Author Topic: SWA Cable and earthing  (Read 521 times)

SWA Cable and earthing
« on: November 04, 2018, 08:13:21 am »
As far as I know, the appropriate way to use SWA for a garden building is twin core SWA, with the sheath acting as earth. Is there any point, benefit or issue with hammering in an earth rod close to the building?

Re: SWA Cable and earthing
« Reply #1 on: November 04, 2018, 09:28:40 am »
I suggest 3 core SWA so that you have the same conductor type for the earth. I don't see any use for an earth spike.

The sheath of SWA can be difficult to get a good connection to.

<rant>You can't buy 3 or 5 core SWA with the right colours for using one as an earth, on single or 3 phase.</rant>
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Re: SWA Cable and earthing
« Reply #2 on: November 04, 2018, 11:08:16 am »
Is the outer allowed to be the earth in building regs?


Re: SWA Cable and earthing
« Reply #3 on: November 04, 2018, 05:33:30 pm »
I'm not a sparky but I think I can answer this.  No, you shouldn't use the sheath of swa for earthing.  You should use 3-core.  The armour should still be earthed at each end though, which it should be if you use the standard gland kit on a metal enclosure.  If it's a plastic box, you should use one of those circular brass thingys at each end and connect it to earth.

I always use the separate earth connection for the armour anyway and connect it to the earth bar, even if it's a metal box.

3-core swa will be brown/black/grey.  You should sleeve the cores as follows when you use 3-core swa for single phase (from tlc-direct):

Core Colour Changes
Brown is old Red (Live)
Black is old Yellow
Grey is old Blue

Single phase wiring
Grey core is Sleeved or Taped with Blue for Neutral

Yellow core is Sleeved or Taped with Green/Yellow for Earth (should this be black ?)

The above cores must be identifiable

Re: SWA Cable and earthing
« Reply #4 on: November 04, 2018, 05:42:25 pm »
Hmmmm the more I investigate the more it seems to indicate what I did 20 years ago was probably the right thing, that is, DON'T connect the armour to the earth in the outbuilding, but use a new earth rod close by.

eg this discussion https://www.electriciansforums.co.uk/threads/swa-2-core-or-3-core.11235/

Although stuff like this https://www.edwardes.co.uk/en/blog/swa-cable-everything-you-need-to-know suggests otherwise

Kim

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Re: SWA Cable and earthing
« Reply #5 on: November 04, 2018, 05:50:36 pm »
I only know enough to know that exporting earths is a bucket of spiders, and the correct approach will depend on the earthing arrangement in the existing building.

A new local earth rod is probably the correct approach if the existing building has a TT earth, but if it's a (more common in the UK) TN system of some kind, I expect that some combination of  a) testing  and  b) maths  will be required to determine the correct approach.

AIUI appropriately marked 3-core SWA is the way to go.
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Re: SWA Cable and earthing
« Reply #6 on: December 30, 2018, 07:53:35 pm »
Steel Wire Armour is normally used as the earth connection. If you're connecting into a plastic terminal box then a brass frying pan on the gland gives you the connection to fix to. You'll need to check the earth loop impedance to make sure that it's low enough to give the required disconnection time. A new version of the Wiring Regulations has just come out so I don't know what the requirements now are.
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Re: SWA Cable and earthing
« Reply #7 on: December 30, 2018, 09:22:14 pm »
FWIW, I went with 6mm 3 core and got a sparkie to connect it up. All my fittings (15, count 'em in two circuits for the ceiling, several in the porch-y bit) worked and passed muster.