1. Visiting from the US? Why not try DIYnot.US instead? Click here to continue to DIYnot.US.
    Dismiss Notice

ring final continuity test ??

Discussion in 'Electrics UK' started by barnies, 16 Jan 2011.

  1. barnies

    barnies

    Joined:
    3 Dec 2010
    Messages:
    71
    Thanks Received:
    0
    Country:
    United Kingdom
    Hello,

    im just reading a book just now and there is something ive just read about the ring final continuity test i dont understand, say when u cross the live to the neutral of the same cable and not the opposite neutral the books says " that the resistance will start to go higher as u measure at the sockets the further u go away from the CU?

    why does that happen?
     
  2. Sponsored Links
  3. echoes

    echoes

    Joined:
    27 Mar 2007
    Messages:
    1,173
    Thanks Received:
    137
    Location:
    Surrey
    Country:
    United Kingdom
    Because the length of conductors whose resistance you are measuring is increasing the further away from the consumer unit you go: resistance is proportional to length.

    It does not happen when the phase and neutral from the opposite cables are connected because you should be measuring the resistance of the same length of conductor at each point.
     
  4. EngStudent

    EngStudent

    Joined:
    8 Jan 2011
    Messages:
    591
    Thanks Received:
    18
    Country:
    United Kingdom
    Your terminology isn't great but I'll give it a shot.

    A ring has two legs. We'll call them leg 1 and leg 2, and therefore L1, N1, L2, N2 for the line & neutral conductors.

    Think about what happens if you connect L1 and N1 together and start testing at socket outlets. To make things easier, we'll say that 1m of 2.5mm² conductor has a resistance of 1Ohm (it's much, much less than this btw). The closest socket to the CU will have the lowest resistance, simply because it's the shortest distance. Lets say the first socket is 1m from the CU.

    From 1 probe, down L1, back up N1 and into the 2nd probe on your test meter is 2m, so using our fictional values the meter will give a reading of roughly 2Ohms (plus a few milli Ohms due to the resistance of connections, test probes etc). Next socket, let's say 3m from the CU. Again, down L1, back up N1, 3 * 2 * 1 = 6Ohms, and so on and so forth.

    Connecting L1 with N2 and L2 with N1 means that wherever you are on the circuit, the loop from 1 probe to the other is equal distance, thus giving equal readings.

    HTH.

    Edit: If I remember correctly readings should be within 0.05Ohms of each other (or am I thinking of something else?)
     
  5. echoes

    echoes

    Joined:
    27 Mar 2007
    Messages:
    1,173
    Thanks Received:
    137
    Location:
    Surrey
    Country:
    United Kingdom
    Is much clearer than what I wrote!
     
  6. barnies

    barnies

    Joined:
    3 Dec 2010
    Messages:
    71
    Thanks Received:
    0
    Country:
    United Kingdom
    ok but why does the cores not be at equal distance if your connecting the same live and neutral of the same cable to each other and when u cross them over on each other they are at equal distance?
     
  7. EngStudent

    EngStudent

    Joined:
    8 Jan 2011
    Messages:
    591
    Thanks Received:
    18
    Country:
    United Kingdom
    Because if you connect L1 to N1 and L2 to N2 your meter will measure the path of lowest resistance. So it'll be a low reading near the CU as it's going up & down L1 and N1, once you get to the middle you'll get your highest reading as it's the furthest distance to L1 and N1 as well as L2 and N2. Once you start getting closer to the CU the path of lowest resistance will be up & down L2 and N2.

    When you cross them over, you're effectively forming a loop of the entire ring. Lets say the ring is 50m long. Starting at socket 1 again, it's 1m down L1, but that's connected to N2, which has to go all the way around the ring - 49m, giving a fictional reading of 50Ohms. Socket 2 was 3m from the CU. Again, it's 3m down L1 but that's connected to N2, which has to go the long way around - 47m, giving - yep, 50Ohms.

    Basically leg 1 is connected to the first socket and leg 2 the last. By connecting L1 and N2 you're reading the entire loop - up L1, through the CU, down N2, back through all other sockets on the ring to the same socket that you've connected your probes to.
     
  8. 1john

    1john

    Joined:
    3 Oct 2009
    Messages:
    3,351
    Thanks Received:
    343
    Location:
    West Glamorgan
    Country:
    United Kingdom
    And this is why radials are better.
     
  9. barnies

    barnies

    Joined:
    3 Dec 2010
    Messages:
    71
    Thanks Received:
    0
    Country:
    United Kingdom
    and also if the same cable was connected to each other L1-N1 and L2-N2 that is double the distance that has to measured yea?
     
