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 3134 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


?
  • 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.


  • 0
    "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."


    Ditto.


    "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."


    Agreed, a bit of "An Old Chestnut" you tend to find a few of those over the years - mind boggling!
  • 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
  • 0
    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 personally installed rings with singles rather than T&E cables , two examples come to mind. The first was in a small machine workshop where steel trunking and metalclad sockets were installed around the walls. The bulk of T&E cable would soon clutter up the limited capacity of the trunking especially if the trunking contained other circuit cables as well.


    The second example was an office where plastic dado trunking was installed, again using singles. The capacity was limited as it was three compartment dado trunking.


    Z.


  • 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?"
  • 0
    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.


  • 0
    Yes you`re right Graham the R1 + R2 we record is a (often theoretical) point exactly half way around the ring (which is the worst place)

    The cross connected ring would give readings substantially similar when the test is done at any point.

    Of course theory only because you would have real world measuring inaccuracies.

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

    That was the point of my original working out a few years back when I noticed the word "exactly" being used quite a lot.

    I only realised that any radial connected to the ring (ie a spur) should be measured without the crossover or an error would creep in because it would be added to near ring midpoint reading rather than an actual position of connection point.

    More of a thought exercise really because we wouldn`t normally be working with rings and spurs approaching safe limits in reality anyway.
  • 0
    ebee:

    More of a thought exercise really because we wouldn`t normally be working with rings and spurs approaching safe limits in reality anyway.


    Well ...


    When you have to pop down to the wholesaler's to get another reel of 2.5 mm² ...


    It depends upon the size of the property. You could easily use up 20 m just getting to the first sockets on the second floor.


  • 0
    ebee:


    I only realised that any radial connected to the ring (ie a spur) should be measured without the crossover or an error would creep in because it would be added to near ring midpoint reading


    Ahhh, now I understand what you're getting at.


    So, in a ring with spurs, the correct statements for verification should be correctly stated (ignoring measurement inaccuracies and dirty contacts of switches and socket-outlets etc.) as:


    (a) At any point on the ring, (R1+R2) ≤ (r1+r2)/4; and

    (b) For any spur off the ring, (R1+R2) ≤ (R1spur+R2spur)+(r1+r2)/4


    ... and if (b) written as a formula or equation (R1+R2)=(R1spur+R2spur)+(r1+r2)/4, this is often an over-estimate unless the spur is close to the mid-point of the ring.


  • 0
    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 ?


Community Rules & Guidelines