Ring Continuity Test

[ebee]

OK, What if someone has connected a boiler controls on a ring via a 13A socket (yes I`ve seen it done) - that`s one item for a start by the way.

PS thanks John for the 1.61 v 1.67 clarification

 

[JohnW2]

OK, What if someone has connected a boiler controls on a ring via a 13A socket (yes I`ve seen it done) - that`s one item for a start by the way.

Indeed, at least 'partially', but on needs at least two things connected gto the CPC to create any 'parallel paths'.

PS thanks John for the 1.61 v 1.67 clarification

You're welcome

Kind Regards, John

 

[EFLImpudence]

Those are undoubtedly fairly 'correct figures, but where did you see them.

I worked it out from the 1mm² and 10mm² which looked more accurate at 44 and 4.4 respectively although obviously they both are likely rounded as well.
Then I realised - as per my following post -

Anyway, why use the VD figures rather than simply 2.5:1.5 which will be (is) the ratio of the resistances?

that it didn't matter what values were given for Volt-drop - or anything else - the ratio of the sizes is all that matters regarding the ratio of the resistances

.

The figures in Appendix 4 are rounded to 29 and 18.

So why did you imply that Jupiter might be wrong and that the value might be 1.61?

 

[Murdochcat]

That won't prevent the apparent CPC resistance being affected by the sort of parallel paths that the rest of us have been discussing.

Not sure I agree with that.

But I’m sure you will disagree but if everything is disconnected at the CU you are certainly eliminating lots of possible parallel paths

 

[SUNRAY]

.

So why did you imply that Jupiter might be wrong and that the value might be 1.61?

I haven't attempted to check the calculations but I think John has already explained his statement.

Looking at published mΩ/m figures doesn't help either12.1/7.41 = 1.633

 

[EFLImpudence]

I haven't attempted to check the calculations but I think John has already explained his statement.

Did you not read my post?

You only need the two sizes to work out the resistances.

Looking at published mΩ/m figures doesn't help either12.1/7.41 = 1.633

Of course not - but 2.5:1.5 does.

 

[JohnW2]

I worked it out from the 1mm² and 10mm² which looked more accurate at 44 and 4.4 respectively although obviously they both are likely rounded as well.

As you say, that 'looking more accurate' is really just 'cosmetic', given that they are undoubtedly rounded - there's no reason to think that copper conveniently has a resistivity that results in a conductor having a 'nice round resistance' in the units we have chosen to use!

In other words, I don't think you could be at all confident that the 44 for 10mm² is any closer to the actual truth than is 18 for 2.5mm² and/or 29 for 1.5mm².

Then I realised - as per my following post - that it didn't matter what values were given for Volt-drop - or anything else - the ratio of the sizes is all that matters regarding the ratio of the resistances

I agreed with that back in post # 11.

So why did you imply that Jupiter might be wrong and that the value might be 1.61?

That 'implication' is in the eyes/mind of the reader - it was not intended.

Kind Regards, John

 

[JohnW2]

I haven't attempted to check the calculations but I think John has already explained his statement.

Quite. Perhaps I was wrong to bother, but I was merely trying to anticipate some clever clogs jumping in and saying that the Bs 7671 VD figures resulted in a ratio other than 5:3.

Looking at published mΩ/m figures doesn't help either12.1/7.41 = 1.633

...and even those figures you have quoted will be 'rounded (to one and two DP respectively - few, if any, things in nature have 'totally round' values (in the units we chose to use).

Kind Regards, John

 

[JohnW2]

Not sure I agree with that. .... But I’m sure you will disagree but if everything is disconnected at the CU you are certainly eliminating lots of possible parallel paths

If by "disconnecting everything" you mean disconnecting all CPCs from the earth bar then, yes, that would eliminate the effects of any parallel paths which involved three or more circuits, but I doubt that would apply to many such paths.

However, more to the point, why bother? I really can't think of any situation in which one would have reason to be concerned about a CPC resistance lower than one might have expected.

 

[SUNRAY]

Did you not read my post?

Yes I did and I responded to

So why did you imply that Jupiter might be wrong and that the value might be 1.61?

viz:

 

[SUNRAY]

As you say, that 'looking more accurate' is really just 'cosmetic', given that they are undoubtedly rounded - there's no reason to think that copper conveniently has a resistivity that results in a conductor having a 'nice round resistance' in the units we have chosen to use!

In other words, I don't think you could be at all confident that the 44 for 10mm² is any closer to the actual truth than is 18 for 2.5mm² and/or 29 for 1.5mm².

One has to be aware, as I'm absolutely sure you are so this may be for the benefit of others, that the % error between 43.50 and 44.49 is smaller than 17.50 and 18.49. As such looking at the figures for larger size wire and calculating down should produce more accurate

Quite. Perhaps I was wrong to bother, but I was merely trying to anticipate some clever clogs jumping in and saying that the Bs 7671 VD figures resulted in a ratio other than 5:3.

