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Farm TT quandary

I have been asked to replace the existing damaged T&E switched supply to a light in a steel framed barn on a farm and extend the switched supply to two adjoining steel frame barns and put a flood light in each.

The light supply and switch are in a nearby old brick built building, the main supply is three phase PME without any RCD protection and the barns are used to house cattle (approx 100) . The steel support posts (22) are set in concrete in holes dug into sandstone. The barn floors are concrete on sandstone and will have straw bedding on top which gets wet.

I am going to stick an RCD in the supply to the lights. TT ing the farm is not an option.

My quandary is what is the least risk option:

1- Ignore BS7671 and keep the metalwork isolated from the supply earth as at present due to the risk of step voltage in the event of a lost neutral. Recently a DNO contractor did manage to loose a phase while working on a supply pole, but the barn metalwork will be extraneous so not a compliant solution,

2- Treat the metalwork (22 support posts) as the TT earth with the risk of step voltage around the posts until the RCD trips;

3- TT the barn lighting circuit with a separate earth. It will very very difficult to get a Ra lower than the barn supports due to the sandstone around most of the farm so potential for step voltages again and problem of finding an accessible place away from animals;

4 Just bond everything to the PME earth, hope the number of posts reduces the step voltages around each to a low level and accept the risks, or

4 - Something else I have not thought of ?

I would normally use SWA and there is a 8 metre catenary involved, can anyone recommend a better alternative as it will be close to 30M across three barns. Even though I will be in a cage on a tele handler I am not keen on trying to install SWA along the roof beams over 20ft up if there is an easier option, I will be using girder clips to secure the cable.

Parents

0 Zoomup over 3 years ago

kfh:

Thank you for all your comments, option 2 seems to be the favourite. I will see if the farmer will splash out on two RCDs. The explanation of voltages on earthed metal, series/parallel resistances, is obvious when someone points it out?, my old aching remaining grey matter takes a long time to get up to speed these days. It looks like I will also be struggling with SWA at height and getting the BBB out for TT earth conductor sizes and working out whether I can use the armour as an earth conductor and the CPC or will I need three cores? Given the small cost difference it will probably be three but I have not priced SWA in the last few months?.
On all the other issues raised:
I did TT a different metal barn on the same farm a few years ago using the posts as earth, no livestock, and the Ze at the barn was less than 20 Ohm on 8 bonded posts.
It was a dairy farm for many years but the farmer gave up on dairy a few years ago as it was not profitable and he was effectively paying to produce milk. Also the company who bought the milk would reject a days production with made up reasons very now and again. He now takes holidays as he is no longer tied to the farm 365 days a year.
Interestingly there was no grid in the floor of the milking parlour, the supply was PME and I did not see any Green or even G/Y bonding to the stall metalwork anywhere. The concrete was probably laid on the sandstone with no DPC and would always be wet form the washing down twice a day.
The livestock may have insulating hooves when dry. But the bedding becomes wet and soiled within a day of being laid so for part of the time they are on wet bedding and have wet hooves. With the salts etc in urine I suspect they will be very conducting.
The farmer is a tenant and the landlord is responsible for the electrics in the rented buildings but the farmer is responsible for the buildings he has constructed and any additions he requires. The farm electrics had an unsatisfactory EICR last year. I am told the inspector was happy with the bits I had installed?. The farmer has not heard about what work or when any rectification work will be done .
Kevin
Thanks for the update Kevin. Don't forget to strap yourself to the cage when working at high level.
Z.
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0 Zoomup over 3 years ago

kfh:

Thank you for all your comments, option 2 seems to be the favourite. I will see if the farmer will splash out on two RCDs. The explanation of voltages on earthed metal, series/parallel resistances, is obvious when someone points it out?, my old aching remaining grey matter takes a long time to get up to speed these days. It looks like I will also be struggling with SWA at height and getting the BBB out for TT earth conductor sizes and working out whether I can use the armour as an earth conductor and the CPC or will I need three cores? Given the small cost difference it will probably be three but I have not priced SWA in the last few months?.
On all the other issues raised:
I did TT a different metal barn on the same farm a few years ago using the posts as earth, no livestock, and the Ze at the barn was less than 20 Ohm on 8 bonded posts.
It was a dairy farm for many years but the farmer gave up on dairy a few years ago as it was not profitable and he was effectively paying to produce milk. Also the company who bought the milk would reject a days production with made up reasons very now and again. He now takes holidays as he is no longer tied to the farm 365 days a year.
Interestingly there was no grid in the floor of the milking parlour, the supply was PME and I did not see any Green or even G/Y bonding to the stall metalwork anywhere. The concrete was probably laid on the sandstone with no DPC and would always be wet form the washing down twice a day.
The livestock may have insulating hooves when dry. But the bedding becomes wet and soiled within a day of being laid so for part of the time they are on wet bedding and have wet hooves. With the salts etc in urine I suspect they will be very conducting.
The farmer is a tenant and the landlord is responsible for the electrics in the rented buildings but the farmer is responsible for the buildings he has constructed and any additions he requires. The farm electrics had an unsatisfactory EICR last year. I am told the inspector was happy with the bits I had installed?. The farmer has not heard about what work or when any rectification work will be done .
Kevin
Thanks for the update Kevin. Don't forget to strap yourself to the cage when working at high level.
Z.
Cancel
Vote Up 0 Vote Down

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Farm TT quandary

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