..and to you.
I would not have thought that one should assume that the outer sheath of buried SWA inevitably remains intact.
Why not? Presumably not for ever but ... Continuity test ? No sheath ?
Rodents, moles, other underground creatures, sharp stones + ground movement (maybe due to plant/tree root growth), other 'environmental' phenomenon and, of course, the possible helping hand of human beings!
That being the case, if the SWA enters a premises, its armour presumably has to be considered 'liable to introduce a potential', and therefore constitutes an extraneous-conductive part.
No sheath ? Are plastic boxes better, after all?
Incomplete/compromised sheath - c.f. a buried plastic box with a crack in it. Once there is any breach of the sheath/box, electrolyte-ridden water will eventually do its work.
The armour will obviously be bonded to the installation's MET to some extent by virtue of the 'earthing' system of the installation.
Oxymoron ?
I was trying exceptionally hard to choose wording that wouldn't stimulate '5 pages'
The CPCs are primarily there to achieve the functionality of 'earthing'. However, they will inevitably also 'bond', albeit via a conductor of inadequate CSA (per regs) for main protective/ equipotential bonding.
However, if the connection between the SWA and CU (or whatever) is in T&E cable, the connection to the MET will be via the T&E's CPC - at worst only 1.5mm² (and unlikely to be more than 4mm² in a domestic setting). Since these connections are inadequate as main equipotential bonding conductors, should additional separate MPB conductors be installed in such situations? (something I haven't heard of being done routinely)
To what would you connect it? Would you remove some of the presumably still intact sheath so that it could become a simultaneously accessible conductive part where it enters the premises? I can see what you are saying, if completely rotted and so in need of replacement, but surely, with intact sheath, the only touchable part would be a glanded metal box which will not be in contact with the ground.
It's not literally the CSA armour which is the issue as far as the house is concerned, but, rather, what that armour is (deliberately) connected to - namely the earthing system of the installation, particularly any parts of the earthing system (and their exposed-conductive-parts) 'electrically closer' to the connection with the SWA armour than to their connection with the MET....
... in practice, I see no real hazard since, even if you accept my argument that the armour should be treated as an extraneous-c-p, the 'bonding' (if I dare call it that!) to the MET via the installation's earthing system should ensure equipotentiality within the premises under nearly all circumstances. However, regs-wise, we would not normally be allowed to rely on a conductor which could be as small as 1.5mm² to achieve that equipotentiality. I suspect that if you had just a fairly short length of water supply pipe within a premises before it changed to plastic for the rest of the house's pipework, you would still feel obliged to main bond it - but I wouldn't expect you to try arguing that, because there was not much of the extraneous-c-p (or things connected to it) to touch, a 1.5mm² main bonding conductor would be adequate, would you?
Kind Regards, John