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Discuss calculation check before i rip the flat to bits. in the Electrical Wiring, Theories and Regulations area at ElectrciansForums.co.uk.

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  1. telectrix
    Offline

    telectrix Scouser and Proud of It Respected Member

    Location:
    cheshire/staffordshire
    Business Name:
    Telectrix
    hi folks, got a CU change to do. flat rewired circa '70's. wylex 3036 fuses.

    my concern is earthing conductors. at present they are 6.0mm. TN-S.

    Pefc is 700A. not sure of main fuse size, so have taken worst scenario at 100A.
    referring to tables, @ 700A the disconnect time for a 100A BS1361 is 2 secs. so the adiabatic gives a S of 6.91 for the MEC.

    thoughts. please.
     
  2. Murdoch
    Offline

    Murdoch Electrician's Arms

    Location:
    Woking
    Hum ... Tricky one. Really need to know the dno fuse sizes and tails
     
  3. telectrix
    Offline

    telectrix Scouser and Proud of It Respected Member

    Location:
    cheshire/staffordshire
    Business Name:
    Telectrix
    tails from isolator to flat CU are 10mm. can only check DNO fuses if the seal fairy comes.

    and there's no sign of damage over last 30 odd years. flat is minimal load. 1 cooker cct. 1 socket cct, 2 lighting. shower is off boiler which is communal. total load is probably < 20A.average.
     
    Last edited: Sep 13, 2018
  4. Murdoch
    Offline

    Murdoch Electrician's Arms

    Location:
    Woking
    If the tails are only 10 mm I'd say crack on once the fairy has told you the fuse size.

    Common sense applies
     
    • Like Like x 2
  5. telectrix
    Offline

    telectrix Scouser and Proud of It Respected Member

    Location:
    cheshire/staffordshire
    Business Name:
    Telectrix
    if the fuse is 60A, then the adiabatic gives a MEC size of 2.19mm.so well in with 6.0mm. as you say, murdoch, common sense prevails.
     
    • Like Like x 1
  6. happyhippydad
    Offline

    happyhippydad Member Electrician's Arms

    Location:
    Gloucestershire
    Gosh Tel... I think in this case I'd be very, very tempted to employ the seal fairies. It sounds like a lot of upheaval for the customer based on the assumption that it's a 100A fuse.
     
  7. telectrix
    Offline

    telectrix Scouser and Proud of It Respected Member

    Location:
    cheshire/staffordshire
    Business Name:
    Telectrix
    even at 100A for the fuse.the 6.0mm is borderline, so i'm going to go ahead and leave it with an observation on the EIC.
     
    • Agree Agree x 1
  8. PEG
    Offline

    PEG Forum Mentor

    Location:
    Manchester
    Come on,Tel,surely you have a little box somewhere with 60A stickers in it...;)
     
    • Funny Funny x 2
  9. Wilko
    Offline

    Wilko Electrician's Arms

    Location:
    Berkshire
    Business Name:
    Wilko Electrics
    An old TNS supply with 10mm tails will almost certainly have been no bigger than 60A fuse, in my humble opinion.
     
    • Informative Informative x 1
  10. mhar
    Offline

    mhar Electrician's Arms

    Location:
    devon
    The fuse is almost bound to fall out when you do the cu change, why not make sure it is only a 60A?
    I never seek authorisation to reduce a fuse rating, only to increase it.
    As a matter of course my DNO replaces 100A fuses with 80s whenever they come accross them, often the meters are only rated at 80A anyway.
     
    • Agree Agree x 2
  11. Lucien Nunes
    Offline

    Lucien Nunes Mercury Arc Rectifier Respected Member

    Location:
    London
    Q: Has anyone EVER seen thermal damage to a 6.0mm² EC on a TN-S install of 100A or less? Or even to a 7/.036 for that matter?
     
    • Agree Agree x 4
    • Like Like x 2
    • Informative Informative x 1
  12. Andy78
    Offline

    Andy78 Forum Mentor

    Location:
    Kingston upon Hull
    Never seen any thermal damage on any earthing conductor, and seen a lot smaller than 6mm.
     
    • Agree Agree x 2
    • Like Like x 1
  13. Murdoch
    Offline

    Murdoch Electrician's Arms

    Location:
    Woking
    Sometimes it seems the seal fairy has changed the DNO fuse ....
     
    • Like Like x 1
  14. happyhippydad
    Offline

    happyhippydad Member Electrician's Arms

    Location:
    Gloucestershire
    I bet you thought twice before writing that Murdoch:D
    I wish there was a 'brave/risky/but we all agree' button!
     
  15. Midwest
    Offline

    Midwest Electrician's Arms

    Location:
    Oxfordshire
    Then when they meter gets changed to one of those new fangled smart meters, they'll put in a 100A main fuse.
     
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