Armoured Cable connection to CU

[JohnW2]

Its even worse than that homslaw!
Taking the figure of a 8.9 ratio between steel and copper conductance, that would mean that if you needed a 10mm bond, you need 89mm of armouring to get the same conductance, if you look at the tables for 2C swa you are looking at 95mm before that is acheieved!

Whilst I agree with all that is being said, before people start interpreting too widely, it may be worth reminding ourselves that it only applies to the situation in which one is contemplating using SWA armour for Main Protective Bonding, which I imagine would be a fairly unusual situation. I would have thought that in the majority of situations, one is going to be using armour only as a CPC, in which case an adiabatic calculation (using the 'K factor', which is much smaller than the conductance ratio) is applicable. An exception, of course, would be, say, the supply to an outbuilding which had extraneous-conductive-parts (which needed MPB) - in which case it would, indeed, seem that it would normally be impractical to provide the MPB with SWA armour.

Kind Regards, John.

 

[Adam_151]

Yeah, but the specific situation we were debating was use of SWA to extend a TNCS equipotenial zone, wasn't it john?

 

[JohnW2]

Yeah, but the specific situation we were debating was use of SWA to extend a TNCS equipotenial zone, wasn't it john?

Was it? It's an interesting point, and was the main reason for my last post. On the face of it, I see no need/point (in terms of either engineering or regulations) to extend, per se, the house's equipotential zone to the garage - unless, I suppose, the buildings are so close that one can touch exposed-conductive-parts in both buildings simultaneously.

That leaves us having to consider an equipotential zone within the garage. The OP does not seem to have mentioned any extraneous-conductive parts in the garage (I guess I have to prepare myself for discussions about damp floors!) and, as far as I can see, it is only the presence of such parts which would create a need (engineering or regs) for those parts to have Main Protective Bonding (MPB). The regs you have cited, which would require an impractically large steel CSA, relate only to MPB. As I've said, if the connection back to the house/MET only serves the function of a CPC (not MPB), then the steel armour could well provide an adequate CSA.

...or have I missed something?

Kind Regards, John.

 

[kai]

It should really be compulsory to make armoured cable with an earth core, the use of two core should really be phased out - the incidence of copper theft these days can lead to main earthing connections being stolen - if the earthing connection is inside the main cable, an unsafe situation is less likely to arise, as the whole cable needs to be removed, not just the easy to steal loose earth wire.

 

[holmslaw]

..

 

[JohnW2]

It should really be compulsory to make armoured cable with an earth core, the use of two core should really be phased out - the incidence of copper theft these days can lead to main earthing connections being stolen - if the earthing connection is inside the main cable, an unsafe situation is less likely to arise, as the whole cable needs to be removed, not just the easy to steal loose earth wire.

I agree with what you say about a separate 'loose' earth wire. However, in relation to what is being discussed here, use of 2-core with the armour as the CPC (where that is possible) does not seem to present any greater risk, if theft occurs, than would 3-core SWA,does it? - no-one is going to steal just the armour of SWA!

Kind Regards, John.

 

[ban-all-sheds]

no-one is going to steal just the armour of SWA!

Having removed some from a length of 10mm² 2 core for which I had no use (but did have use for 10mm² singles) I can attest that it's an utter PITA.

 

[JohnW2]

no-one is going to steal just the armour of SWA!

Having removed some from a length of 10mm² 2 core for which I had no use (but did have use for 10mm² singles) I can attest that it's an utter PITA.

I suspect that if you'd sold it on eBay and then bought 10mm² singles, you may well have at least broken even, and not had to do so much cursing

Kind Regards, John.

 

[ban-all-sheds]

BAS is wrong, the tables in the link are not for sizing main bonding they're for sizing earthing/cpc's.

Indeed I am and indeed they are.

And the adiabatic equation must not be used for directly sizing bonding conductors. On a non pme supply the main earth size can be calculated by using the adiabatic, and then the main bonding can be half the size of the earth, but a minimum size of 6mm.

A useful point to note, should anyone want to use armour as a bonding conductor.