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I just thought of something

ebee
I know it`s many years ago that I queried the age old saying that was taught in college etc as to the r1 & r2 cross connection to form a double loop and the statement was made that this gave the exactly the R1 + R2 reading of the whole ring when taken from any point on the ring.

My statement was that this statement was not quite right and the word "exactly" needs substituting with "substantially" (I think the error was about 6% which as 6% of an already small number was not a great worry and it was still a very good approximation fit for use).


Anyway to add to that,,it just occurred to me . If we leave connected and test at a spur then it adds the spur value to the (nearly) ring value so that`s usually OK too.

However that`s only for spurs near to ring midpoint.!

If we had a spur nearer to one ring end than to midpoint it would therefore give a missleadingly large R1 + R2 value.

Not normally an issue but in extreme cases too pessimistic and causing a headscratch.


Off course field errors and instrument errors give missleading readings too.


I`d say once we done the fig 8 for the ring we should really connect ring ends together then test R1 + R2 from ring origin to each spur end to get as truer reading.


I know, I should get out more


?
  • 0
    gkenyon:
    ebee:


    However this theoretical value still varies 6% around the ring before taking these errors into the mix.

     


    And if the ring is wired in 4/1.5 rather than 2.5/1.5 ?




    At the beginning (or end) of the ring, the value is 36% lower.


  • 0
    Thanks Graham.

    Yes I only did the bog standard 2.5/1.5 calcs.

    Thanks Chris 36% eh sounds big but if you`ve done the sums then that is an eyeopener.

    My initial indignation was the term "exactly" being bandied about in such a manor.

    6% didn`t cause much concern because the numbers are small to start with. Whereas 36% is a different ball of wax
  • 0
    So, I believe the following is the correct method to work out the difference:
    1. At the DB, resistance is at its minimum: r1*r2/(r1+r2)

    • At the mid-point of the ring, resistance is at its maximum (r1+r2)/4

    • Ratio of min to max = (4*r1*r2)/((r1+r2)^2)


    Little maths trick, is that, because theoretically the conductors are of the same length, the factor (length/1000) cancels, so you can use the relevant "milli-ohms per metre" values for the CSA from Table I1 of OSG (E1 of GN1, B1 of GN3) for r1 and r2 in the third expression above, for the cable combination.


    So, we get:
    • 2.5/1.5: ratio min:max is 0.94, or 94 % (6 % difference in readings)

    • 4.0/1.5: ratio min:max is 0.80, or 80 % (20 % difference in readings)

    • 6.0/2.5: ratio min:max is 0.83 or 83 %, (17 % difference in readings) - although in BS 1363 accessories might struggle to get 2 no. 6.0 sq mm in the terminals as it's over-and-above the requirements of the standard.



  • 0
    ebee:

    Thanks Graham.

    Yes I only did the bog standard 2.5/1.5 calcs.

    Thanks Chris 36% eh sounds big but if you`ve done the sums then that is an eyeopener.

    My initial indignation was the term "exactly" being bandied about in such a manor.

    6% didn`t cause much concern because the numbers are small to start with. Whereas 36% is a different ball of wax


    I don't get 36 %, but 20 % ... but of course that again, as you say, is a different ball of wax.


    Basically, the rfc test only yields (r1+rx)/4 at every point, for conductors of the same csa.


  • 0
    That`s a nifty calc Graham, I like it. Will have to study it as I`ve only just peeked at it.

    If memory serves I did the calcs longhand with my sights set on readings at 10%, 25%, 33%, 50% around the ring and at origin or some such figures and made the ring length an easy (abnormal) length for easy maths calcs
  • 0
    I have to admit to doing the "long-hand algebra" which leads to the resistance on the ring being a parabola "nose up", the maximum value being (r1+r2)/4 at the mid-point, and the minimums (zero distance along the ring either way from the DB/CU) being r1r2/(r1+r2).


    Sad I know ...
  • 0
    gkenyon:

    I don't get 36 %, but 20 % ...


    My apologies, I agree with Graham. I put the wrong figure into my spreadsheet.


    I still wonder why anybody would use 4 mm² T&E for a ring.


  • 0
    Chris Pearson:



    I still wonder why anybody would use 4 mm² T&E for a ring.




    Minimum tabulated current carrying capacity Iz to be at least 20 A (Reg 433.1.204) and therefore from Table 4D5, Ref Method 101# or, where correction factors (such as grouping or ambient temp) might mess things up a little, Ref Methods A, 100#, 102#.


    As per earlier comment re 6 sq mm .... you might struggle with some products to drop spurs off a 4 sq mm ring at a socket-outlet or other accessory to BS 1363 because of terminal constraints if the product meets the minimum requirements of the standard.


  • 0
    Yes indeed but I think the "better ones" tend to allow this with adequate terminal holes. 2 x 6mm and a single 2.5 (often undoubled) in a decent terminal holds well but the standard does not mandate this ability. And the rest of the world don`t dig rings like we do. If we had never had them and someone suggested them today the we might have a similar outlook too I think.. 60s and 70s there was, to a degree, a mindset of if you did not do rings you were not a proper electricians (mind you, a popular bit of kit was an "neon tester screwdriver")
  • 0
    gkenyon:
    Chris Pearson:



    I still wonder why anybody would use 4 mm² T&E for a ring.




    Minimum tabulated current carrying capacity Iz to be at least 20 A (Reg 433.1.204) and therefore from Table 4D5, Ref Method 101# or, where correction factors (such as grouping or ambient temp) might mess things up a little, Ref Methods A, 100#, 102#.


    As per earlier comment re 6 sq mm .... you might struggle with some products to drop spurs off a 4 sq mm ring at a socket-outlet or other accessory to BS 1363 because of terminal constraints if the product meets the minimum requirements of the standard.




    Graham, thank you. I see what you mean. I had thought do radial instead, but short of going up to 6 mm² (which as you say, is possible only with the better makes of socket) under those circumstances, it would be necessary either to downsize to a 20 A OCPD, or complete a ring.


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