Tnc-s on agricultural installations

Question

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 .

lyledunn · Accepted Answer

By the way Adrian, great to have you on the forum. New recruits are always welcome! You will find this forum a very useful sounding board with some of the best in our industry prepared to take time to respond to all sorts of queries in a friendly and measured way.

mapj1 · Answer

that is the best practice in the current standard, and if a new chicken shed was being designed from scratch as a new build, or in any new farm building for that matter, then studs can be welded to the floor slab re-bar, and the TNC-s retained. 
 However, as the thread above, this is pre-exisitng, and actually this not a high risk category like a dairy farm, but actually only involves animals whose feet are closer together than those of humans, so the whole reason for that advice really does not apply. 
 Worse there is a credible risk to animal welfare if the additional RCDs for TT conversion are applied. 
 Ergo, those who can see it, and weigh up the balance of risks in that situation, may or may not decide the TNC s is not an immediate danger, or indeed a realistic potential danger - which sounds like the way it is heading - actually I think it is moving towards 
 'not to current standards but quite satisfactory for continued use and no action required' 
 But of course the caveat is that neither of us have actually seen it.... 
 M.

Sparkingchip · Answer

You really need to have a conversation with a network engineer at the DNO to determine the true nature of the DNO earthing system and also determine the generator earthing system, then ensure the earthing system is suitable for use both on and off grid. 
 If you want to finalise your EICR you could code the earthing arrangement FI- Further Investigation required.

Tnc-s on agricultural installations

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