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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:

    PS in my opening statement second to last line my ref to a fig 8 might be misleading. I was refering to the r1 r2 crossover test to give R1 + R2 figure rather than the Fig 8 bridged ring situation which it is designed to find (amongst other things).


    Makes me think that one of our more knowlegable members should sketch, rings, fig 8s, crab claws, lollipops, 3 & 4 leaf clovers, trees etc etc.

    Could make a small book in its own right ?


    Do any of us make a habit of doing the crossover test to see if we`ve absent mindedly done a fig 8 or do we just do the test on an existing circuit by virtue of thinking "I would never do anything so daft!" ?


    When wiring a ring final we know how many cables we have installed. A "pure" ring has two 2.5mm2 T&Es at each socket or wiring accessory. If spurs are added then we may have three 2.5mm2 cables at a socket or accessory, but this makes terminating a bit more difficult. All those cables in one box. I have never wired a ring as a figure of eight either deliberately or accidently. I am quite happy with just an end to end resistance test for rings.


    The worst thing that I have found generally is two rings crossed over at a consumer unit. Ring A Leg L1 to M.C.B. 1, but Ring A Leg L2 to M.C.B. 2 etc. So two M.C.B. have to be turned off to deaden the one ring. This is mainly seen with hot wire fuses. 


    Z.


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  • 0
    ebee:

    PS in my opening statement second to last line my ref to a fig 8 might be misleading. I was refering to the r1 r2 crossover test to give R1 + R2 figure rather than the Fig 8 bridged ring situation which it is designed to find (amongst other things).


    Makes me think that one of our more knowlegable members should sketch, rings, fig 8s, crab claws, lollipops, 3 & 4 leaf clovers, trees etc etc.

    Could make a small book in its own right ?


    Do any of us make a habit of doing the crossover test to see if we`ve absent mindedly done a fig 8 or do we just do the test on an existing circuit by virtue of thinking "I would never do anything so daft!" ?


    When wiring a ring final we know how many cables we have installed. A "pure" ring has two 2.5mm2 T&Es at each socket or wiring accessory. If spurs are added then we may have three 2.5mm2 cables at a socket or accessory, but this makes terminating a bit more difficult. All those cables in one box. I have never wired a ring as a figure of eight either deliberately or accidently. I am quite happy with just an end to end resistance test for rings.


    The worst thing that I have found generally is two rings crossed over at a consumer unit. Ring A Leg L1 to M.C.B. 1, but Ring A Leg L2 to M.C.B. 2 etc. So two M.C.B. have to be turned off to deaden the one ring. This is mainly seen with hot wire fuses. 


    Z.


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