Main bonding at outbuilding

[Darkwood]

What services does your outbuilding have and are they coming in in metal, is the outbuilding get any structural steel?

 

[davesparks]

Good point, although I had foolishly made the assumption from the OP that the need for bonding had already been established!

 

[Richard Burns]

I understand the 50mm required is copper.

I have seen a chart which suggests the SWA is equivalent to 33mm of copper.
www.earthingnuts.co.uk/pdf/pvc_xlpe.pdf

I understand what you mean re isolating the TNCS, however I am reluctant to abandon a good earthing system in favour of a potentially unreliable TT system (in London).

That table is for cpc suitability not bonding, different factors apply which is why you need 425mm² of steel to match 50mm² for bonding.
Check they got the 50mm² right at the main building and do as Darkwood says.

 

[Engineer54]

What sort of distance are we talking about for this 25mm 4 core SWA between the main school's intake and the outbuilding??

 

[Jimmyb]

didn't think swa steel could be used for bonding

 

[Darkwood]

If the SWA has the additional capacity to cover the bonding as well as serve the earthing requirements of the submain it can be used (unlikely if your equip' bonding is 50mm), if it can't you need to run the full requirements of the earth bonding as an additional cable and not a csa that only makes up the shortfall. Also you can use structural steel as a bonding path and link other services to it as long as it is suitably sized to meet the copper - steel comparison as well as been structurally sound through out its length to your primary structural bonding point from the MET.

 

[Jimmyb]

I always thought for a tncs/pme the steel couldn't be used, got me think now

 

[Darkwood]

Im not aware why it can't be used if its size and structural integrity are sound then why not? Its a conductive path of suitable low impedance and structurally solid.

 

[davesparks]

Steel armour can be used for bonding on tncs but you have to consider the heating effect of diverted neutral currents flowing through the armour as this will affect the cables ccc .

 

[Engineer54]

Im not aware why it can't be used if its size and structural integrity are sound then why not? Its a conductive path of suitable low impedance and structurally solid.

Let's just say it's one big MET!! ...lol!!