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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


?
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    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.

     


    The correct statement, is that the test between L and PE at any point on the cross-connected ring (r1+r2)/4 equals the maximum reading if a standard (R1+R2) test were to be performed, that could be measured at the furthest point on the ring, if there were an accessory to measure it at, when the circuit is connected normally.


    But that's a lot of words ...


    (r1+r2)/4 is never actually (R1+R2) at any other point in the ring when connected in normal use, because, as you move closer to the CU around the ring, either way from the furthest point, you end up with (R1+R2) dropping from its maximum of (r1+r2)/4 at the [notional] furthest point, to somewhere approaching zero when you get to the CU itself.


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    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.

     


    The correct statement, is that the test between L and PE at any point on the cross-connected ring (r1+r2)/4 equals the maximum reading if a standard (R1+R2) test were to be performed, that could be measured at the furthest point on the ring, if there were an accessory to measure it at, when the circuit is connected normally.


    But that's a lot of words ...


    (r1+r2)/4 is never actually (R1+R2) at any other point in the ring when connected in normal use, because, as you move closer to the CU around the ring, either way from the furthest point, you end up with (R1+R2) dropping from its maximum of (r1+r2)/4 at the [notional] furthest point, to somewhere approaching zero when you get to the CU itself.


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