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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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    Yes Chris,

    I did the calcssimilar to you.

    When I did the C & G 2391 course our tutors (and some NIC approved contractors) used the term and emphasised "Exactly" and I queiried this.

    So I did the cals (half expecting to be shot down) and to make the arithmetic easy peasy I used a ring of abnormal length to get the figures an easy r1 of 1.5 ohms and r2 of 2.5 ohms.

    Did the calcs at several places and found a 6% (I thnk) difference.

    Bearing in mind 6% of an already small ohm number does not add up to a big figure in real life terms then I was happy to substitue "substantially" in place of "exactly".

    Actual measurement inaccuracies could, in the real world, add up to more than this small figure anyway.

    Years later (this weekend) it struck me that a spur of any length would give much the same reading no matter what part of the ring it was connected to. A spur at 10% or 25% around the ring would give much the same reading as one at 50% around the ring I think.

    In a few very instances it could, in theory, lead to a false high reading.

    Like I said = I really should get out more!


    It`s just that some folk point out "absolute facts" and they get repeated until they "become true" and nobody ever questions them for years and years.

    "40% of body heat is lost thru the top of your head" I think is another such saying..


    "It`s a "WELL KNOWN FACT" saying always tempts me to reply "Oh, a lot of people believe it therefore it bcomes true then?".

    or "Smoking must be healthy cos millions of people do it?"
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  • 0
    Yes Chris,

    I did the calcssimilar to you.

    When I did the C & G 2391 course our tutors (and some NIC approved contractors) used the term and emphasised "Exactly" and I queiried this.

    So I did the cals (half expecting to be shot down) and to make the arithmetic easy peasy I used a ring of abnormal length to get the figures an easy r1 of 1.5 ohms and r2 of 2.5 ohms.

    Did the calcs at several places and found a 6% (I thnk) difference.

    Bearing in mind 6% of an already small ohm number does not add up to a big figure in real life terms then I was happy to substitue "substantially" in place of "exactly".

    Actual measurement inaccuracies could, in the real world, add up to more than this small figure anyway.

    Years later (this weekend) it struck me that a spur of any length would give much the same reading no matter what part of the ring it was connected to. A spur at 10% or 25% around the ring would give much the same reading as one at 50% around the ring I think.

    In a few very instances it could, in theory, lead to a false high reading.

    Like I said = I really should get out more!


    It`s just that some folk point out "absolute facts" and they get repeated until they "become true" and nobody ever questions them for years and years.

    "40% of body heat is lost thru the top of your head" I think is another such saying..


    "It`s a "WELL KNOWN FACT" saying always tempts me to reply "Oh, a lot of people believe it therefore it bcomes true then?".

    or "Smoking must be healthy cos millions of people do it?"
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