power to outbuilding

[Risteard]

Our wholesaler does not stock 2 core SWA as apparently no one buys it, but they don't stock 5 core for TP&N supplies.

And that is the irony.

Although it could unfortunately be said that many run an unnecessary green/yellow single alongside the SWA for a TPN supply.

 

[JohnW2]

Although it could unfortunately be said that many run an unnecessary green/yellow single alongside the SWA for a TPN supply.

... RF was suggesting that "DIs" were to blame for the practice, but they presumably would not often be dealing with 3-phase supplies, would they?

Kind Regards, John

 

[EFLImpudence]

RF was saying that the wholesalers do NOT stock 5 core meaning that those who install 3-phase - not DIYers - do not run a core for the CPC.

 

[JohnW2]

RF was saying that the wholesalers do NOT stock 5 core meaning that those who install 3-phase - not DIYers - do not run a core for the CPC.

Exactly - but Risteard appeared to be suggesting that many do run a G/Y single alongside 4-core SWA - so who are 'they' (presumably not DIYers or DIs)? ... or did I misunderstand him?

Kind Regards, John

 

[RF Lighting]

I don't think I've ever seen a copper earth run in with a TP&N SWA supply where it was not required.

 

[JohnW2]

I don't think I've ever seen a copper earth run in with a TP&N SWA supply where it was not required.

Fair enough. In that case, either I misunderstood Risteard or else his experiences are different from yours.

Returning to single-phase, one question I've asked in one of these threads has not be answered. Do I take it that you accept that, in a PME installation, reliance on SWA armour as a main bonding conductor is not compliant with BS7671unless the SWA is at least 70mm²? If so, does that mean that, if bonding is required (e.g. in an outbuilding with extraneous-c-ps), you would not export the PME earth but, instead, would TT whatever is on the end of the SWA?

Kind Regards, John

 

[EFLImpudence]

You know the various methods as well as anyone.

Other than misunderstandings isn't whether to TT or not usually decided by cost?

 

[JohnW2]

You know the various methods as well as anyone. Other than misunderstandings isn't whether to TT or not usually decided by cost?

Possibly - although whenever the issue is debated here, the arguments are usually to do with issues other than cost.

If one wants to be compliant with what the regs "actually say", in the common situation we see being discussed here, of a domestic outbuilding (with extraneous-c-ps) (and PME), I really can't see anyone in their right mind installing ≥70mm² SWA so that they could use the armour as a bonding conductor. In that situation, presumably they would either TT the outbuilding, use a core of the SWA (if ≥10mm²) or run a separate bonding conductor. Which of those would be the cheapest, I'm not sure - usually TTing, I imagine.

Kind Regards, John

 

[ericmark]

I think you have all pointed out it's not as simple as just connect up a cable and all is fine. I remember a friend being demoted for using an SWA cable with the armour as an earth to feed a container being used as a workshop when some one got a shock when moving it on a crane a few feet to squeeze in another container.

To me to lift a container which still connected to electric power was crazy, but the enquiry felt the cables earth was not good enough in that it relied on the SWA not being pulled out of the gland.

Working with Germans they questioned why we run earth wires strapped to side of cable rather than select a cable with an extra core. I really could not give them a good answer.

Having gone around factories where the earth from the SWA had been lost, clearly some where under the road way the cable had been damaged and water over the years had converted the SWA into iron oxide I can see the reason for using copper earth specially where it is unlikely regular tests are done to ensure the earth loop impedance is still within limits.

One Midlands council use to employ a friend of mine to test and record the insulation resistance of the SWA to earth so if any company digging damaged the cable they could show who had damaged it and claim for cost to repair. It seems damage from diggers was becoming too costly, I was rather surprised I had expected concentric to have been used. May be it is the rust problem why concentric is used by DNO?

But if I had posted the question by now I would be completely baffled by the replies.

 

[ban-all-sheds]

Any such bafflement should be regarded as a good indication of incompetence at circuit design.

