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Distribution circuits between buildings

Graham Warner
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.

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    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
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  • 0
    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
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