agricultural tt earth

[Simon-0116]

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

 

[Rockingit]

Right - at the risk of an amount of ridicule, I'm now confused. I'm either had too many ciders in the sunshine or am just not reading those tables right: We've got 16mm conductors, which means we need a cpc of 1/2 16 = 8mm. We've got TN-S so we need to satisfy 544.1 and therefore need to be somewhere between 6mm and 25mm for a bonding conductor. That table says we've got a csa eq of 49mm, yes? So where's the problem?!

 

[atm84]

Right - at the risk of an amount of ridicule, I'm now confused. I'm either had too many ciders in the sunshine or am just not reading those tables right: We've got 16mm conductors, which means we need a cpc of 1/2 16 = 8mm. We've got TN-S so we need to satisfy 544.1 and therefore need to be somewhere between 6mm and 25mm for a bonding conductor. That table says we've got a csa eq of 49mm, yes? So where's the problem?!

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)

 

[GLENNSPARK]

I won’t bother e-mailing stuff to you in future Glenn, you don’t look at it.
You’ve had that table for well over a year.

yeah....well if your gonna be like that you miserable old goat...

 

[jamie-spark]

Right - at the risk of an amount of ridicule, I'm now confused. I'm either had too many ciders in the sunshine or am just not reading those tables right: We've got 16mm conductors, which means we need a cpc of 1/2 16 = 8mm. We've got TN-S so we need to satisfy 544.1 and therefore need to be somewhere between 6mm and 25mm for a bonding conductor. That table says we've got a csa eq of 49mm, yes? So where's the problem?!

The problem is if there is extraneous conductive parts in the grain shed . Then it requires to be the copper equivalent. So steel 8/1

 

[jamie-spark]

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 )

Are you 100% sure ?
Have a look at the table above

 

[Rockingit]

The problem is if there is extraneous conductive parts in the grain shed . Then it requires to be the copper equivalent. So steel 8/1

View attachment 24726

That's what I mean about not being clear on exactly WHICH csa is being used in the tables - because if some of them don't comply then that suggests they have already been converted steel > copper. And if we've got STEEL at 49, then we've only got a Cu eq of 6mm (which goes back to my original remark about using the adiabatic to see if it complies as earthing conductor or not).

 

[Rockingit]

Are you 100% sure ?
Have a look at the table above

That's for TN-C-S, not TN-S

 

[atm84]

Never 100% certain. Ill whack it into the havacomp and see what that suggests.

 

[jamie-spark]

That's for TN-C-S, not TN-S

Correct


Where did he say it was tns

 

[atm84]

The problem is if there is extraneous conductive parts in the grain shed . Then it requires to be the copper equivalent. So steel 8/1

View attachment 24726

That's what I mean about not being clear on exactly WHICH csa is being used in the tables - because if some of them don't comply then that suggests they have already been converted steel > copper. And if we've got STEEL at 49, then we've only got a Cu eq of 6mm (which goes back to my original remark about using the adiabatic to see if it complies as earthing conductor or not).

They are converted values!

 

[jamie-spark]

They are the actual armouring CSA sizes

 

[jamie-spark]

That's for TN-C-S, not TN-S

Where does the op say tns mate

 

[Rockingit]

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:

 

[jamie-spark]

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:

Lol. We all go on auto pilot. To be fair it may be a typo on both cases

 

[Rockingit]

And in fairness, those tables are still as clear as mud!

 

[jamie-spark]

I don't get what u don't get if you know what I mean

 

[Simon-0116]

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

 

[Engineer54]

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....

 

[Engineer54]

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.

 

[Richard Burns]

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).