I see it a lot where the SWA isn’t serving as a protective conductor which I know it must still be earthed at one end due to being an exposed conductive part. My question is if multiple SWAs are all earthed at the supply end and meet again at a bit of equipment if we then connect all the SWAs together locally is this still classed as earthing even though they are already earthed at the supply? 
The typical construction of SWA PVC (Polyvinyl Chloride) cable is as follows:
| Conductor: | Class 2 stranded plain copper conductor to BS EN 60228:2005 |
| Insulation: | XLPE (Cross-Linked Polyethylene) |
| Bedding: | PVC (Polyvinyl Chloride) |
| Armouring: | SWA (Steel Wire Armour) |
| Sheath: | PVC (Polyvinyl Chloride) |
| Sheath Colour: | Black (Carbon loaded for UV stability) |
| Voltage Rating: | 600/100V |
The S.W.A. gland at the end of the cable can not become live, as it is earthed. It is connected to the earthed armouring. The origin of the cable must be correctly glanded and earthed. Also it is not exposed as it is covered with an insulating shroud.
The gland is a component part of the armouring which is a circuit protective conductor combination. 543.2.1 (v).
I agree with this sparks' comment.....
nrg: The use of a third core as a cpc in SWA cable isn't an alternative to using the SWA as the earth; it's an additional measure, because the SWA would have to be connected to earth (at the supply end at least) anyway, so that the cable is protected from mechanical damage.
Z.
Yes Graham, but as I point out above, it doesn't matter whether it is insulating or not. The Armour is no more "odd" than any bit of trunking or conduit. I think that your remark probably makes the discussion less clear, and it was already pedantic enough.
What happens to the armour at the end of the cable is more pertinent. A "boot" over the end of SWA glands isn't really a "sheath" - that's where the "exposed-conductive-part" comes in.
I did have a discussion recently regards the general use of stuffing glands for SWA to help prevent the armour being an exposed-conductive-part ... whether that complies with the manufacturers' intent instructions (cable and/or gland) is another question?
"Agreed, I think it's an exposed-conductive-part all the time (for LV use), BUT only a cpc when it's connected to cpc's at both ends of the cable (an exposed-conductive-part is permitted to be a protective conductor)."
I suppose it is semantics, but:
As an exposed-c-p, you must think a live conductor is liable to make it live during a fault and a person is likely to contact it?
A cpc to a Class II item is not connected at both ends; does that mean it ceases to be a cpc protecting the circuit?
Yes Graham, but as I point out above, it doesn't matter whether it is insulating or not. The Armour is no more "odd" than any bit of trunking or conduit. I think that your remark probably makes the discussion less clear, and it was already pedantic enough. Poor OPer. It is actually no different to saying that the sheath of T&E is an exposed conductive part, this is not and never has been the intention! I'd love to see you trying to bond these plastic parts! I am afraid I do not understand why these plastic parts (whatever they are called) are assumed to be conductive, this is just an oddity caused by standards being inaccurate and nonsense. Even SWA when extruded has a test between a water bath (for cooling) and the Armour to make sure there are no defects. The same for all wires, and the standard should say the sheath must be continuous and defect free. Why deny reality? As everyone knows both XLPE and PVC (the sheath materials) are excellent insulators. Are plastic pipes now conductive so may be used as CPCs? Of course not!
I think that Graham and I are interpreting the OP differently.
Granted that a fault in any of the SWA cables could travel both ways and by virtue of the wee bits of G/Y back down the other cables, but if a core is being used as the CPC and there is a fault beyond the termination of the armour, how does that current get into any of the armour?
It is not a requirement that Earth or CPC conductors are sheathed,
David, if the armour is earthed at one end of the cable, and ADS is used, equally easy to argue it's not a cpc, because it's not connected downstream, and cpc's are to be run to each point in wiring, and be part of, or installed adjacent to, the wiring system containing the live conductors.
I presume you are not proposing that the armour may sometimes be left totally unearthed.
Yes, there are occasions when this might happen, BUT not, of course, if the cable is buried, where the armour must be both earthed and suitable to act as a protective conductor.
Therefore when earthed it acts as a CPC for the internal circuit in case of, for example, striking by a spade.
Or an exposed-conductive-part ... BS 7671 is not clear on this. If the armour is earthed at one end only, it's not providing cpc function to the connected equipment or circuit downstream of the cable, and therefore another core (or single core cable) is the cpc ...
How can it be an extraneous-conductive-part? Extraneous-c-ps are not earthed.
Apologies, mis-type, exposed-conductive-part as above. cpc's definitely are earthed in TN, TT and IT systems ... via the MET and the main earthing conductor!
Mr.Jack would be more correct in his thinking of it as an exposed-c-p which has to be earthed so we are back to square one - earthing and a CPC.
Agreed, I think it's an exposed-conductive-part all the time (for LV use), BUT only a cpc when it's connected to cpc's at both ends of the cable (an exposed-conductive-part is permitted to be a protective conductor).
Now the sheath plastic is not conductive, so it fails this test, whether it is "adequate" insulation for 230V (which it is).
It is not ... the sheath, conductive or otherwise, is there for mechanical protection only. It has no defined electrical properties and these are not tested in cable standards.
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