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TT installation - Why is there a minimum size requirement for earth conductors?

Discussion in 'Electrics UK' started by a546345, 27 Jul 2020.

  1. Taylortwocities

    Taylortwocities

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    The usual way is to run 16mm Main earth from CU to a separate MET. This makes disconnect and testing of the electrode easier. the conductor from the MET to the electrode is sized as per BS7671.
    If/when supply type changes it’s easy then to run the appropriate earth (usually 16mm) from the MET to the DNO’s earth
     
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  3. a546345

    a546345

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    I have 2 RCD/RCBOs between the tails and the end of each circuit cable - a 30mA/ms and a 100mA/ms. So both would need to fail I think to get a current > 100mA ish for longer than it takes the RCD to trip.
     
    Last edited: 28 Jul 2020
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  4. a546345

    a546345

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    Existing is 10mm2 from rod to MET and plan was to change it to 16mm2.
     
  5. JohnW2

    JohnW2

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    Assuming that you are talking about the 'earthing conductor' from MET to the DNO earth terminal, as has been said, the requirement is different (from TT), and potentially very different, for the reason explained by bernard, the minimum earthing conductor CSA with TN-C-S (PME) being 10mm.

    As bernard has said, with TN-C-S that 'earthing conductor' is part of the same loop that could, under certain supply-side fault conditions, carry the same very high currents as the main bonding conductors - hence the requirement that the main earthing conductor have at least the same CSA as required for main bonding conductors (per Table 54.8 of regs). The same potential issue does not arise with TN-S, so the requirement is then no different from TT.

    Kind Regards, John
     
  6. JohnW2

    JohnW2

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    Thats what I have. In fact, my earthing conductor (from MET to earth electrode) is also 16mm² - totally unnecessary at present, but I did that because the path of that conductor goes past the 'cut-out' (such as it is with an overhead supply!), so that if anyone ever wanted to change to TN-C-S (which I have been offered), they merely have to snip that cable and connect it to the DNO's 'earth'.

    Kind Regards, John
     
  7. JohnW2

    JohnW2

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    Indeed - and, as I have pointed out, in determining the minimum size for 'protective conductors' in general (of which an 'earthing conductor' is one example) the calculations require that, for TN-S and TN-C-S systems, the usually single protective device (MCB, RCBO or fuse) does operate when it should - so having two RCDs/RCBOs in the path is, in many senses, better than that!

    In any event, as has been said, unless you have an exceptionally low-resistance earth rod, just 2.5mm² would be adequate even if neither of your RCDs/RCBOs operated.

    Kind Regards, John
     
  8. JohnW2

    JohnW2

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    As has been said, provided that none of it is buried (in earth) (which is unlikley), without protection against corrosion and mechanical damage, 10mm² is more than enough.

    Kind Regards, John
     
  9. EFLImpudence

    EFLImpudence

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    Also:

    Just because you do that, for no electrical reason, does not mean that the protective conductor sizes have to be increased.

    The sizes are based on the "required" csa of the live conductors (for TN-S and TN-C-S).
     
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  11. Taylortwocities

    Taylortwocities

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    @a546345 did he say why he felt it necessary to increase the size of conductor? That would help our understanding.
     
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  12. a546345

    a546345

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    I will ask him why he wants to change the cable. Before I do, here is a pic of the rod. Would you say it was protected against mechanical damage and corrosion?

    Cheers.


    IMG_20200729_171950.jpg
     
  13. JohnW2

    JohnW2

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    The figures that TTC quoted related only to buried (in the ground) earthing conductors (which yours isn't). If not buried, then the minimum for any 'protective conductor' (which is not part of a multi-core cable) is 2.5mm² if mechanically protected, otherwise 4mm² - both of which are clearly well under your 10mm².

    I can but repeat that your 10mm² conductor is more than adequate.

    Kind Regards, John
     
  14. Taylortwocities

    Taylortwocities

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    The cable is protected, it is inside the conduit. I would worry about damage to the rod though. I’d drive it further in so the terminal box is flat on the pavers.
     
  15. AdrianUK

    AdrianUK

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    Back in those days..... was an earth electrode common?
     
    Last edited: 29 Jul 2020
  16. AdrianUK

    AdrianUK

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    Forget the above..... I replied to wrong thread! :rolleyes:

    I meant to reply to the "With no functioning RCD, does/did a TT electrode achieve anything?"
     
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