This discussion is locked.

You cannot post a reply to this discussion. If you have a question start a new discussion

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 AJJewsbury over 3 years ago

The barn has 22 steel support posts, so if a high level flood light became faulty wouldn't the 240 Volts to earth be divided approximately by 22 at each post.
The current would be divided rather than the voltage. To divide the voltage the posts would have to be in series rather than in parallel. Check my maths, but as I see it:
Let's say each post has a resistance to Earth of 100Ω, and the supply has an impedance of 0.1Ω on the L side.
22 lots of 100Ω in parallel (1/R = 22x 1/100Ω) gives a total resistance to Earth of about 4.5Ω (and presume the bonding conductors connecting the posts together have a negligible impedance). Adding in 0.1Ω for the L side gives a loop impednace of 4.6Ω. For Uo of 230V that's then an Earth Fault current of 230V/4.6Ω = 50A.
If all the posts have the same resistance then the the current is split equally between all 22 posts, so 50A/22 = 2.27A through each post.
Thus each post, by Ohm's Law, will develop a voltage of 2.27A x 100Ω = 227V. Less than 230V … but not by much.
Rework with other arbitary numbers if you like (e.g. 20Ω at each post gives 207V for me). The big issue is the low impedance on the L side - it's really difficult to match that using electrodes in soil, even if you're using loads of them in parallel.
- Andy.
Cancel
Vote Up 0 Vote Down

Cancel

Reply

0 AJJewsbury over 3 years ago

The barn has 22 steel support posts, so if a high level flood light became faulty wouldn't the 240 Volts to earth be divided approximately by 22 at each post.
The current would be divided rather than the voltage. To divide the voltage the posts would have to be in series rather than in parallel. Check my maths, but as I see it:
Let's say each post has a resistance to Earth of 100Ω, and the supply has an impedance of 0.1Ω on the L side.
22 lots of 100Ω in parallel (1/R = 22x 1/100Ω) gives a total resistance to Earth of about 4.5Ω (and presume the bonding conductors connecting the posts together have a negligible impedance). Adding in 0.1Ω for the L side gives a loop impednace of 4.6Ω. For Uo of 230V that's then an Earth Fault current of 230V/4.6Ω = 50A.
If all the posts have the same resistance then the the current is split equally between all 22 posts, so 50A/22 = 2.27A through each post.
Thus each post, by Ohm's Law, will develop a voltage of 2.27A x 100Ω = 227V. Less than 230V … but not by much.
Rework with other arbitary numbers if you like (e.g. 20Ω at each post gives 207V for me). The big issue is the low impedance on the L side - it's really difficult to match that using electrodes in soil, even if you're using loads of them in parallel.
- Andy.
Cancel
Vote Up 0 Vote Down

Cancel

Children

No Data

Farm TT quandary

Community Rules & Guidelines