Rules and Guidelines
Want to participate in the discussions? Please read our Community Rules and Guidelines. Need some help? Visit Support and Answers.

  • State Not Answered
  • Locked Locked
  • Replies 31 replies
  • Subscribers 32 subscribers
  • Views 3147 views
  • Users 0 members are here
Options
You may also be interested in
This discussion has been locked.
You can no longer post new replies to this discussion. If you have a question you can start a new discussion

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


?
Parents
  • 0
    Isn't that interesting - GN3 tells lies!


    Only where the conductors are the same size is the resistance in steps 2 and 3 uniform around the circuit. So line/neutral cross-connection does give uniform results but line/CPC does not. That is assuming, of course, that T&E is used, but I don't quite see why you would install a ring with singles - better to have radial.


    I have drawn up a wee spreadsheet which illustrates the point using, roughly, an 80 m ring in 2.5 mm² T&E. That gives r1 = 0.6 Ω and r2 = 1.0 Ω. If a socket is placed as close as possible to the DB, the test resistance is 0.375 Ω  as opposed to the expected 0.4 Ω, with is 93.75% or as ebee says, a 6% error.


    The position of a spur is irrelevant except that ones near the centre of the ring will exaggerate the peak there, and ones near the origin will flatten the curve.

    Ring.xlsx
Reply
  • 0
    Isn't that interesting - GN3 tells lies!


    Only where the conductors are the same size is the resistance in steps 2 and 3 uniform around the circuit. So line/neutral cross-connection does give uniform results but line/CPC does not. That is assuming, of course, that T&E is used, but I don't quite see why you would install a ring with singles - better to have radial.


    I have drawn up a wee spreadsheet which illustrates the point using, roughly, an 80 m ring in 2.5 mm² T&E. That gives r1 = 0.6 Ω and r2 = 1.0 Ω. If a socket is placed as close as possible to the DB, the test resistance is 0.375 Ω  as opposed to the expected 0.4 Ω, with is 93.75% or as ebee says, a 6% error.


    The position of a spur is irrelevant except that ones near the centre of the ring will exaggerate the peak there, and ones near the origin will flatten the curve.

    Ring.xlsx
Children
No Data

Community Rules & Guidelines