What size SWA for main 80A supply?

[JohnW2]

For the sake of a few extra ££ it's not worth buying the "better" parts imho.

Do you perhaps mean ...
"...it's not worth not buying the "better" parts ....."
?

Kind Regards, John

 

[d000hg]

It's obvious a matter of a risk assesment and deciding 'where to draw the line'.

Rally and racing drivers do, of course, where crash helmets, in addition to all the other safety factors built into their cars.

In terms of ordinary cars/drivers, you might argue that it is "taking things too far" to have seat belts AND air bags AND crumple zones AND collapsible steering columns/wheels etc etc. - but I don't think many people would argue that! Perhaps more relevant to the context of this discussion, if it were known that there was a significant risk that, under certain circumstances, one of those safety features might 'fail', that would be an argument for having others (as 'belt and braces').

I think your premise may not be totally correct. I would say that the armour of SWA is "designed" to provide mechanical protection and also an earthed sheath that will facilitate operation of a protective device in certain situations of cable 'penetration'. It so happens that the armour is usually (but not always - see our wiki) electrically adequate to be also used as the cable's 'earth' conductor - but I would not personally say that it was "designed" to have that role.

However, as has been said, you are right that the armour alone usually is adequate, and compliant with regs, and that a good few people believe that is the appropriate and 'correct' way to do things. However, as I said earlier, the only advantages of doing it are slight benefits in terms of cost and convenience - there is no electrical reason for not having an 'earth' core.

Kind Regards, John
Regards, John

Good points John, thanks. I suppose not every SWA necessarily needs to carry an earth at all, or am I mistaken there? e.g. in my case I could earth the cabin where the cable comes into the building?
And if I was using a small SWA for outside lighting, likely there would be no need for an earth at all?

 

[JohnW2]

... deleted - present 'send' too soon!

 

[JohnW2]

Good points John, thanks. I suppose not every SWA necessarily needs to carry an earth at all, or am I mistaken there?

It doesn't necessarily have to 'carry an earth' to anywhere - but the armour has to be earthed (at one end or the other) in order for it to work as SWA (in the event of penetration).

e.g. in my case I could earth the cabin where the cable comes into the building?

I'm not totally clear as to what you mean. However, a not uncommon arrangement with outbuildings is to earth the SWA armour at the house end, then to be very careful not to connect that armour to anything at the outbuilding end but, instead, derive the earth for the outbuilding entirely from a local earth rod.

And if I was using a small SWA for outside lighting, likely there would be no need for an earth at all?

The lights may well not need an earth but (a) as above, if the SWA were being used as SWA, the armour would have to be earthed (presumably at the house end) and (b) the regs theoretically require a CPC ('earth') to be run to every point in an installation (e.g. a light fitting), even if it is not currently required - presumably in case what is at the end of the cable is subsequently changed to something which does require an earth.

Kind Regards, John