...and even those figures you have quoted will be 'rounded (to one and two DP respectively - few, if any, things in nature have 'totally round' values (in the units we chose to use).

Kind Regards, John

I was merely agreeing with the error that you clearly explained exists in other parts of the documentation. However a very quick calculation with pencil so possibly not perfect, the extremities of the rounded figures: 12.1/7.41 do not reach 5/3 so it appears something is not right.

I think I have a drum of 2.5/1.0mm² T&E, If I think about it I'll measure the resistance.

 

[JohnW2]

One has to be aware, as I'm absolutely sure you are so this may be for the benefit of others, that the % error between 43.50 and 44.49 is smaller than 17.50 and 18.49. As such looking at the figures for larger size wire and calculating down should produce more accurate

For a start, you seem to have got that back to front. Per BS 7671 figures, the 44 (with the smaller potential percentage error due to rounding) id that for 1 mm² cable, but the 18 figure (with the larger potential percentage error due to rounding) is for 2.5 mm² cable - so, for the comparison of 1 mm² and 2.5 mm² cables, the 'the more accurate' figure (the one with the smaller potential error due to rounding) would be expected to be the one for the smaller size of conductor (the opposite of what you suggest).

However, it gets more complicated, dependent on what actual CSAs one compares, since, at least in the BS 7671 figures, the degree of rounding varies They don't round to a certain number of decimal places but, rather, to two significant figures.

Hence, for 10mm² cable, the figure is given to one decimal place, namely 4.4 mΩ/m (for two conductors), the potential range is 4.35 to 4.44999, a difference of nearly 0.1, which is about 2.27% of 4.4. However, for 4mm² cable, the BS7671 figure is rounded to 'zero decimal places', namely 11 mΩ/m ( (for two conductors), hence a range of 10.5 to 11.4999, a difference of nearly 1.0, about 9.09% of 11 - hence this time with a smaller potential percentage error with the larger size of conductor.

I was merely agreeing with the error that you clearly explained exists in other parts of the documentation. However a very quick calculation with pencil so possibly not perfect, the extremities of the rounded figures: 12.1/7.41 do not reach 5/3 so it appears something is not right.

Yes, although the discrepancies are fairly small, it is hard to explain why, from any one source, the ratio of resistances for two different CSAs should not always be exactly the inverse of the ratios of the CSAs.

The actual figures (for any CSA) may well vary a bit from source to source, since they might assume slightly different resistivities, not to mention the fact that different sources quote resistivities (or cable resistances) at different temperatures - but the ratio for two CSAs should still be 'as expected' for a given source.

Kind Regards, John

 

[SUNRAY]

For a start, you seem to have got that back to front. Per BS 7671 figures, the 44 (with the smaller potential percentage error due to rounding) id that for 1 mm² cable, but the 18 figure (with the larger potential percentage error due to rounding) is for 2.5 mm² cable - so, for the comparison of 1 mm² and 2.5 mm² cables, the 'the more accurate' figure (the one with the smaller potential error due to rounding) would be expected to be the one for the smaller size of conductor (the opposite of what you suggest).

Yes you are absolutely correct, what happened was terrible editing, I corrected one part but not the other.

 

[ebee]

Whoops. I hope I`m not seen as a "Clever Cloggs" (OK sometimes I might be - and sometimes more I might come across as that in addition0 it is not usually my intention. I saw what I thought might be a typo and mentioned it to John (I have learned to respect John and his posts so much that I did a recheck on my calculator using 2.5 to 1.5 ratios just to remind myself).

Similarly, when I mentioned the fig 8 R1 + R2 tutors speak then I`lll admit I`m one of those sometimes awkward gits - Initially, for quite a while, I took the "Exactly" statement as true but I always with everything in life like to check for myself. When I am told a fact I like to "fact check it" not because I am arrogant enough to doubt a tutor etc but it helps me understand a principle therefore it helps me actually remember it. So if it`s something I can work out myself then I usually do at some point.
The thing about the fig 8 thingy was I noticed a difference of about 6.25% but I also noted a few tutors that used the word "Exactly" but always emphasised it too.
If they have merely said it was the same and not also emphasised it then I wouldn`t have bothered, reason being that 6.25% is not a great error plus you also could field errors due to how hard you hold the meter lead connections, ambient temp and rounding errors on the meter too and the ring resistances are usually so small that 6.25% of a tiny amount is even more so a tiny amount.
It was not a massive principle but rather I had wondered why the term "exactly" was used and indeed had I missed something. I published it on the IET forum, probably this forum too and I thought that enough people see it and many of them are more knowledgeable than me then someone will get back to me with my error, that never happened so I took that to mean that I was probably right or thereabouts.

So if I have offended John or anyone else then I am sorry, it was never my intention.

 

[bernardgreen]

Sometimes a "good" Earth continuity test can be masking a serious double fault in the CPC

If there is any doubt about the measured R of the CPC then a wander lead should be used to verify that each and every socket has an effective Earth