Earthing a metal shed

[roukel01]

Morning Folks.

Doing some work at a customers house and they've bought one of these Metal Sheds from B&Q. He wants power to it. The plan is to run a 4mm SWA to it for a socket and lighting. With the shed being metal, I'm going to earth it, however, should it be a 10mm earth back to the board, or can I get away with earthing it via the swa?

Many Thanks.

 

[chrish_1977]

Depends how far the shed is from the building, might be easier just to put an earth stake in.

 

[roukel01]

its no more than 5 meters away

 

[Farmelectrics]

then the armour would be ok

 

[Farmelectrics]

plus an internal earth on the swa sockets will be fixed to shed anyway which is were your swa will be glanded into so the shed will be well earthed

 

[Steve_P]

Watch what fittings you put in the shed, even though they keep the rain out, they sweat more than a fat plumber and get full of condensation.

 

[chocca]

they sweat more than a fat plumber and get full of condensation.

Oh dear GOD! Totally unexpected, never heard it before and so funny I nearly blew my entire brew down my nose. Thanks for that...

 

[roukel01]

Watch what fittings you put in the shed, even though they keep the rain out, they sweat more than a fat plumber and get full of condensation.

Haha, yeah we noticed the condensation. Everything installed is weatherproof!
Went to have a look this morning, there are Icicles hanging from the roof!

 

[malcolmsanford]

For an armour to be suitable as a bonding conductor you will need to use the 8 times rule guys, and that means your SWA must be equivalent to 80mm in this case

I would just have a check to see if your 4mm armoured is that ............

Not sure what distance as got to do with it though

 

[Engineer54]

Though i hate to bring it up, is your house supply PME/TNC-S?? I'd also check if the steel shed is earthy in it's own right too (depends if it has a concrete base that the sheds been bolted too)!! So maybe an IR test to see if it's over 23K ohms or under !! If the answer to both is Yes, ...then you'll need that 10mm bonding connection....

 

[Deleted member 26818]

Yes 10mm² bonding or TT the shed.

 

[brman]

There is quite a good little article on this, including SWA armour equivelent sizes, at SWA as a CPC

I've not read it fully but it appears to make sense......

 

[Deleted member 26818]

That's a paper written by John Peckham, very useful
It shows which SWA cables can be used with the armour as a CPC.
Unfortunately, it does not show any SWA cables where the armour can be used as a bonding conductor.

 

[brman]

That's a paper written by John Peckham, very useful
It shows which SWA cables can be used with the armour as a CPC.
Unfortunately, it does not show any SWA cables where the armour can be used as a bonding conductor.

I am obviously missing something. This guide does say

3. If the SWA is used as a main equipotential conductor other than on a PME system it must have a copper equivalent CSA, for CSAs over 25mm, of not less than half the cross sectional area of the associated earthing conductor and not less than
6mm2 to comply with 544.1.1. If the installation is PME then the copper equivalent CSA of the SWA must comply with table 54.8.

Apart from the typo (I assume ", for CSAs over 25mm, " should read "under 25mm") that implies that the swa can be used for bonding. Any SWA normally used in domestic has a copper equivelant CSA of >10mm2.
What am I missing?

 

[Deleted member 26818]

No the ratio of conductivity between copper an steel is about 8:1.
Copper is eight times more conductive than steel.
As such for the SWA to have copper equivelance, the CSA of the armour would have to be in the region of 80mm².
With 2core 70° C PVC cable the minimum CSA of the conductors would be 70mm², 3core, 50mm², 4core, 25mm².
Of course, if you are using conductors with those CSAs, the chances are that you would require a larger than 10mm² bonding conductor.