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Tnc-s on agricultural installations

Adrian G

Hi all.  I am currently doing an Eicr for a large poultry site.  There is 8 broiler sheds.  These are chicken that are specially rared for meat production that we all end up eating.  Now there is a 3 phase 200a supply tnc-s earthing arrangement. The tnc-s earth remains throughout the whole site. Section 705 of bs 7671 and reg 705.415.2.1 at bottom there is note saying that unless a metal grid is laid in the floor, the use of pme earthing facilities as a means of earthing for the electrical installation is not recommended. Looming everyone's thoughts on how you would code this on Eicr. Do u feel it needs changing and converting to tt. Converting to tt brings it own problems with upfront rcd which may give nuisance tripping and power is essential in high density poultry houses to keep ventilation and heating running.  At certain stages of the birds life if power failure for about 30mins can end the lives of thousands of birds .

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  • 0

    Does anyone think that it would be benificial to add an earth electrode to each submains whilst also keeping the tncs earth.

    If by an electrode you're thinking of a 4' rod banged into the ground, then I'd vote no. If you had huge steel frame sheds with each upright sunk feet into the ground then perhaps. To be effective you'd want the product of the electrode resistance and neutral current to be < 50 - e.g. something in the order of 0.25Ω for a 200 amp supply - maybe a bit higher if you were confident the loads would always be reasonably balanced across phases, but you're probably still looking at 'reasonably difficult' territory, Also consider the potential rise of the ground around the electrode(s).

       - Andy.

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  • 0

    Does anyone think that it would be benificial to add an earth electrode to each submains whilst also keeping the tncs earth.

    If by an electrode you're thinking of a 4' rod banged into the ground, then I'd vote no. If you had huge steel frame sheds with each upright sunk feet into the ground then perhaps. To be effective you'd want the product of the electrode resistance and neutral current to be < 50 - e.g. something in the order of 0.25Ω for a 200 amp supply - maybe a bit higher if you were confident the loads would always be reasonably balanced across phases, but you're probably still looking at 'reasonably difficult' territory, Also consider the potential rise of the ground around the electrode(s).

       - Andy.

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  • 0 in reply to AJJewsbury

    Or if there is ever work to replace a concrete floor and there is  accessible rebar, then a weld on tab may be worth considering, to bring things to the modern level, but only if easy, for the  zeroing of any cross-floor step voltages, The benefit is negligible.

    However there should already be at least  one private electrode, to ensure that there is always  some terra-firma reference when on genset, even if the DNO mains cable earth path has been removed to effect a repair.

    Mike

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