Earthing or bonding ?

Now we are really getting into trouble! An exposed conductive part is (BS7671): Conductive part of equipment which can be touched, which is normally not live but could become so under fault conditions.

Now the sheath plastic is not conductive, so it fails this test, whether it is "adequate" insulation for 230V (which it is). Now consider metal plumbing which is bonded to the installation MET. Is this an exposed conductive part? No because it is not part of the Electrical installation, although it is connected. SWA is not considered an Exposed Conductive Part.

Now the actual question, which is slightly confused. Are you asking if the SWA armour is an Earthing conductor under your conditions, then yes. If it is only connected one end then No. Even if you put an Earth G/Y cable alongside an SWA, the armour can still act as an Earthing conductor too, the current will share all the parallel conductors depending on the resistance of each.

It is not a requirement that Earth or CPC conductors are sheathed, although they should be marked at the ends if not. The SWA sheath PVC is exactly the same as wire insulation, although it may be thicker, but it is only there to prevent corrosion of the wires. Look at trunking and conduit. As much SWA is 90 degree XLPE the same applies, the sheath is insulating. Whilst on that point, sometimes Insulated and sheathed cables get the name "double insulated" Whilst this is true, the sheath is classed as mechanical protection, because it could be damaged and the double cannot be guaranteed. The same goes for SWA but even if exposed the armour is not dangerous under any normal conditions and the likelihood of contact (buried for example) is effectively zero.

David Stone said:

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.

David Stone said:

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.

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!

David Stone said:

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?

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.

Construction of an SWA Armoured Cable

The typical construction of SWA PVC (Polyvinyl Chloride) cable is as follows:

"Bedding" is NOT a "sheath".

If SWA glands are used, the armour must be earthed at one end. If the armour is "gapped" at the other end, then it's not a cpc, but

If SWA is not buried, nothing in BS 7671 requires the armour to be earthed when carrying SELV or PELV.

But I think we can agree that it would usually be earthed when carrying LV, because there is no insulation over the top of it, and it is therefore an exposed-conductive-part of the equipment (cable is equipment in BS 7671 terms, just as an enclosure is equipment).