Why is it....

[unseen]

Flex comes with all 3 cores insulated, but Twin+Earth and it's brothers all come sans insulation on the earth?

Sorry if it's a stupid/daft/dumb question, just had one of those "why don't they...." moments whilst I was fitting the wall lights last week. And if you don't ask you'll never learn (So I kept getting told!)...

Cheers

Rob (having a tea total new year... Thanks dear for working NYE/NYD *sigh*)

 

[wotan]

The answer is in the question...flex has all three conductors insulated because it's flexible...Twin and earth is used in none movement situations, and has solid conductors, so it does not need to be flexible.

Wotan

 

[ban-all-sheds]

SWA tends not to move around much either

So we'd better look for a different explanation...

 

[davelx]

SWA tends not to move around much either

So we'd better look for a different explanation...

..which may, or may not, have something to do with the fact that in T&E the Earth lies directly between the two live conductors, whereas in flex and SWA they are arranged in a triangle. Think about what happens if the cable or flex gets damaged. (We're leading the student to help him work out the answer for himself )

 

[GaryMo]

Cost and probably the same reason that the CPC is smaller than the live conductors.

 

[martinxxxxxx]

This has been asked before here.

Basically they are saying, if you hit a nail through, you are likely to hit the cpc as well as a line conductor, thereby activating the protective device.

Seems reasonable and explains the swa mismatch.

Martin

 

[bernardgreen]

If the insulation on the Live of a fixed cable is damaged in damp conditions then the damp area becomes Live. With a bare CPC conductor there is a better chance of the damp area already being in contact with the CPC when the fault occurs and thus a better chance of the RCD being operated with the leakage to the CPC than through a person.

On the other hand a bare CPC in damp conditions will corrode faster than the protected Live and Neutral conductors. So do not run such cable in damp areas.

 

[Taylortwocities]

In flex cables, all cores are stranded. In SWA, all concutors are also stranded. My guess is the cable standards require all conductors for stranded have to be insulated.

In BS7671 all live conductors must be double insulated. The makers don't sleeve cpc because they do not have to = lower cost.

This is different in some countries. I believe that in Australia the earth conductor is sleeved.

 

[GaryMo]

In BS7671 all live conductors must be double insulated. The makers don't sleeve cpc because they do not have to = lower cost.

I was lead to beleive that t&e wasn't DI and that the outer sheath was mechanical protection only.

That being, it's a case of double insulated or single insulated combined with mechanical protection - singles in conduit for example.

 

[ColJack]

SWA tends not to move around much either

So we'd better look for a different explanation...

and SWA doesn't come with an earth core BAS.. the SWA itself is used unless you sleeve one of the other conductors..
and unless you buy different SWA to me, the SWA itself isn't sheathed is it..?

 

[GaryMo]

You can get 5 core SWA which comes with cores coloured as green/yellow, blue, brown, black and grey.

 

[holmslaw]

..

 

[Lectrician]

Thats it, it's just cost cost cost.

 

[ColJack]

I've seen SY in 5 core, but never SWA...
SWA is usually 2 core , 3 core, 4 core, 7 core and 11 core...

 

[GaryMo]

I've used 5 core SWA.

http://www.eland.co.uk/electrical-c...able61/steel-wire-armoured-cable-swa-pvc.html