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Discuss Submain Supply 100a 100mA in the Electrical Forum area at ElectrciansForums.co.uk.

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  1. Tom Christopher
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    Tom Christopher Guest

    Installing a 25mm 3core swa to feed an exisitng consumer unit due to the SEB moving the supply.

    Planning to use a 100A 100mA time delayed rcd to feed the split load rcd board.

    Sound ok?
     
  2. Ian1981
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    Ian1981 Forum Mentor

    Location:
    North east
    Why a 100mA S type rcd upfront?
    Is it a TT system?
    Will the dual rcd board accommodate a 25mm into the earth bar?
    Will there be a switch fuse isolator fitted If the supply is over 3 meters from the cut out?
     
    • Like Like x 2
  3. Dillb
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    Dillb Electrician's Arms

    Location:
    Nottingham
    Why would it need that? Nothing in BS7671 about it, and the DNO may not specify it.
     
    • Optimistic Optimistic x 2
  4. Ian1981
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    Ian1981 Forum Mentor

    Location:
    North east
    433.3.1 item (iii)
    A device for Overload protection need not be provided at the origin of an installation where the distributor provided an overload device and agrees it affords overload protection to the part of the installation between the origin and main distribution point where further overload protection is provided.
    The general consensus is that the DNO will allow 3 meters and no more for their cut out fuse to protect the tails.
    Also 434.3
     
    • Like Like x 1
  5. Dillb
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    Dillb Electrician's Arms

    Location:
    Nottingham
    So as I say its speculation if its needed, nothing to say in this instance that the DNO require it fitting.
     
  6. spinlondon
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    spinlondon Forum Mentor

    Location:
    Harlow Essex
    My understanding is that the DNO will not generally allow their fuse to be used to protect any part of a consumer’s installation.
    The 3m stipulation relates to the BS7671 requirement that overload protection must be provided within 3m of a reduction in the CCC of a conductor.
     
  7. Ian1981
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    Ian1981 Forum Mentor

    Location:
    North east
    This is northern powers requirements anyway 1388B8A2-0046-4E6C-88C4-944582FF8636.png
     
    • Like Like x 1
  8. Tom Christopher
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    Tom Christopher Guest

    PME

    Yes dual board will take a 25mm

    SWA will be made off into a metal enclosure with the 100a 100ma time delayed rcd. This will be mounted next to the main supply, so tails less than 3m

    SWA will be above the plaster board so wont it need rcd protection?

    I thought time delay to save both RCD trippings if there is a fault?
     
  9. Ian1981
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    Ian1981 Forum Mentor

    Location:
    North east
    Tho that is a regulation in the context you describe that has Nothing to do with the DNO tails
    Read 433.3.1 and 434.3 or not
     
    • Agree Agree x 1
  10. spinlondon
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    spinlondon Forum Mentor

    Location:
    Harlow Essex
    RCD protection for cables concealed above ceilings and below floors is not an acceptable method for providing extra protection.
    SWA however is.
     
    • Agree Agree x 1
  11. spinlondon
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    spinlondon Forum Mentor

    Location:
    Harlow Essex
    Not aware we were discussing DNO tails?
    BS7671 does not apply to DNO equipment.
     
  12. Murdoch
    Online

    Murdoch Electrician's Arms

    Location:
    Woking
    ^^ swa doesn’t need rcd ...
     
    • Agree Agree x 1
  13. Andy78
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    Andy78 Respected Member

    Location:
    Kingston upon Hull
    There is no need for RCD protection for those reasons if using earthed SWA. If it did, then you RCD you selected would not have been suitable anyway.
     
    • Agree Agree x 2
  14. buzzlightyear
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    buzzlightyear please let me back in to the prison cell. Electrician's Arms

    Location:
    star command
    new sub mains from tncs to tt then .
     
  15. westward10
    Online

    westward10 In echoed steps I walk across an empty dream. Staff Member Moderator

    Location:
    Northamptonshire
    We can only confirm if this is okay if you let loose what the earthing system is.
     
    Last edited: Jun 25, 2018
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