It is important to recall the reasons TT is recommended on farms. The most critical case it is is to do with step voltages for long 'wheelbase' animals like cows and horses, where a shock from the front legs to the back is a serious risk - dairy cattle can be put off from producing milk by small voltages between things like rails touched with a wet nose and with the other  contact to milking machines and so on. In areas with an earthen floor control, also common in stables and riding arenas to  control  the gradients that lead to  step voltages is almost impossible if there is current in the earthing network from diverted neutral currents.

Equally for a farm that is arable, or where there are only livestock that are not especially sensitive to electricity - and generally birds feet are not - after all the stunner electrodes are usually applied to the head or neck, the only risks to consider from TNC-S are just the same ones that would apply to people working in any other industrial setting involving water and a mixture of indoor and outdoor working, perhaps like a garage or workshop or industrial unit.

This may still favour the use of TT over TNC-s but the driver is less significant - and something not being recommended in new designs is very much not the same as 'must be dug up and changed' - especially if there is uncertainty about RCD tripping of life critical air and water- which I presume is the issue with the birds.

When such places are wired TT, there is usually a hierarchical approach where the incomer splits to more than one coarse and time delayed a RCD of perhaps 300mA - more than one so that an alarm can be raised using power from  by one if the other trips, and livestock critical services can be duplicated - loss of half the fans in a barn  of birds is not as bad as loss of all.... Then under these two zones tighter RCD cover such as 30mA instant is only applied to circuits where it is really required and in a way that any one circuit tripping is not a total loss.

I;d not suggest  more than an advice about not to current best practice unless there are other factors you have not mentioned, like a dairy round the back...

Mike.

• Thank you for your reply.  This is a poultry only farm. No other livestock on site.  What you are saying about selectivity of rcds and alarms is already in place.  More than one rcd per shed and good battery back up alarm systems. The farmer is willing to leave it right and will do whatever I advise as he has eicr to do every 3 years.  I had though about keeping tncs on all sub mains and converting each shed to tt. Installing a separate earth rod for each poultry house and isolating the pme earth.That way we don't have 1 upfront rcd but maybe 2/ 3 upfront withing each shed.  If i did this then there is heating pipe work which is metal running from boiler houses that would be on a tncs earthing arrangement and bonded from that earth system running into another shed which would be tt earthing and bonded from that earting system. Would this cause any dangers from different potentials?If the is no apparent danger from the installation as it stands all on tncs then maybe it is best left alone. The is always dangers with tncs in any installation if distributers neutral was ever lost the all exposed conductive parts have the potential to become live.  

Given that the electric supply is essential and losing the supply for more than thirty minutes will result in the deaths of thousands of chickens, I presume there is a generator that will start and maintain the supply if the main RCD trips?

Yes auto standby generator on site

  If i did this then there is heating pipe work which is metal running from boiler houses that would be on a tncs earthing arrangement and bonded from that earth system running into another shed which would be tt earthing and bonded from that earting system.

That sounds to me that any PME potential would be transferred into the TT shed via bonding - completely defeating the purpose of TT'ing the sheds in the first place. You'd have to TT everything that shared extraneous-conductive-parts.

I presume there is a generator that will start and maintain the supply if the main RCD trips?

Wouldn't the generator normally be connected upstream of RCDs? Otherwise ADS is rather compromised if power is re-applied after a safety device has opened in order to disconnect.

   - Andy.

Yes generator is at origin of supply and covers whole site.  

And yes I was thinking there would be different potentials and perhaps leaving it worse than current state.  As it is an exsisting installation would maybe as in earlier comment best to leave all on tncs and make a note that is isn't to current best practice but it doesn't warrant a coding.  

If plumbing and other metallic services are common to the buildings you cannot really separate the earthing zones - as you say, they will end up some indeterminate voltage apart, and bonding removes that but it also restores the TNCs earthing,

Again, if it was being done from new you could specify plastic pipework, but then you could just as easily  specify  a welded length of stud-rod  to the re-bar grid in the concrete floor and keep the TNCs.

I've only ever done it on my own property, but its not hard to do any time before the cement mixer arrives. Actually so long as there is no DPC below it a large area of reinforced concrete in contact with earth is a surprisingly good electrode both for mains and RF (though the tendency to shatter the floor slab precludes the technique for use in lighting systems !)

Mike.

What is the earthing system when the generator is running and supplying the essential services off-grid?

I'm not entirely sure but I I'm guess it is centre tapped. Tncs.  Has it own earth electrode to earth the frame work. 

Never say never, but private generator TNC-s is very rare, I strongly suspect it is really intended as TN-s

i.e. once earth and neutral separate at or near the genset, they stay in separate wires and are never combined again (the 'C' in TNCs) all the way to the loads.

By the way I use  large and small letters to indicate the distribution side and the load side but this is based on the convention from  another poster on this forum and is not a universal recognized notation. 

(So TNSCS-s would be a street with an earth Electrode at the transformer (T), and then some old style separated Earth and Neural cable  (S) for a bit, then a repair in in the road using modern concentric with a combined N and E PEN style,  (C) and then back to the old cable (S), then on the load side of the main fuse separated (s.)

If you seem to have 2 phases, then it may be split phase centre tap, but if it is 3 phases then it will be a star, with the centre  being neutral, and that neutral will be earthed in one place and one place only if it is TN-s

Mike