SWA from supply to C.U questions

[ZEDEZ]

Up until now every installation I've worked on has involved the C.U being within 3m of the supply.

The job I'm working on at the moment is a domestic rewire that has had the supply moved to a box on the outside of the property, and the new C.U will be in a utility room about 4 - 5m away from the supply.

The supply is TN-S with a BS 1361 100 a main fuse fitted. What I'm planning on doing is installing a metal clad 80 A switch fuse at the supply. The customer has already had a 2 core 25mm SWA cable run from the supply box to the new C.U location.

No separate Main Earth has been run with the SWA to the new C.U position though which is my main concern - though after doing some research it seems to be indicated that using the steel armour of the SWA as the main earth should be fine so long as it's PVC SWA (PDF linked below has a useful table):

http://www.earthingnuts.co.uk/pdf/pvc_xlpe.pdf

If anyone here has experience with this and could confirm either way that'd be great.

If it is compliant, my next question would be how to correctly terminate either end of the SWA to the MET and C.U Earthing terminal - I'm thinking of using a banjo along with a couple of short lengths of 16mm.

EZ.

 

[yellowvanman]

Generally the SWA should be fine, but you can confirm that with the adiabatic equation. I think the times when the SWA is not enough is when you're using much much bigger conductors.

The link you gave is for a site that supplies earthing nuts, I always use them nowadays, makes life much easier.

 

[davesparks]

Use the adiabatic equation to confirm that the armour is ok to use as the cpc.
It will not be suitable to form part of the main bonding of a PME supply so you will likely need to run the bonding to the MET instead of the cu earth bar.
Terminate the SWA exactly the same as you always would with a suitable gland and earthing connection (banjo, earth nut etc)

 

[telectrix]

what he said ^^^^^. i usually fit a small adaptable box below the SW-F and 1 below the CU. fit a coupler and bushes between box and unit, run tails through and add a bit of GN/Y of appropriate CSA to the banjo (or earth nut).

 

[uksparks]

I've got a very similar issue with an extension, I'm moving the CU to the revers side of a wall and about 2m along, I think without measuring it wi be under 4m but over 3m.

How much do you stick to the 3m rule, it seems redicilous to put a switch in for the sake of 1m, it's all visible and you g to be clipped along a garage wall.

 

[telectrix]

I've got a very similar issue with an extension, I'm moving the CU to the revers side of a wall and about 2m along, I think without measuring it wi be under 4m but over 3m.

How much do you stick to the 3m rule, it seems redicilous to put a switch in for the sake of 1m, it's all visible and you g to be clipped along a garage wall.

think yourself lucky that the tails will be surface so you don;t need to RCD protect them as if they were buried. bung a 80A SW_F in and use 16mm tails.

 

[uksparks]

I'm going back this week to start mounting the new board etc do will have a good measure, I say if it's under 3.5m I'll just put new ones in and not bother with a fuse as I feel it's pointless, anything over and I will but there's got to be an element of common sense, it makes no sense adding additional joints for really no benefit.

 

[davesparks]

Which DNO is it and what do they say on the matter?

I was surprised when I last had a meter moved by UKPN when their paperwork stated that they allow tails up to 4 metres long

 

[uksparks]

It's WPD, I've not spoken to them but will give them a buzz about it.

 

[Deleted member 9648]

It's WPD, I've not spoken to them but will give them a buzz about it.

Good luck with that.

"I'm afraid your question does not come up on my computer Sir, I'm Maxine...is there any thing else I can help you with?"

 

[ZEDEZ]

Thanks YVM!

 

[ZEDEZ]

Thanks Telectrix, I think this is the way I'll go !

EZ

 

[ZEDEZ]

Thanks for the advice Davesparks.

Curious - why wouldn't the armour be suitable to form part of the main bonding of a PME supply?

Regards
EZ

 

[Deleted member 26818]

Because the conductivity of steel, is about one ninth, that of copper.

 

[davesparks]

Thanks for the advice Davesparks.

Curious - why wouldn't the armour be suitable to form part of the main bonding of a PME supply?

Regards
EZ

It may not be suitable as a bond for other types of supply too, but with PME it is pretty much guaranteed.
As to why, it quite simply won't be big enough.
While the size of a steel cpc is calculated using the adiabatic equation the size of a steel bonding conductor needs to be of a size which affords equivalent conductance to the required copper bond, Cu being 8x more conductive than steel. So if you need a 10mm Cu main bond then you need an 80mm steel main bond.

 

[Deleted member 26818]

Would not be happy with a 1:8 ratio.

 

[davesparks]

Would not be happy with a 1:8 ratio.

Yeah, just checked the book, it's 8.5 according to GN8