Bit quick advice please

[hightower]

Thanks for your help everyone. On phone at minute so will respond properly when i get on a computer.

 

[Richard Burns]

If you were going to install a rod it would have to be a very good rod connected to the MET of the installation and the cost would be prohibitive unless you have very good soil as the resistance would likely need to be in the region of one ohm. The container should be bonded to the MET as well.
Placing a rod at the containers will reduce potential differences at the container but the rod may take the installation current on an incoming broken neutral.

 

[hightower]

Okay, on a computer so I'll try and put all the relevant details in a single post.

Two containers that are insulated and have refrigeration units, to be installed about 20m away from a factory unit in the factory car park. Supply is TN-C-S so my initial thoughts were stop the supply earth at the isolators and have an RCD to protect with a rodded earth. But then my thoughts were we need to avoid putting it on an RCD so there is less chance of the units tripping out the feed and the stock within going bad.

However, the units being big metal things got me thinking what happens if the DNO loses a neutral. That's why I thought about rodding it alongside the supply earth, but I'm not really sure what that'll achieve. I think the electrical supply is totally separate from the unit, as in Class II, however, is the unit still classed as an extraneous conductive part and therefore needs bonding?

 

[Wilko]

Happy to be shown the error of my ways, but if it's to be an extended TNCS then I'd be inclined to bond the metal container back to main building met.

 

[hightower]

^^^ That's exactly what I'm thinking as well @Wilko

 

[hightower]

But TN-C-S means it's gonna be a 10mm. Don't suppose anyone has a table of SWA and their armour CSAs (in copper equiv.) to see if the armour can be used? Otherwise guess it's a case of getting the calculator out.

 

[hightower]

According to this table:

http://www.askthetrades.co.uk/hosted_images/Armour CSA.pdf

6mm 4-core has a copper equiv. CSA of 16.8mm so my thoughts are I can take a 10mm cable straight from the feed terminated in the isolator and bolt it on to the metal frame, and use the armour as my bonding return to the MET?

 

[Leesparkykent]

According to this table:

http://www.askthetrades.co.uk/hosted_images/Armour CSA.pdf

6mm 4-core has a copper equiv. CSA of 16.8mm so my thoughts are I can take a 10mm cable straight from the feed terminated in the isolator and bolt it on to the metal frame, and use the armour as my bonding return to the MET?

no if another metal is used for bonding then it needs to offer at least equivilant conductance. The carbon content of steel varies so you have to use the guidance in GN8 which I believe is in the region of 8.5. So if 10mm copper is deemed adequate then the steel armour would need to have a cross sectional area of at least 85mm. It would be better to use 5core SWA or run in a seperate bonding conductor. Remember if the seperate bonding conductor is to be buried you would have to use 16mm.

 

[hightower]

So what are the copper equivalent columns in that link referring to then? Reason we've used 4-core is because it's what the factory owner had lying around and he's on a cost saving exercise. Will have to have a look at pulling in a separate conductor.

 

[Leesparkykent]

So what are the copper equivalent columns in that link referring to then? Reason we've used 4-core is because it's what the factory owner had lying around and he's on a cost saving exercise. Will have to have a look at pulling in a separate conductor.

They are for earthing not bonding.