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.  

• 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.