70°.


I don't know why the figures are lower than you expected.
Does the relevant volt drop not equate to 19.51 x 1.2?
That is if you were using the cpc.

Thanks for your help lads

EFLImpudence said:

The cables are 'cold'. Using the 70° values will give inaccurate results.

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Sure, in terms of this exercise, that's true. However, I was asking about what figures/calculations electrician's 'routinely use', in the real world, when one's interest is likely to be in the 'worse case' in-service VD or Zs - in which case, as I said, it would seem appropriate to use 'operating temperature' figures (even though, as you say, those temperatures are probably rarely attained in practice). In any event, as you go on to say ....

EFLImpudence said:

In my mind, that is why the maximum allowed Zs is reduced to allow for temperature difference rather than adjusting the measured figure.

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I had forgotten that the regs themselves and the OSG take opposite approaches (which must confuse some people) - but they do both end up considering the situation at 'operating temperature'. The regs quote 70°C resistance/VD figures, but 'uncorrected' 'maximum Zs' ones (relating to those 70°C resistivities), whilst the OSG assumes 10°C (not 20°C, I'd forgotten that) resistivity figures but then gives 'adjusted maximum Zs figures' to take into account that resistance relates to the lower temperature. In theory, those two approaches should end up with much the same answer (give or take the 10/20°C difference). Taking the present example, the difference in resistances at the two temperatures is about 1.20 (23.5/19.51) and the 'correction factor' used for the OSG Zs figures is 1.24 - so much the same. I'm sure that some must get confused by this difference between BS7671 and OSG, but, as I said, both end up (by different 'routes') effectively giving the maximum Zs at 'operating temperature'.

EFLImpudence said:

I don't know why the figures are lower than you expected. Does the relevant volt drop not equate to 19.51 x 1.2? That is if you were using the cpc.

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It was only the difference between the 20°C (R1+R2) (i.e. 19.41 m&#937 and the corresponding 70°C figure (~23.5 m&#937 that felt larger 'than I expected' - but that was only because I was foolishly relying on 'intuition', rather than calculations or tables

Kind Regards, John

robertseymour said:

I am currently doing the above course and need a by of guidance please just to see if I'm doing this right or wrong any help and calculations / formula would be greatly appreciated the question (doing a continuity of a ring final circuit the small r1 rn is 0.43ohms and the cable size is 2.5/1.5.which is19.51mohms per metre resistance Work out the length, the r1 rn and the r2,and the zs ) I got 22metre cable length ,r1=0.1. r2=0.71 rn=0.1. And struggling wit zs also.

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first calculation for length = res per m/res total = 19.51/1000=0.019/.43=22.6 metre is this incorrect ?

You know my views on the OSG.

Anyway, with regard to the points you make, obviously it does not matter which way round the temperature difference is considered, people are supposed to know what they are doing.

I would think that reducing the maximum Zs value is easier than correcting all the measurements taken, bearing in mind that the meter can only measure what is there.

However, in practice it is not so often necessary.
When calculating or measuring the Zs it is far below the BGB given maximum maximum, for example, my own guide is that if it is less than a quarter that figure then I do not need to do any adjusting as all will be well regardless.


I presume that the maximum maximums in the BGB are so that the installer can calculate whatever allowance for temperature is seen fit.
None at all for a 6A lighting circuit, for example ???

JohnW2 said:

EFLImpudence said:

robertseymour said:

r1 rn is 0.43ohms and the cable size is 2.5/1.5.which is19.51mohms per metre resistance

Click to expand...

Yes, but r1 and r2 are 2.5mm² so the 19.51 is not the correct value.

Click to expand...

Indeed, that figure appears to be part of the question, but goodness know exactly where it came from ... using 70°C figures (answers would be a bit less at 20°C), one metre of 2.5/1.5mm² cable ought to have an (R1+Rn) of 18 mΩ and an (R1+R2) of 23.5 mΩ (9 + 14.5) - so I'm not sure what 19.51 mΩ is all about! It sounds like an (R1+Rn) at a temperature around 100°C (its about 19.0 mΩ at 90°C) !

Kind Regards, John

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it's a twin and earth so I have used 2.5/1.5 tabulated value of19.51 John/ metre if that makes sense

robertseymour said:

it's a twin and earth so I have used 2.5/1.5 tabulated value of19.51 John/ metre if that makes sense

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Yes, it makes sense now. See above discussion - I suppose I should have done, but I hadn't twigged that they/you were quoting 20° figures rather than the 'operating temperature' (70°) figure (about 23.5 mΩ/metre) which appears in BS7671.

Kind Regards, John