earthing the armour on SWA

[ban-all-sheds]

Yes, if I understand your point correctly, then I agree. If one installs SWA when one doesn't actually need SWA (which might be the case, for example, with an outdoor run entirely clipped to a wall), then there is theoretically no need to earth the armour (although I imagine that most people would still earth it.

I think most people would. I would, if it could be done.

But if there is no need then that is a circumstance in which it would be appropriate to not do it.

 

[JohnW2]

Yes, if I understand your point correctly, then I agree. If one installs SWA when one doesn't actually need SWA (which might be the case, for example, with an outdoor run entirely clipped to a wall), then there is theoretically no need to earth the armour (although I imagine that most people would still earth it.

I think most people would. I would, if it could be done. ... But if there is no need then that is a circumstance in which it would be appropriate to not do it.

Yes, I agree - at least that there would be no regulatory need to earth the armour if there was no regulatory need for the armour to be there in the first place. However, we seem to agree that most of us probably would earth it - just as we probably would usually earth a metal enclosure even if all the live conductors within it were protected by two layers of 'insulation'.

Kind Regards, John

 

[ricicle]

Of course the armour would not be connected as such at the load end if we were creating a separate TT install. But the armour obviously still needs earthing at the supply end.

 

[EFLImpudence]

I think you have missed the point.

The question was "Under what circumstances would it be appropriate to NOT earth the armour on SWA cable?"

BAS has said "when the armour isn't necessary".

 

[JohnW2]

Of course the armour would not be connected as such at the load end if we were creating a separate TT install. But the armour obviously still needs earthing at the supply end.

Indeed - provided the cable needed to be SWA. However, BAS has chosen to raise the (very unusual) situation of SWA being used where SWA was not actually required. In that situation, provided that the cable has an earth core, I think he's right that there is technically no requirement that the armour should be earthed - just as there is technically no requirement for a metal conduit to be earthed if it only contains DI (insulated+sheathed) cables. However, as has been said, most of us would want to earth the metal in all these situations, whether there was a'requirement' or not.

Kind Regards, John

 

[ban-all-sheds]

However, BAS has chosen to raise the (very unusual) situation of SWA being used where SWA was not actually required.

Well - it's not so much that I chose to raise it, more it being the answer to "Under what circumstances would it be appropriate to NOT earth the armour on SWA cable? ".

provided that the cable has an earth core

I did miss that as also being necessary - remiss of me.

 

[JohnW2]

However, BAS has chosen to raise the (very unusual) situation of SWA being used where SWA was not actually required.

Well - it's not so much that I chose to raise it, more it being the answer to "Under what circumstances would it be appropriate to NOT earth the armour on SWA cable? ".

Fair enough - perhaps I should have said that you 'chose to give the answer'. No-one forced you to Mind you, I think most of us would feel it 'desirable' to earth it, even if not actually required by regs, so one might say that this made it 'appropriate' - so it could be argued that, in your (and many others') opinion, it is never particularly 'appropriate' to not earth the armour!

provided that the cable has an earth core

I did miss that as also being necessary - remiss of me.

You may be being unnecessarily hard on yourself! As we've discussed before, much as one might expect them to (in case of penetration etc.) the regs do not seem to demand that every cable has a CPC, the nearest seemingly being the requirement that 'every point' in an installation be provided by a CPC. Hence, if the place to where the SWA was going was provided with some other CPC, then it would seemingly not be essential, reg-wise, that the SWA (without earthed armour, because it didn't need to be SWA) had an 'earth core'. Again, however, I think that most of us would want the SWA to include a CPC (armour and/or core), whether 'required' or not.

Kind Regards, John

 

[ban-all-sheds]

Fair enough - perhaps I should have said that you 'chose to give the answer'.

I gave the correct answer.

Why would I (knowingly) give an incorrect one?

There was no choice involved, apart from the one about not lying.

 

[JohnW2]

Fair enough - perhaps I should have said that you 'chose to give the answer'.

I gave the correct answer. Why would I (knowingly) give an incorrect one? There was no choice involved, apart from the one about not lying.

