70mm 4 core SWA

Discussion in 'Electrics UK' started by SethDavy, 16 Sep 2006.

  1. SethDavy

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    Does BS 7671 require a 70mm 4 core SWA have to have a separate earth conductor, or is it legal to just use the armouring within the cable?
     
  2. RF Lighting

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    Not quite the answer you were after, but this may help you to work it out.

    http://www.batt.co.uk/images/pics/7529ArmourResistanceBS5467.pdf

    Select the actual csa of the cable from manufacturers data or BS 6346 for PVC cables or BS 5467 or 6724 for XLPE cables.

    Then apply to this formula.Cable equivalent CSA =k2/k1 X actual SWA section.

    Where k1 is the phase conductor k value from table 43A BS7671 and k2 is CPC k value from tables 54D and E BS7671.

    This will give you an equivalent cable csa, sheathed in the same insulation as the cable you are dealing with.
     
  3. ricicle

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    I personally would run a seperate cpc with this size of cable.The size would be S/2 = 35mm.This size of cable would be either for a long run (high impedance armouring-steel higher resistance per meter than copper), or for a large single load with high fault current,again higher I2r means more heat under fault conditions or as a sub main needing to ensure that x number of circuits do not have a failed/poor earth if corrosion were to occur

    Ricicle
     
  4. Spark123

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  5. RF Lighting

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    I went looking for that table in the wiki, but looked under cable types > SWA :confused:

    Could you put a link to it there aswell?
     
  6. Spark123

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    OKay, I've put a little link at the bottom :D
     
  7. SethDavy

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    Thanks for your replies. If I have done my sums right, I get Rf's formula to
    CSA = K2/K1 x actual SWA section
    51/115 x 70 = 31.043
    If this is right, the answer is less than the 35mm that ricicle suggests.
    The cable used is BS6343, so Spark123's table won't apply.
    We have just completed an installation in an industrial unit. The customers landlord's electricians installed the 70mm incomer. Also this cable has been jointed. They haven't used a steel enclosure and glands, just with what looks like lugs and bolts and a lot of tape. I feel that there should be a separate earth but the landlord's people are having none of it.
     
  8. Spark123

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    The formula you need to use is K1s/2K2= Req min SWA size
    Do you mean BS6346?
    70mm² has an armour CSA of 138mm², k1 = 115, k2 = 51, required size is 79mm² so is compliant with 54G.
    Bit OTT for a garden shed or garage though :D
     
  9. fattony

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    Are you saying that once the armour csa reaches a certain size a seperate earth is not required? I never knew that I would have run a seperate earth but dont use cables that size.

    At what point do you stop using 3 core and start running a seperate cpc?
     
  10. Spark123

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    One of the main logical reasons for using 3 core on single phase is so it automatically complies with table 54G. In 3 phase and neutral installations though you may need to run an additional earth cable in adjacent.
    You can use the armour as the earth providing it complies with table 54G or the adiabatic subject to normal design procedures i.e. if it is to be used as part of a main eq bonding in PME then it needs to be 10mm² (or DNO requirement) copper equivalent etc.
    Firstly before you buy the 3 core you can determine wether or not the armour is sufficient to act as the CPC (or bonding conductor etc), either by 54G OR the adiabatic. If the armour on a 2 core is sufficient then why buy 3 core?
    If 2 core does not comply on a single phase circuit then you can buy 3 core and are guaranteed for it to comply with 54G.

    edit;
    Have added a 70ºC SWA chart into the Wiki.
     
  11. ban-all-sheds

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    1) So that you don't have to rely on the integrity of the armour, either the quality of its connection via the glands, or its long-term condition if any water gets in and rusts it?

    2) To use one core as a main bonding conductor for PME installations?
     
  12. Spark123

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    Point taken on a cable corroding after being damaged by spade etc. You could argue the cable glands need to be connected to the CPC at both ends or even regular intervals for the armour to be effective in the event of it corroding due to water ingress possibly at more than one point, although as the armoured strands are galvanized this may be ott.
    Is it a fair point to make that you cannot take into account every eventuallity however hard you try, the cable should not get damaged if installed correctly either in the ground or on a tray, if a cable is damaged by spade, impact etc it should be repaired before being back filled. The glands if made off correctly and the cable being secured correctly mean they shouldn't get pulled out.
    Wouldn't the armour comply with being used as part of a main equipotential bond as long as its csa is greater than 10mm² copper equivalent (providing 10mm² acceptable by DNO)?
     
  13. ban-all-sheds

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    Good point.
     

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