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Distribution circuits between buildings

Proposed scenario:  200A 3ph DNO supply to a 15 yr old building, upgraded from the present 100A.  From this building a couple of Distrib ccts, one to a small commercial kitchen / dining room / sports hall / laundry room all in one separate building (they were farm buildings many years ago), another to a further separate building now a modest sized meeting room, with electric showers included within the toilet facility.

DNO supply is as far as can be ascertained, TN-C-S.  Local transformer serving a number of local properties is on a pole about 120m away from "our" pole, from which drops a cable to run underground a few metres to the 'new' building.


Existing supply to the two outbuildings described above is from a separate 100A 3ph DNO feed from the same pole, the service head being in an old barn, thence distrib ccts in SWA.  The Earth from this supply is exported to these two buildings via the armour.  There's also a small earthing conductor to a local earth rod beside this old barn.  It's' proposed to utilise this 100A supply for a new development elsewhere on the site.


Question:  normally we're not permitted to export the earth from TN-C-S in one building to another, but make the remote building TT.  This is fine - except that it's quite possible (but impossible to check) that the water mains to each of the 3 buildings concerned might be in iron pipes.  Thus as soon as the water bond is connected in each building, their main earth terminals would be effectively linked by the mains water pipes, therefore exporting the TN-C-S earth by a different route.  Is this an issue and what could be a solution?  OK, I'm over-thinking again, but the question must be asked.

  • Assuming these buildings are more like dwellings, and contain nothing that makes the use of TNC-s difficult, like a pool, an EV charger or caravan connection point or provision for livestock for examples, then you can distribute the TNC-s  CPC, indeed it may be safer and less confusing if you do, given the problems of the shared metallic services.

    BUT, the export cables must be sized to carry the full potential diverted current, so CPC cross-section same or bigger than the bonds you need for the water pipes or whatever in the building. So if your SWA sub mains are thin, you may wish to export a 10mm2 CPC alongside. Now is also a good moment to ask if any outside taps in TNC-s buildings  have at least one insulating pipe fitting in the line, not actually a reg but a strong recommendation of the ENA.

    If you may have pools, EV charging, animals sheds or whatever, then a 300mA delay RCD at the origin and running the whole site as TT with as much grounded metal as you like may be easier.

    regards Mike
  • I agree there's no general prohibition on "exporting" PME from one building to another - it's just often avoided where the other building isn't suitable for creating an equipotential zone (e.g. outbuildings with damp floors etc) or where there isn't an apatite for the cost of running larger bonding-sized conductors between buildings.


    A TT installation fortuitously picking up a PME earth likewise isn't a great issue unless you're specifically trying to avoid PME (e.g. for some special locations/installations).


    You might need something a bit larger than 10mm² for bonding for a 200A supply though - as all main bonding conductors for PME have to be sized in relation to the DNO's supply N even if they serve outbuildings with smaller submains.


       - Andy.
  • Graham Warner:

    it's quite possible (but impossible to check) that the water mains to each of the 3 buildings concerned might be in iron pipes. 

     


    you could do continuity tests on the pipes with a wander lead to check. 


  • OlympusMons:
    Graham Warner:

    it's quite possible (but impossible to check) that the water mains to each of the 3 buildings concerned might be in iron pipes. 

     


    you could do continuity tests on the pipes with a wander lead to check. 




    Quite so - but the impossibility lies in the elimination of all parallel paths in a safe manner, at least without shutting down the whole site, which is not feasible.

    GW.


  • If I am being naïve, doubtless somebody will say so. Yes, the CPC in a distribution circuit to another building must be at least as big as a MPBC, but does it ever need to be bigger than the earthing conductor of the whole installation?
  • Quite so - but the impossibility lies in the elimination of all parallel paths in a safe manner, at least without shutting down the whole site, which is not feasible.

    In this case (if you really did need to prove you could setup a TT island that was fully independent of PME earths) a positive result would be a lack of continuity - so you actually don't want to eliminate parallel paths for the test.

    Yes, the CPC in a distribution circuit to another building must be at least as big as a MPBC, but does it ever need to be bigger than the earthing conductor of the whole installation?

    The c.p.c. in the distribution circuit only has to be sized for a main bond if there are extraneous-conductive-parts in the other building needing to be main bonded - otherwise it only has to serve as a c.p.c.


    Occasionally an earthing conductor may be smaller than a main bond - not for PME - but for TN-S systems the earthing may be sized according to the supply line conductors which might occasionally be small, or via the adiabatic which may allow smaller still; in TT systems the earthing conductor may be as small as 2.5mm² even though main bonding has a minimum of 6mm².


       - Andy.

  • That rather depends on the current paths. If the CPC finds itself and the shared plumbing form a lower resistance path than the street main and service cable it could carry more than half of the neutral current for the building, and neutral current for any other buildings on the same branch. It is safest, but in most cases overkill  to rate the CPC based on the current rating of the incoming neutral. In the drawing below, building A may end up passing current dependant on the load in building B.
    7ab959d3e9f1a1e790a90e2d7209171f-original-divertedneutral.png
  • Well, unfortunately there is a pool as well - originally part of our property but now belongs to neighbour, having been sold off several years back - but apparently with a contractual obligation that we continue to supply the juice to their changing room/showers/water treatment kit etc.  Changing room in same row of buildings as kitchen/laundry (above) and pool itself open-air, about 4m from the building.

    If, as the helpful comments above (MANY THANKS, fellows) suggest, I need not worry about exporting the PME earth, given correct sizing for Eq-P Bond, would making just that pool room a TT island with 100mA delay RCD at the start of its 32A 3ph distribution circuit be advisable/acceptable?
  • Depends a  bit what else is in the pool room, and if anything  brings in an 'earth' from one of the PME zones. Plastic pipe fittings may be your friends.

    What is the 3 phase load - if it is 3 showers  it may be possible to thin it to one 3 phase+N  30mA RCD and an emergancy light or two.


    Mike.
  • but apparently with a contractual obligation that we continue to supply the juice to their changing room/showers/water treatment kit etc.

    Are you responsible for the electrical installation within "their" buildings or just the supply of electricity thereto? (seems like rather an odd situation in any event). If the latter and they already have a supply with a PME earth, and you're just re-jigging the cables on your side, then I'd be tempted not to worry about the pool's requirements - as that'll be their responsibility - they can create a TT island or not as they prefer.

       - Andy.