Discuss Main bonding at outbuilding in the Commercial Electrical Advice area at ElectriciansForums.net

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londonlec

Hi all,

I have a commercial premises (school) that requires installation of main bonding at a block which is an outbuilding from the main school.

Now the bonding in the main school is 50mm, however the cable feeding this outbuilding is just a 25mm 4core.

So what size should I be using?!

Many thanks in advance.
 
Assuming 50mm is the minimum as required by the regs for the incoming supply then the answer to your question is 50mm all the way back to the MET.
 
Is the existing set up correct? What is the nature of the supply? TNS/TNCS
 
Assuming your TNCS is PME then the main bonding needs to be selected in accordance with table 54.8 (regulation 544.1.1)
This gives you a bonding conductor size based on the size of the supply neutral, this is the DNO's incoming neutral, not necessarily the size of the consumers tails and definitely not the size of the sub-main feeding the building.
 
Assuming your TNCS is PME then the main bonding needs to be selected in accordance with table 54.8 (regulation 544.1.1)
This gives you a bonding conductor size based on the size of the supply neutral, this is the DNO's incoming neutral, not necessarily the size of the consumers tails and definitely not the size of the sub-main feeding the building.

Many thanks for your reply.

But given the supply cable to said outbuilding is a 25mm SWA I won't have enough CSA in the armouring to make up 50mm (hope this makes sense).
 
That 50mm is copper size not steel.

If you are going to use the armouring as part of the main bond you need to calculate the CSA of steel which would be equivalent to 50mm copper, I think you'll find this comes out at around 425 square mm of steel. And then you need to assess whether the armouring is suitable with regards the heating effect of currents flowing through it as a result of being used as a main bond.

I can strongly recommend a getting a copy of guidance note 8 if you are going to design earthing and bonding systems.
 
It might be worth considering what would be involved in isolating the outbuilding's earthing from the main building and weighing up the cost of this compared to the cost of running 50mm copper all the way back to the MET.
 
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).
 
Why is it potentially unreliable, a correctly designed and installed TT system is no less reliable than its TN equivalent. It is your job to design and install a reliable system.

The other option available is to run 50mm copper back to the MET, If someone thought to leave a draw rope in the duct carrying the SWA then that may not be as difficult as it first sounds.
 
Good point, although I had foolishly made the assumption from the OP that the need for bonding had already been established!
 
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.
 
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.
 
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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.
 
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 .
 

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