Posting a message to the forum will remove the above advertisement

Discuss High r1, rn, r2 etc on ring final. in the Electrical Forum area at ElectrciansForums.co.uk.

Please make sure you checkout our forum sponsors, many do discounts for members and, they keep the forum free to use.
  1. Marcus Vaughan
    Offline

    Marcus Vaughan EF Member

    Location:
    Sheffield
    Hi All.

    I had a job which was going to be a 'quicky' fault finding and repairing a couple of sockets, then it came to the testing.....

    The consumer unit gave me no cause for alarm but was all labelled up wrong, so it turned out there were far more sockets and square meterage on this ring final than I first thought.

    So I get, on end to end continuity (2.5/1.5 T&E)
    r1=1.41 ohms, rn=1.43 ohms, r2=2.38 ohms. Then from that fig 8 test a range of R1+R2 from 1.53 to 2.61 ohms.

    Way too high. Nulled the leads (again). Checked again...

    If it was just one of those end to end readings that was too high, I would set about looking for a high resistance connection, but as the r1 is so similar to rn and the r2 is about right too - it seems unlikely that there is a high resistance joint in all three?

    If I reverse input those readings to get a estimated cable length it comes out at around 190 metres!

    The house is not particularly old - about 10 years I think. The floor area of the circuit is round about the 100 square metre mark, maybe more, and includes ground floor and first floor rooms - so the cable length could well be pretty high. On the limits at guess - but not way over as I have found.

    Splitting this all down would be no easy task, and I've been told the wiring was 'inspected' three years ago during a change of ownership. So I'm looking to avoid the disruption, if possible. If anyone has any advice or ideas I'd certainly appreciate it - big time.

    Are there any factors which might have given me misleading high readings?
     
  2. stantheman
    Offline

    stantheman Regular EF Member

    Have you recently used a wander lead and still have your meter nulled for that?
     
  3. Murdoch
    Online

    Murdoch Electrician's Arms

    Location:
    Woking
    If you were using a plug in tester for the figure of 8 then dirty contacts can do exactly what you describe.

    Have you allowed for spurs?

    What were the actual Zs readings like?
     
  4. Marcus Vaughan
    Offline

    Marcus Vaughan EF Member

    Location:
    Sheffield
    Hi Stantheman. I had just used the wander lead. Pretty sure I 're-nulled' though.

    Dirty contacts - possible. Some of the sockets are past their best.

    Live Zs was coming in at just over 1.1 ohms on a few 'DIY' spurs.

    Thanks for prompt responses - I'll be back there on Thursdayto try and resolve what's going on. It's good to be armed with ideas....
     
    • Like Like x 1
  5. Murdoch
    Online

    Murdoch Electrician's Arms

    Location:
    Woking
    When you get unusually high Zs readings on sockets, you'll find that plugging in and unplugging the tester several times will, in 90% of cases lower the actual Zs readings
     
    • Agree Agree x 3
  6. wirepuller
    Offline

    wirepuller Trusted Advisor

    Location:
    south uk
    As long as Zs readings are acceptable I wouldn't lose any sleep over it. As long as you are sure your continuity readings are correct just note it on your MW.
    There's a site I work at wired about 10 years ago where all the power circuits are 4.0 32a radials on RCBO's, Zs is invariably higher than that f0r the OCPD on the longer runs.....I just note that the RCD element of the device meets the disconnection time. It's poor initial design, but as long as there are no issues with the use of the circuit or fault protection just accept it and crack on.
     
    • Agree Agree x 1
  7. robo83
    Offline

    robo83 Regular EF Member

    Location:
    London
    I had similar situation, after going round and checking the termination of all the sockets and removing every Fascia. I realised my lead for my megger was damaged and wasn't making the correct termination with the meter :(
     
    • Funny Funny x 2
  8. stantheman
    Offline

    stantheman Regular EF Member

    Usually best to take readings from the terminals themselves as to avoid poor contact issues.
     
    • Agree Agree x 1
    • Disagree Disagree x 1
  9. Risteard
    Offline

    Risteard Electrician's Arms

    Location:
    Derry, Ireland
    Business Name:
    Walsh Electrical Services
    I'm not sure I'd agree. I think it would definitely need to be established why such readings were being obtained. It doesn't seem entirely credible that the circuit is indeed that length (but who knows?).
     
  10. davesparks
    Offline

    davesparks Trusted Advisor

    Location:
    guildford
    Did you test both sockets of the double sockets? I've often found that one side is different to the other.
     