This is all very pedantic - but, yes there was ("choice involved"). No-one is suggesting that you didn't give the correct answer, or that you should have (knowingly) given an incorrect answer. However, there was "choice involved" in your deciding to give a (correct) answer - as I said before, no-one forced you to answer!

Kind Regards, John

 

[Spark123]

The British Standards for SWA say in the informative part that the armour should be earthed at the supply end.
You could maybe argue that the armour isn't exposed but I'll argue that the glands that it connects to are hence they need to be earthed (when using ADS as the method of protection).

 

[ericmark]

Because parallel cables will always have some inductance or capacitance any unused cores should be earthed. OK I know often we don't earth unused cables with low voltage but really we should. So with any supply which is tied to earth unused conductors should be earthed.

The SWA should be earthed even if the supply is PME and the destination is TT. The question is earth to what?

Even using SWA for a SELV supply it should be earthed but again to what?

Sorry I don't have the answer. I would earth to supply earth but not sure if that is correct or not. Aerial cables are more of a problem as who knows what the voltage will be. I remember a crane in fact at the time the tallest free standing in Europe as Sizewell 'B' power station and the base was well and truly earthed. However in high winds we got inch long sparks from the hook to earthed re-bar. All joints were bolted there was no way that hook was not earthed by the crane structure and cables to earth.

So suppose we should ask that daft guy who tried to fly a kite in an electrical storm. Insulation has a limit to how much voltage it can resist before breakdown and unless to tie some part of the installation to earth how can we limit the voltage insulation has to stand?

 

[JohnW2]

The British Standards for SWA say in the informative part that the armour should be earthed at the supply end. You could maybe argue that the armour isn't exposed but I'll argue that the glands that it connects to are hence they need to be earthed (when using ADS as the method of protection).

If one was 'unnecessarily' using SWA and, for that reason, not planning to earth the armour, one probably would not use SWA glands, and therefore neither the armour nor anything connected to it would necessarily be exposed.

However, as I've said, as far as the regs are concerned, what if it were exposed? If there are at least two layers of insulating material (or even one layer of 'reinforced'insulation) between it and the live conductors, it doesn't count as an exposed-conductive part (otherwise metal-cased Class II items could not exist), and therefore, as far as regs are concerned, it doesn't have to be earthed, does it?

Whatever, I think that most of us are agreed that we would want to earth the armour, even if the armour is not 'required', but AFAICS that's not what the regs say.

Kind Regards, John

 

[JohnW2]

Because parallel cables will always have some inductance or capacitance any unused cores should be earthed. OK I know often we don't earth unused cables with low voltage but really we should. So with any supply which is tied to earth unused conductors should be earthed.

What you say is obviously technically true, even though the inductance and capacitance will (unless one is talking about miles and miles of cable) be so low as to not cause any hazard. However, I'm not aware of any such requirement (to earth unused cores,or armour) in the regs.

The SWA should be earthed even if the supply is PME and the destination is TT. The question is earth to what?

As has been discussed, as far as regs are concerned seemingly only if the cable is required to have armour. If it doesn't, and there is an earth core in the cable (or a separate CPC), then, as has been discussed, I don't think there is a regulatory requirement for the armour to be earthed at all (even though you, I and most others would probably earth it).

Kind Regards, John

 

[Spark123]

I don't think the insulation between the armour and the cores of a SWA count as double insulated, some of it I can pick away with my thumbnail and in the bigger sizes can just have a ribbon.

 

[JohnW2]

I don't think the insulation between the armour and the cores of a SWA count as double insulated, some of it I can pick away with my thumbnail and in the bigger sizes can just have a ribbon.

One could argue that, given that I'm not sure what mechanical properties something has to have to qualify as 'insulation'. However, as I said, if one doesn't use SWA glands and the ends of the armour are not exposed (e.g. if the whole of the SWA enters enclosures through a stuffing gland), it presumably can't count as an exposed-c-p.

Whatever, this whole discussion is a little silly and hypothetical since I don't think (m)any of us would dream of not earthing the armour, whether armour were required or not.

Kind Regards, John