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Topic Title: TNS On farms
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Created On: 02 January 2013 08:50 PM
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 02 January 2013 08:50 PM
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davel81

Posts: 3
Joined: 05 December 2012

Hi

Its that old chessnut of earthing and farms again!!

Can anybody advise / clarify

Can you export TNS around farm buildings, or is it best to TT each building?

I am about to do some work on a farm which is currently TNC-S!! . They are having a wind turbine put in and are thus having transformer put in by the DNO. This will be feeding the complex with a TNS supply (seperate earth supplied from the neutral terminal of the transformer). I have to do some lights etc into one of the buildings where they have exported the TNCS and was going to change this to TT, but if this is going to chage to TNS next week, am I waisting my time??

THANKS
 02 January 2013 09:00 PM
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slittle

Posts: 3550
Joined: 22 November 2007

I've got a couple of TNS Poultry farms. Never had an issue (to date)

What sort of farm is it ?

Stu
 02 January 2013 11:02 PM
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davel81

Posts: 3
Joined: 05 December 2012

HI,
A few pigs, ducks, poultry in the part where i am going to be working. they sublet another part off for cows. I assume that it ok to export the tns (providing the ZS is ok)
 03 January 2013 09:36 AM
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AJJewsbury

Posts: 11539
Joined: 13 August 2003

No problem using true TN-S - it's the best of both worlds. The problem most of us have (on shared public distribution systems) is what's presented as TN-S is often really TN-C-S under the pavement
- Andy.
 03 January 2013 10:18 AM
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tillie

Posts: 789
Joined: 03 May 2006

Hi , I am glad this question has popped up because I am in the near same situation.

Have been asked to price for an installation in an old barn which is to be converted into a workshop ( no animals present ).

It is a metal framed barn about 80 mtrs from the main house.

Not seen the job yet but thinking about all possible scenarios before I see it next week.

If the main supply is Tns would it be recommended to use the supplied earthing terminal.

If the supply is Tncs would it be recommended to TT the barn or use the Tncs earth supplied.

Advice appreciated.
 03 January 2013 10:18 AM
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tillie

Posts: 789
Joined: 03 May 2006

Hi , I am glad this question has popped up because I am in the near same situation.

Have been asked to price for an installation in an old barn which is to be converted into a workshop ( no animals present ).

It is a metal framed barn about 80 mtrs from the main house.

Not seen the job yet but thinking about all possible scenarios before I see it next week.

If the main supply is Tns would it be recommended to use the supplied earthing terminal.

If the supply is Tncs would it be recommended to TT the barn or use the Tncs earth supplied.

Advice appreciated.
 03 January 2013 10:49 AM
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AJJewsbury

Posts: 11539
Joined: 13 August 2003

Have been asked to price for an installation in an old barn which is to be converted into a workshop ( no animals present ).

It is a metal framed barn about 80 mtrs from the main house.

I don't see a fundamental objection to using PME, although I'd ask how well you'll be able to construct an equipotential zone. Walls & roof sound easy - but what about the floor. If it's concrete without a d.p.c. and no buried grid you can connect to (as might be imagined for an old barn), then the floor might well remain obstinately earthy regardless of bonding. If I could imagine class 1 hand tools with large metal work pieces stood on the floor, I'd be very tempted to TT it.

- Andy.
 03 January 2013 11:13 AM
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Parsley

Posts: 1032
Joined: 04 November 2004

UKPN latest earthing guidance document states, all low voltage networks shall be assumed to be PME even if originally constructed using SNE cables. I guess the installation could be fed from it's own TX.

As Andy states concrete floors may introduce an earth potential which may be a problem with animals, The note to 705.415.2.1 states PME isn't advised unless there is an earthed grid laid in the floor.

UKPN's document states that PME may be provided on farms if.

A)There is no extraneous metal work
B)The building electrical installation meets the bonding requirements of BS7671.
C)The building is not occupied by animals or, if it is occupied by animals a bonded earth grid is installed in the floor.

It then goes on to state TT should be used if these conditions can't be met.

Regards
 03 January 2013 03:03 PM
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Delbot321

Posts: 77
Joined: 06 November 2012

I think there needs some clarifcation on TNCS - there are 2 versions of it TNCS(PME) and TMCS (PNB).

As Parsley has posted above the PME version is not likely to be complaint on a farm so it will almost certainly need to be converted to TT.

However using a PNB supply shouldn't be an issue. The supply from a transformer is usually classed as TNCS(PNB) as there will be a NE link in the intake pannel or similar just before the main switch. The criteria used but the supply authority/RECs is if it feeds more than 1 customer and there is a CNE cable then it it is PME.

You will find that alot of larger leisure centres and swiming pools will be supplied directly from the transformer as a single customer this is so that there is no PME into the pool but they don't need to use an RCD for a TT supply which could be a neusance.

The situation you describe for your farm is much the same. Avoid bringing a PME earth into they system and you don't need to convert to TT.

As a side not its worth remembering that TNCS(PME) needs much larger main bonding cables where as TNCS(PNB) need only meet the standard criteria for TNS or TT earthing.
 03 January 2013 06:33 PM
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weirdbeard

Posts: 1562
Joined: 26 September 2011

Originally posted by: davel81

This will be feeding the complex with a TNS supply (seperate earth supplied from the neutral terminal of the transformer). I have to do some lights etc into one of the buildings where they have exported the TNCS and was going to change this to TT, but if this is going to chage to TNS next week, am I waisting my time??



Hi davel81, are you sure it's going to be TN-S, if the suppliers earth terminal is going to be directly connected to the neutral it sounds to me like it will continue to be TN-C-S?

From the consumers side of the installation I don't think it makes much difference if the suppliers neutral/earth link is installed at the cutout position or at the transformer.
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