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Discuss CPC size of SWA Armouring in the Electrical Forum area at ElectrciansForums.co.uk.

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  1. Sparkyphil83
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    Sparkyphil83 New EF Member

    Location:
    Hull
    Evening,
    Just joined upto this forum after a search online for some info, hopefully someone can help me?
    I have taken over a job from another contractor who has run in a 50mm 4 core swa from a substation to a TP&N DB roughly 100m away.
    My concern is that it appears they have not run a separate earth and are relying on the armouring as the CPC. I believe it should have a 25mm earth ran in alongside the SWA?.
    My other problem is that the cable is now burried underground and has a tarmac road above it ect.
    What are my options?
    Thanks in advance
     
  2. westward10
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    westward10 In echoed steps I walk across an empty dream. Electrician's Arms

    Location:
    Northamptonshire
    Why do you believe it needs a separate cpc.
     
  3. Ian1981
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    Ian1981 Electrician's Arms

    Location:
    North east
    Armour will be acceptable as the cpc however the length could be a factor as to achieving your disconnection time, calculations required IMG_2860.JPG
     
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  4. Andy78
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    Andy78 Trusted Advisor

    Location:
    Kingston upon Hull
    Why don't you ask for the cable calculations from the previous contractor ? Although I suspect it'll be reet.
     
    • Agree Agree x 1
  5. Sparkyphil83
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    Sparkyphil83 New EF Member

    Location:
    Hull
    Thanks guys appreciate the help, I'll double check the calcs tomorrow regarding disconnection time
     
  6. davesparks
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    davesparks Trusted Advisor

    Location:
    guildford
    Presumably you have calculated that a 25mm copper conductor is required and not just guessed this?
     
  7. westward10
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    westward10 In echoed steps I walk across an empty dream. Electrician's Arms

    Location:
    Northamptonshire
    I agree it would be interesting to know the reasoning as opposed to quoting answers.
     
  8. Sparkyphil83
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    Sparkyphil83 New EF Member

    Location:
    Hull
    Davesparks i was referring to table 54.7 of bs7671 which states if S>35 and the protective conductor is the same material as the line conductor this is S ÷ 2
    S = 50mm
    50 ÷ 2 = 25mm
     
  9. Ian1981
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    Ian1981 Electrician's Arms

    Location:
    North east
    A perfectly acceptable method used the other being a calculation so no big deal
     
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  10. davesparks
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    davesparks Trusted Advisor

    Location:
    guildford
    So no calculation then, just using the Table which gives an oversized cpc. Not that there is anything wrong with using the table for design work, but when assessing compliance of an existing setup you should do the proper calculation with the adiabatic equation to make certain if the answer from the table isn't acceptable.
    S is not the same material as the line conductor in this case so why have applied that part of the table anyway?

    Did you even go as far as assessing the suitability of the armour based on the table 54.7 requirements? Or did you only look at the required size of a copper cpc??
     
  11. davesparks
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    davesparks Trusted Advisor

    Location:
    guildford
    But if assessing someone else's design, as in this case, it is not acceptable to condemn their design based only on the table when they may have used the adiabatic equation.
     
    • Agree Agree x 1
  12. Ian1981
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    Ian1981 Electrician's Arms

    Location:
    North east
    True but I know were the op is coming from regarding his 25mm
     
    • Agree Agree x 1
  13. cliffed
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    cliffed Regular EF Member

    Location:
    Lancs
    Is there any main Earth bonding in the building
     
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