    • Like Like x 2
  11. Marcus Vaughan
    Offline

    Marcus Vaughan EF Member

    Location:
    Sheffield
    Hi All. Thanks for responses. I have spent the morning changing a few accessories - including 3 sockets and a FCU. The readings are coming down - it an still getting various Ze of between 0.56 and 1.26 ohms. In fact there is a group that is compliant then a step change to a group that are not. I'm starting to think a kitchen extension is to blame? Interconnections? Now to retest properly from the board.
     
  12. davesparks
    Offline

    davesparks Trusted Advisor

    Location:
    guildford
    If Ze is varying then that is another matter entirely, that's the DNO's problem.
     
    • Like Like x 1
  13. Marcus Vaughan
    Offline

    Marcus Vaughan EF Member

    Location:
    Sheffield
    Sorry - I didn't mean Ze, no idea why I wrote that. I meant R1+R2.
     
  14. Marcus Vaughan
    Offline

    Marcus Vaughan EF Member

    Location:
    Sheffield
    Well - I returned. Found even more sockets on the ring. Mapped it out. Used IR test to confirm everything disconnected. r1, rn, r2 all still high (the same as before). Tested back and front of sockets and swapped a few out there were a little 'resistive'.
    All new R1+R2 readings - very uniform but at 0.89 ohms give or take 0.05 for those on the ring (couple of spurs a teeny but more). Add a 0.3 Ze on a 32 amp type B.......

    It's non-compliant. I think the house was at close to maximum upon design. Add an extension, add some electric velox windows, add this, add that...

    Now trying to come up with a solution that won't destroy the place...
     
  15. wirepuller
    Offline

    wirepuller Trusted Advisor

    Location:
    south uk
    I'm still not sure whether the effort is justified, unless I missed it what's the Zs? Are disconnection times met?
    It's an existing circuit that may not be compliant but is unlikely to cause a problem based on information given. I think you are making a mountain out of a mole hill

    Edit...just noted you've stated 1.1 ohms on spurs? Within required max for the OCPD. I cant see the point in it.
     
  16. Marcus Vaughan
    Offline

    Marcus Vaughan EF Member

    Location:
    Sheffield
    Max Zs is 1.29 ohms on a B32. Correct for temperature I usually work to 1.10. So no - disconnection times not met.

    Mountain out of a mole hill? I thought disconnection times were there for a reason - I mean how do I write it up. Is it not in -unsafe condition?
     
  17. wirepuller
    Offline

    wirepuller Trusted Advisor

    Location:
    south uk
    I assume it's on an RCD? Bs 7671 permits an RCD to take care of earth fault protection on a TN system, in which case your max Zs becomes 1667 ohms. I'm not saying that would be usual practice, but this is a circuit only marginally over required criteria. I just cant see the point in major disruption and expense for a circuit like this. Alternatively loadings permitting, stick it on a 20a OCPD
     
  18. Marcus Vaughan
    Offline

    Marcus Vaughan EF Member

    Location:
    Sheffield
    Max Zs is 1.29 ohms on a B32. Correct for temperature I usually work to 1.10. So no - disconnection times not met.

    Mountain out of a mole hill? I thought disconnection times were there for a reason - I mean how do I write it up. Is it not in -unsafe condition?
     
  19. Marcus Vaughan
    Offline

    Marcus Vaughan EF Member

    Location:
    Sheffield
    Ring Finals have to be on a 30amp or 32amp MCB. - 433-1-204.

    Yes. RCD protected. What about overload and disconnection times?

    The customer wants to know 'is it safe'?
     
  20. wirepuller
    Offline

    wirepuller Trusted Advisor

    Location:
    south uk
    It's up to you to determine whether it's safe, your name on the form, I'm just saying what I'd do. Regarding the 30/32a device for a ring final, if you feel the load is OK for a 20a then put it on the form as a departure and justify it with the reason you've given that it is a poorly designed circuit and reducing the OCPD rating is making it safer.
    Personally I'd list the high circuit readings on the form with explanation, leave it on a 32a and state that the RCD is meeting disconnection times and that I consider the circuit poorly designed but safe for continued use.
     
  21. Marcus Vaughan
    Offline

    Marcus Vaughan EF Member

    Location:
    Sheffield
    Thanks Wirepuller. Seems like sound advice. It's obviously not a new build or initial verification - I'll talk it through with the customer - who will probably want to go down the changing the MCB route.
     
Loading...
Similar Threads - High ring final Forum Date
RCD Tripping Hell with high load New Member Introductions Sunday at 9:20 AM
High zs on low bay lighting circuits Commercial Electrical Talk Sep 3, 2017

Share This Page

  • Electricians Directory Post a Domestic Job Post a Commercial Job