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Discuss Zs-Ze = R1+R2 - doesn't it?! in the Electrical Forum area at ElectrciansForums.co.uk.

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  1. Marcus Vaughan
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    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
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    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.
     
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  3. wirepuller
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    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
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    LankyWill Electrician's Arms

    Location:
    Northants
    I use it I can't see what all the fuss is about, it has its uses.
     
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  5. Zdb
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    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.
     
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  6. Wilko
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    Wilko Gorillas are Great 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 :)
     
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  7. davesparks
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    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.
     
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  8. Marcus Vaughan
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    Marcus Vaughan EF Member

    Location:
    Sheffield
    Great idea - but can I take it with me?
     
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  9. Andy78
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    Andy78 Trusted Advisor

    Location:
    Kingston upon Hull
    Not even if it asks nicely.
     
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  10. Marcus Vaughan
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    Marcus Vaughan EF Member

    Location:
    Sheffield
     
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  11. Marcus Vaughan
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    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
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    Marcus Vaughan EF Member

    Location:
    Sheffield
    I've been told.
     
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  13. Andy78
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    Andy78 Trusted Advisor

    Location:
    Kingston upon Hull
    Nothing stopping you dead testing at an available socket.
     
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  14. Dave OCD
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    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
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    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?
     
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