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Earthing and Bonding

Former Community Member
Former Community Member
Hi All


Working on a scheme whereby we have a private transformer serving the site - TN-S arrangement. This comes into the building, we then split to feed a switchboard in one part of the building which is existing, the second split heads to a new build part which has the second switchboard. This new build contains swimming pool. The supply to the new switchboard is via a busbar 3L+N+PER. Question relating to earth bars - we currently have the main earthing bar at the intake point which is at the existing switchboard location - we then had cable feeding out to the new earth bar at the new switchboard location. Contractor is proposing that they remove this interlinking earth cable between bars, and take a bond directly from the new switchboard earth bar to the new earth bar in the room. Does this sound feasible - to me doesnt sound right, but I can't find anything specific to say not.


Many thanks
  • 0
    I am no expert so take my answer with a pinch of salt....


    I believe that if you want to take the bonding for the swimming pool from a remote sub distribution board, the earth for that sub distribution boards supply needs to be sized for both Earthing and bonding. 


    Quite a few years ago - like 10 or 12 - I asked a similar question on the old platform for this forum and I was given a series of brilliant replies - mostly by the same guys who haunt this forum now - and they went on to explain and expand on this statement. 


    Now I'm sure if I tried hard enough I could expand on that satement - but darned if I can explain it at the moment....at the time it was explained very well, I did the necessary calculations and installed the cable, but I cannot for the life of me remember what I did. I've forgoton. (The NICEIC area inspector even checked it - so it was done properly)


    So I'll put my ignorance out there


    - being sized for earthing - I can work this out

    - being sized for bonding - I can work this out


    Being sized for both earthing and bonding.......... well now what - add the two sizes together I suppose. I know  asked this same question last too............darned if I can remember the answer. 



    I'm sure one of the guys will expand on my answer soon.
  • 0
    Contractor is proposing that they remove this interlinking earth cable between bars, and take a bond directly from the new switchboard earth bar to the new earth bar in the room. 


    Does the contractor consider that the existing combined earthing/bonding cable is too small for its functions? Why not ask the contractor for  his reasons?


    Does 312.2.1.1 Fig 3.8 help? The protective conductor (PE) normally carries very small currents in comparison with normal load currents, but may carry high currents during fault conditions. 


    Z.


  • 0
    Is any of this applicable?

    IET Forums - Earthing arrangement for two buildings (theiet.org)


    "What is the supply and earthing arrangement of the building that you intend adding the chiller to - knowing that will give you the best starting point but generally speaking regardless of one or two transformers, there is usually no real issue providing the supply from one substation to multiple buildings at LV - or as an alternative, having multiple HV and LV inputs to the same building


    Regards


    OMS"



    Z.
  • 0
    Then there is 542.1.3.1 but that does not add much that we don't already know..


    Z.
  • 0
    Being sized for both earthing and bonding.......... well now what - add the two sizes together I suppose. I know asked this same question last too............darned if I can remember the answer.

    It only has to satisfy both requirements simultaneously - no need to add - so if it has to be say 2.5mm2 for a c.p.c. and 10mm2 for a bond, a 10mm2 will do.

      - Andy.
  • Former Community Member
    0 Former Community Member
    Zoomup:
    Contractor is proposing that they remove this interlinking earth cable between bars, and take a bond directly from the new switchboard earth bar to the new earth bar in the room. 


    Does the contractor consider that the existing combined earthing/bonding cable is too small for its functions? Why not ask the contractor for  his reasons?


    Does 312.2.1.1 Fig 3.8 help? The protective conductor (PE) normally carries very small currents in comparison with normal load currents, but may carry high currents during fault conditions. 


    Z.




    Thanks for the replies so far everyone - just to clarify, when I say remove this cable, its from the design (to save cost) and not an existing link. Essentially saying that the aluminium conductor within the interlinking busbar becomes the earthing and bonding conductor. So what i'm reading so far is that if the aluminium CSA is sufficient size to equal the main earthing conductor it should be okay to use this as both the protective earth and the bonding conductor?


    The arrangement would be main earth into main earth bar from DNO, bond from this earth bar to panel supplying the remote switchboard fed by interlinking busbar, bond from remote panel to local earth bar in the room, with all local bonding to that earth bar. 


    Any other thoughts welcome


    Many thanks


  • 0
    This type of thing perhaps?


    An P.E.  bus bar and perhaps the outer of the system being earthed as well?

    https://www.youtube.com/watch?v=2rX-ITLtFrc


    Z.




  • 0
    If it helps at all, where an installation spans several buildings, it's usual to have a "main earth terminal" for each building (often called a building earth marshalling terminal in subsidurary buildings to distinguish it from the installation's MET) - the idea being that all c.p.c.s and bonding conductors are brought together within each building - so minimising the potential difference between parts within each building.  So there's little need to have separate bonding and earthing conductors between buildings - as they'd be connected together at both ends anyway - the only exception I can think of is where the main cable's c.p.c. (e.g. steel wire armour) isn't sufficient for a bonding conductor and so another G/Y is run alongside - in parallel.


       - Andy.
  • 0
    KC83:

    Essentially saying that the aluminium conductor within the interlinking busbar becomes the earthing and bonding conductor. So what i'm reading so far is that if the aluminium CSA is sufficient size to equal the main earthing conductor it should be okay to use this as both the protective earth and the bonding conductor?


    Generally, but not necessarily. I think that one has to be precise about the nomenclature. If "main earthing conductor" is the earthing conductor of the entire installation, I cannot think of any circumstances when any bonding would be larger than it. So earthing conductor selected i.a.w. Table 54.7 and main protective bonding conductor selected i.a.w. Table 54.8.


    If "main earthing conductor" means the link between the two parts of the building, it could be smaller than the bonding - see Andy's garden shed example above.


    For a combined conductor, just select the bigger of the two.


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