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Sizing of flylead to SWA

Graham Warner
Sorry about idiot-level question. 

3 no Distribution circuits, 3 phase, one each in 4 x 16, 4 x 25 and 4 x 35mm XLPE/SWA and each with separate 16mm cpc alongside.  The armour will of course be in parallel with the external cpc but ignored for sake of R1+R2 etc. however doing belt-and-braces by using Earthing Nuts at both ends.  Other than treating the armour as extraneous part to be supplementary bonded, in which case presumably the 4mm (544.2.1) rule applies, is there any proper guidance on the size of the fly lead to these Earthing Nuts?  Cable sizes quoted above are required primarily from voltage drop considerations.

These circuits are taken from a larger distribution circuit, overall on TN-C-S with additional earth rod at the end of the primary Distrib cct, which is in the middle of a field.  There are no other parts or services that require Supplementary Bonding.

Thanks for your advice.
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    Welcome. Do not worry about "idiot" questions - at the least they may help others and provoke a bit of thought.


    Actually the apparently simple ones are sometimes the ones that trigger a century run of posts, showing it was not simple at all


    So where would I start ?

    Do you know the upstream fuse size or MCB that would need to operate if these links were required to do their duty ?.- I'm imagining a cable part severed  giving an open armour to the supply but a  live to armour fault near the load end may be the worst credible fault case.

    I'd base it on that and a bit of adiabatic and see it it looks sensible.


    I presume all 3 start at the same place but have widely spaced endings - there is no CPC current sharing at the load end...

    Mike.
  • 0
    In view of the presumably very short lengths and thus trivial material cost, and would avoid "over thinking" this and simply go for no smaller than the phase conductors. Larger is fine if that is what you have a short piece of in the van.
  • 0
    I thought armour was an exposed conductive part, not an extraneous conductive part?
  • 0
    I'd like to think it is not exposed except perhaps at the glands, ,and is part of the installation, not really extraneous. 

    M.
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    Usually the armour of a cable is a cpc, whether or not you use one of the cores of the cable as a cpc as well. It definitely must be if the cable is buried.


  • 0
    Thanks for giving that elusive (to me anyway) bit of clarity.

    Correct presumption Mike, load ends are in camping cabins / feeder cabinet.  I'll go for 16mm to match the external cpc - well within MCB rating.

    This is actually voluntary work for a youthwork charity in my retirement, so no van and no spare cash.

    G.
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    There really is no point in going for a larger size than the live conductors, there is no credible fault that could require this, except an imperceptible change in R2. Whether 16 mm is adequate is presumably your actual question, and it will be, as under fault conditions we can allow a suitably high conductor temperature, using the adiabatic equation. Why have you got a 16mm CPC as well as the SWA? I assume that each cabin has an RCD-protected CU, 30 mA would be wise with equipment probably being extended outside. The only time where the current could get higher than a line conductor is if there are metal service pipes between multiple bonded points, PME, and a broken neutral. I assume this is not your case?
  • 0
    I understand the OP question slightly differently to the answers given above, I think:; Isn't he asking about the size of fly lead between the main earth bar in the consumer unit and the banjo (or in this case earth nuts) of the SWA gland?


    Is there a way of defining what size this should be?


    Considering the casing of the DB is metal, every other SWA gland should also have a fly lead and the earth bar is connected to the DB outer casing.


    Its a good question.......... I don't think it needs to be more than say 2.5mm and a blue crimp for the 16mm and 4mm and a yellow ring crimp for the 25mm and 35mm - but that's just pulled streight out of the air for no reason what so ever.

    It just ensures a good earth path exists but on a metal DB - its not the only earth path - all the other SWAs have earths between the earth bar and the gland plate too - providing multiple paths back to the earth bar.
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    Tatty, given that the fly lead is being added because the banjo connection to chassis isn't guaranteed to be sufficient, and in fact, in the absence of any suitable hard data, has to be assumed to provide no contact, then the fly lead might need to take the full fault current. So using 2.5mm for 16mm is a bit like connecting the cpc of a 16mm T&E to the earth bar using a final few inches of 2.5mm
  • 0
    Yes, if your " piranha" type thingy is your only means of connection and the cpc is only from armour then you need to consider the equiv copper content of the armour and then the fly-lead size and its correct connection to the earth nut (those little eyelets on the surface of the nut hex might not be robust enough)

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