 

[JohnW2]

To me to lift a container which still connected to electric power was crazy, but the enquiry felt the cables earth was not good enough in that it relied on the SWA not being pulled out of the gland.

Yes, as I observed somewhere, relying on SWA means relying on 'plumbing fittings' to maintain continuity - although that presumably is very rarely a problem (in the absence of 'crazy behaviour')!

Working with Germans they questioned why we run earth wires strapped to side of cable rather than select a cable with an extra core. I really could not give them a good answer.

If one wants/needs to run a copper earth, it is obviously 'neater' to use a core (if it has large enough CSA). A separate earth conductor without any mechanical protection would presumably be more vulnerable to mechanical damage than a core (protected by armour), particularly if buried.

Having gone around factories where the earth from the SWA had been lost, clearly some where under the road way the cable had been damaged and water over the years had converted the SWA into iron oxide I can see the reason for using copper earth specially where it is unlikely regular tests are done to ensure the earth loop impedance is still within limits.

That is the main argument I've seen against relying on SWA armour. I haven't a clue as to how common a problem it is.

Kind Regards, John

 

[EFLImpudence]

Well, it's been around a while but as the "armour" isn't really much good for mechanical protection perhaps it should be copper as well.

I am not sure why you don't trust the plumbing fittings. Would you prefer the armour to be twisted together and held by a big screw connector?

Perhaps outside runs should be singles in copper pipe then the plumbing fittings would be more appropriate and bonding not a problem.

 

[JohnW2]

Well, it's been around a while but as the "armour" isn't really much good for mechanical protection perhaps it should be copper as well.

Copper 'armour' would afford a lot less mechanical protection than steel. If that were not believed, and the only point of the 'armour' was to provide an 'earthed sheath' (to make it very likely that penetration would result in a protective device operating), then I don't think SWA would ever have come into being - we'd just be using cable with an outer earthed copper foil, or somesuch.

I am not sure why you don't trust the plumbing fittings. Would you prefer the armour to be twisted together and held by a big screw connector?

I wouldn't go as far as saying that I don't trust the glands - but it is a pretty unusual (unique?) way of establishing an electrical connection. Tightening a brass gland nut onto a whole pile of steel strands is perhaps not ideal, and I have certainly come across gland nuts which were (had become?) quite 'loose'', haven't you?

Perhaps outside runs should be singles in copper pipe then the plumbing fittings would be more appropriate and bonding not a problem.

That would seem electrically appropriate, although I don't know how the mechanical protection afforded by copper pipe would compare with that afforded by steel strand armour. As you say, CSA would be fine for bonding. Even 15mm copper pipe, with 1mm wall thickness, would have a CSA of about 23mm². FWIW, some of the outside wiring I inherited in my house consisted of T+E within copper pipe (with standard plumbing fittings).

Kind Regards, John

 

[ban-all-sheds]

Tightening a brass gland nut onto a whole pile of steel strands is perhaps not ideal

What about tightening a brass bush onto a steel tube?

That would seem electrically appropriate, although I don't know how the mechanical protection afforded by copper pipe would compare with that afforded by steel strand armour.

There are a number of situations where MICC is a valid alternative to SWA.

 

[JohnW2]

Tightening a brass gland nut onto a whole pile of steel strands is perhaps not ideal

What about tightening a brass bush onto a steel tube?

My point didn't primarily relate to the materials. Tightening a bush/whatever onto any sort of tube is a different kettle of fish from tightening it onto a pile of strands (of whatever).

That would seem electrically appropriate, although I don't know how the mechanical protection afforded by copper pipe would compare with that afforded by steel strand armour.

There are a number of situations where MICC is a valid alternative to SWA.

Indeed - but, as I said earlier in my post, if the mechanical protection provided by SWA armour were not regarded as being worthwhile, it would probably never have come into being, and we would just be using cables with an outer earthed foil or suchlike.

Kind Regards, John