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Discuss agricultural tt earth in the UK Electrical Forum area at ElectriciansForums.net

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Just been to look at a job subing to grain drying firm to help then installing cables, lighting etc. The mains is 3ph in house, across newly tarmac to a small out house were it goes to db, then off to variouse sub dbs. The main swa from house is 16mm 4 core, swa as earth. ( No 300 / 500ma main switch in house.) From here it will run to the main board to the new grain shed. There electrican says put 300ma main switch on db in grain store and make it to tt, (as they havent priced to altet anything) But I thought it should be a source. So I proposed making main swa in to tt system / eart rod / 300 ma, then all of farm protected and wont trip the current farm house. Ah this is a arable farm, no livestock ever present, am I right or totally wrong and should keep my thoughts to my self
 
Correct :)


Where did he say it was tns

Apologies..... I completely misread post #4!! Only three letters so brain scanned it as TN-S, not as typo'd TN-C-S !!!

I'll get my coat..............:dunce2:
 
And in fairness, those tables are still as clear as mud!
 
Hi thanks for all comments will ask electrician on site and decide the best way. I belive he had decided to use the armourings as earth and not tt it, due to no livestock. Just didnt like saying without asking and I now have a gn 7 book to look at to see the regs. Thanks
 
There ain't one. The old boys got muggled up. The armouring of a 16mm2 4 core is sufficient as a cpc and bonding conductor but I always run a separate conductor as don't like to have to rely on a gland. (however by regulations you would be fine not to run a separate conductor- testing permitted and satisfactory results obtained obviously)

So, ....Who's got it ''muggled up'' ??

TNC-S will need the armouring to be a minimum CSA of 80mm....
 
Hi thanks for all comments will ask electrician on site and decide the best way. I belive he had decided to use the armourings as earth and not tt it, due to no livestock. Just didnt like saying without asking and I now have a gn 7 book to look at to see the regs. Thanks

As already advised you will need to pull in a 10mm separate bonding conductor, the armouring on that 4 core SWA does not comply for extending a PME equipotential Zone.
 
The csa comparisons for earthing and bonding are different.

Generally the tables you find such as that posted by Engineer54 are for assessing the compliance of a steel conductor (the armour) for use as a circuit protective conductor. in which case you are using kCu/kSt as the conversion factor, (with the associated conditions of the conductors and the csa limits).

When considering steel as a bonding conductor then you are only considering the comparative conductance of the different metals and the copper to steel resistivity ratio is 1 to 8.5 (1 to 1.68 for Cu/Al).
 
Which equates to, ...you can't use the armouring of a SWA cable as a combined CPC/Bonding conductor unless the steel has a suitable CSA to match/exceed the conductance of the minimum required copper conductor. ...In this case, 10mm.
 

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