Posting a message to the forum will remove the above advertisement

Discuss Zs-Ze = R1+R2 - doesn't it?! 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. The title is a slight red-herring to provoke a response....

    Looking through previous threads here and elsewhere tells me that the formula in the title just isn't true - that you cannot just measure Zs at a socket outlet (live test) and subtract a measured Ze to get an R1+R2 value for your minor works certificate.

    Fair enough - I thought - Ze requires the earthing conductor to be temporarily removed, parallel paths etc - so I can see why it wouldn't work....

    So - it seems odd then that my MFT seems to encourage doing just exactly that - entering a Zref value (Ze or Zbd) then doing your live Zs as normal and it will actually subtract the Zref and give an "R1+R2".

    Am I missing something here?
    I don't get it?
     
  2. Andy78
    Online

    Andy78 Trusted Advisor

    Location:
    Kingston upon Hull
    If your MFT is encouraging you to do anything, you should stop talking to it and get a holiday.
     
    • Funny Funny x 10
    • Like Like x 1
    • Winner Winner x 1
    • Dumb Dumb x 1
  3. wirepuller
    Offline

    wirepuller Trusted Advisor

    Location:
    south uk
    My megger ELI meter has the same function. Just because it's there doesn't mean you have to use it.
     
  4. LankyWill
    Offline

    LankyWill Electrician's Arms

    Location:
    Northants
    I use it I can't see what all the fuss is about, it has its uses.
     
    • Like Like x 1
  5. Zdb
    Offline

    Zdb Regular EF Member

    Location:
    UK
    From what I've read, Zs minus Zdb will sometimes give you an accurate R1+R2 (provided that there are no parallel paths), but it's really bad practice because you can't guarantee that you're getting true readings.
     
    • Bad Spelling Bad Spelling x 1
  6. Wilko
    Offline

    Wilko Electrician's Arms

    Location:
    Berkshire
    Business Name:
    Wilko Electrics
    The 3 things are slightly different so the best method is to perform 3 measurements, IMHO :)
     
    • Agree Agree x 1
  7. davesparks
    Offline

    davesparks Trusted Advisor

    Location:
    guildford
    My MFT has the same function, it seems like it could be used as a quick reference whilst carrying out testing to check that a measured Zs correlates to the R1+R2 previously measured.
     
    • Like Like x 1
    • Agree Agree x 1
  8. Marcus Vaughan
    Offline

    Marcus Vaughan EF Member

    Location:
    Sheffield
    Great idea - but can I take it with me?
     
    • Funny Funny x 1
  9. Andy78
    Online

    Andy78 Trusted Advisor

    Location:
    Kingston upon Hull
    Not even if it asks nicely.
     
    • Funny Funny x 1
  10. Marcus Vaughan
    Offline

    Marcus Vaughan EF Member

    Location:
    Sheffield
     
    • Creative Creative x 1
  11. Marcus Vaughan
    Offline

    Marcus Vaughan EF Member

    Location:
    Sheffield
    Thanks for the replies. Great stuff.

    I'll just carry on as I think I should - if I add a socket to a Ring Final then I'll go for the full cross connection R1+R2 test (and Ze/polarity). Remove the doubt. It takes a while though. It's just that background feeling of not wanting to disconnect the circuit from the consumer unit unless I have to. But that's what I'll do!
     
  12. Marcus Vaughan
    Offline

    Marcus Vaughan EF Member

    Location:
    Sheffield
    I've been told.
     
    • Like Like x 1
  13. Andy78
    Online

    Andy78 Trusted Advisor

    Location:
    Kingston upon Hull
    Nothing stopping you dead testing at an available socket.
     
    • Like Like x 1
  14. Dave OCD
    Offline

    Dave OCD Electrician's Arms

    Location:
    Cornwall
    Business Name:
    Hendry Electrical Services
    Marcus you don't have to disconnect at the DB to do the cross connection/figure of 8 test - just do it at a socket with both ring legs in it.
     
  15. Zdb
    Offline

    Zdb Regular EF Member

    Location:
    UK
    If you're extending the ring and not adding a spur you could always do end to end tests at the socket outlet and calculate your R1+R2. That way you don't have to disconnect anything at the D.B.

    I think it's okay to use this method?
     
  16. Dave OCD
    Offline

    Dave OCD Electrician's Arms

    Location:
    Cornwall
    Business Name:
    Hendry Electrical Services
    Andy beat me to it - but I explained it better. :D
     
    • Agree Agree x 1
  17. Dave OCD
    Offline

    Dave OCD Electrician's Arms

    Location:
    Cornwall
    Business Name:
    Hendry Electrical Services
    No you have to do the cross connecting as well to check it's actually a ring , then each socket should have pretty much the same reading apart from spurs which will obviously be higher.
     
    • Agree Agree x 2
    • Like Like x 1
  18. Marcus Vaughan
    Offline

    Marcus Vaughan EF Member

    Location:
    Sheffield
    Dave - sometimes I can't see the wood for the trees. Brilliant and simple at the same time. Thankyou. To me it makes sense to cross connect at the last new socket going in - and test at the consumer unit. I could always do a quick live Zs test on this last socket once it's in to ensure 'all is good'.
     
    • Like Like x 1
  19. Dave OCD
    Offline

    Dave OCD Electrician's Arms

    Location:
    Cornwall
    Business Name:
    Hendry Electrical Services
    Happy to help mate. :)
     
  20. buzzlightyear
    Offline

    buzzlightyear Electrician's Arms

    Location:
    star command
    op have got a onsite guide!
     
  21. Marcus Vaughan
    Offline

    Marcus Vaughan EF Member

    Location:
    Sheffield
    Dave - sometimes I can't see the wood for the trees. Brilliant and simple at the same time. Thankyou. To me it makes sense to cross connect at the last new socket going in - and test at the consumer unit. I could always do a quick live Zs test on this last socket once it's in to ensure 'all is good'.
    Sorry?
     
  22. buzzlightyear
    Offline

    buzzlightyear Electrician's Arms

    Location:
    star command
     
  23. buzzlightyear
    Offline

    buzzlightyear Electrician's Arms

    Location:
    star command
    read the on site guide if you have one, it will show you about testing .
     
Loading...
Similar Threads - R1+R2 doesn't Forum Date
Lighting Circuit r1+r2 values. Electrical Forum Jul 17, 2017
extractor fan issues (as in, it doesn't work very well) New Member Introductions Sep 10, 2017

Share This Page

  • Electricians Directory Post a Domestic Job Post a Commercial Job