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 |
Graham I am not really disagreeing with you, but there is only one very special condition where the armour of SWA should not be connected at both ends and be part of the CPC. This is where it is desired to separate two differing Earthing systems. Everywhere else there is no reason at all not to terminate the armour with the correct brass gland, and make the SWA part of or all the CPC. In answer to the OP the answer to his question is YES. Why there is a reluctance to use the armour as a CPC is simply a case of misunderstood or bad practice. John Peckham wrote an advice paper a few years ago, pointing out that the armour has sufficient cross section to use as the ONLY CPC up to 95mm if I remember correctly, although in many cases an additional G/Y is installed where it is unnecessary. As SWA always has all the current carrying conductors in the same magnetic loop, heating is not a problem.
In the case of this TT isolation for outbuildings, the armour is electrically disconnected, but even you assume that the sheath prevents touching of two independent Earthing systems! My preferred way of separation would be to use a plastic box, a brass gland as usual, but simply not connect the banjo, and ensure that the boot cannot be removed with some self amalgamating tape, thus meeting all the requirements in one go.I would also add a simple warning label pointing out the separation is at the box.
I can feel the need for a new GN, on installation practice for the tricky cases and slightly unusual cables etc. to go with the OSG, it might be quite popular. I have noticed that may electricians not familiar with SWA fit the wrong sized glands, and that makes proper termination difficult as the armour is fanned out far too much and has to be overlong. BW25 gland on 2.5 SWA being common, it should be a BW20S (small one!).
Now the sheath plastic is not conductive, so it fails this test,
So we're relying on something that has no defined insulating properties to prevent contact - so what about BE steel conduit - the paint prevents anyone directly touching the steel - so does that make mean conduit is no longer an exposed-conductive-part? Or back to the flush steel box - is it OK to rely on (possibly damp) plaster to prevent contact?
- Andy.
Ah it is the simplest questions that raise the most problem cases.
In reality of course the SWA outer and the paint on the conduit do greatly reduce scope for a large area shock, so even if nicked or scratched still change a possible fatal shock into an unpleasant but survivable, if vocabulary expanding, experience. But with uncertain performance it is in the same league as an improved floor covering, like using gravel at substations, to drain well and to discourage inadequate footwear choices - does not reduce the shock current to zero, still very much worth doing.
If we did not use the phrase elsewhere, we could have called it additional protection...
It is perfectly possible to terminate SWA, and for that matter other cables with armours or shields, much in the manner of split con, combing the armour wires to a terminal and only clamping to the outer for strain and water sealing, and while it may raise an eyebrow from the purists, it is not electrically dangerous, and can be made mechanically and environmentally superb. (it can also be stuffed up royally, but then so can SWA glands - I have seen a few pull right out at the armour. ) It's not great for EMC as it introduces several tens of nH in the earth loop, but at 50Hz no one cares.
Back to the original post.
If those glands are graunched into a metal plate, ideally with a star washer under the nut, then the linking green wires are not needed - all the glands are already connected.
If the quality control is poor, and there is a risk the panel may be painted, or the nuts only finger tight, then the links may make sense as a redundant path.
Mike.
meanwhile M20 (BW20 BW20s ) and M24 (BW25) finetooth internal star washers ( https://www.accu.co.uk/711-internal-fine-tooth-locking-washers?page=1 ) are worth the investment, as well as the M4s.
If they wanted to make life better, they could tighten the rattle fit tolerance on the nuts AF dimensions at the same time and ditch the banjo for something more like the piranha.
M.
416.1.
We already have basic insulation around the individual wires within the conduit - the question is whether something that stops you directly touching the metal of the conduit is sufficient to stop it being classed as an exposed-conductive-part.
- Andy.
The other point to consider is the conditions prevailing during service, not just at installation time. We use SWA underground because we expect that the cable could be damaged by digging tools - at which point the armour becomes exposed (perhaps as well as the digging tool itself) - whereas for plastic conduit we'd only select it if the risk of damage in the intended environment was negligible.
- Andy,
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