  10. Sponsored Links
  11. Spark123

    Spark123

    Joined:
    2 Jun 2005
    Messages:
    14,841
    Thanks Received:
    474
    Location:
    Cumbria
    Country:
    United Kingdom
    You have to consider what you are trying to achieve by the interconnections. Below is a (badly drawn) schematic of the two interconnections.
    For simplicity reasons I will use 1 ohm for each resistor, however in reality on a ring final circuit each resistance will be a fraction of this and not equal but the theory is the same.
    In terms of R1 and Rn as the cores are of the same length and CSA the resistance of the conductors of each run should be equal hence both the resistors between nodes 1 and nodes 2 are of equal value, similarly with the resistors between nodes 2&3, 3&4 etc.

    With the incorrect version if you take a measurement between nodes 1 you will have two resistances in parallel, 2Ω to the left and 14Ω to the right. As these are in parallel the overall resistance will be (2x14)/(2+14)=1.75Ω.
    If we move along to nodes 2 and measure the resistance the result will be (4x12)/(4+12) = 3Ω
    nodes 3
    (6x10)/(6+10) = 3.75Ω
    nodes 4
    (8x8)/(8+8) = 4Ω
    nodes 5
    (10x6)/(10+6) = 3.75Ω
    nodes 6
    (12x4)/(12+4) = 3Ω
    nodes 7
    (14x2)/(14+2)=1.75Ω

    As you can see from above, the resistance increases towards the centre.

    For the correct version the measurement across nodes 1 will give two lots of 8 ohms in parallel. This will give you (8x8)/(8+8)=4Ω
    The measurement across nodes 2 will also give you the same (8x8)/(8+8)=4Ω, similarly for all the rest and the same as checking across the interconnections at the CU.

    You can try this with varyring resistances around the ring (keeping the resistances of equal values or proportional between the nodes) if you wish.
     
  12. bernardgreen

    bernardgreen

    Joined:
    3 Nov 2006
    Messages:
    24,752
    Thanks Received:
    2,433
    Location:
    Bedfordshire
    Country:
    United Kingdom
    If you use that logic on other things then a 1950's car is better than a 1990's car. Better only because it less complicated and therefore easier to test and repair.
     
  13. 1john

    1john

    Joined:
    3 Oct 2009
    Messages:
    3,351
    Thanks Received:
    343
    Location:
    West Glamorgan
    Country:
    United Kingdom
    Yes. but any car from 2000 onwards is easily plugged in and tested, it isn't even intrusive. Motor vehicles aside we are talking about wiring which has mostly stayed the same, it hasn't become more developed, sophisticated or more efficient between 1950 and 1990, the sceince is the same. What has happened though is that in that time frame millions of ring final circuits have been poorly installed and badly modified. I think that logic is best used on a ring final circuit, not so much on the MASSIVE advancments in automotive engineering. You cannot really apply it to the advancments in medicine either as we are keeping onder people alive for longer. I cant think of anything to compare it to for an analogy which suits it as there is nothing else about which was poorly designed from the outset with the only option to improove it being to abolish it, but adopted and held in such high regard by people who didn't understand it.

    I dont want to start a long argument or a 4 page thread about ring vs radial, my veiws are that radials are alot safer and superior, they are quicker to install, easier to commision and test and easier to fault find on, they are much safer in as much as it is harder for them to be incorrectly installed or altered. A fault on a 'live' conductor will make it's self shown and known to the user of the installation and will not leave the installation still functioning except in a dangerous situation. Anywhere a ring final circuit is can be fed from the same amount or cable with 2 x 20A mcbs or less 4mm2 cable on a 32A breaker.
     
  14. ban-all-sheds

    ban-all-sheds

    Joined:
    27 Aug 2003
    Messages:
    69,783
    Thanks Received:
    2,858
    Location:
    London
    Country:
    United Kingdom
    Barnies - your life will involve being able to communicate with people in writing - quotes, exams, email with customers etc.

    You have to learn the difference between a mobile phone and a computer keyboard.
     
  15. cumbriahandyman

    cumbriahandyman

    Joined:
    23 Nov 2010
    Messages:
    1,256
    Thanks Received:
    43
    Country:
    United Kingdom
    How very sad...

    :rolleyes: :rolleyes: :rolleyes:
     
  16. EngStudent

    EngStudent

    Joined:
    8 Jan 2011
    Messages:
    591
    Thanks Received:
    18
    Country:
    United Kingdom
    He's right though. This is an internet forum, not a text message. There's no need for lack of grammar or 'txt tlk'. To be an electrician you need to have a reasonable understanding of the English language and if you're asking a question on an internet forum it's only polite to use good English.
     
  17. dingbat

    dingbat

    Joined:
    29 Sep 2003
    Messages:
    2,846
    Thanks Received:
    68
    Location:
    West Midlands
    Country:
    United Kingdom
    Oh, go on... it's been a couple of days at least! :D
     
Loading...

Share This Page