Need to main bond SWA armour?

[Smugley]

Was talking to someone today about a radial from the CU to a single garden IP rated socket. TN-S supply.

I was thinking that I would run 3-core SWA from the garden socket into an indoor adaptable box and from there in T+E back to the CU. The SWA runs underground across a lawn.

In the adaptable box I was just going to link the armour to the circuit CPC to provide a return earth path. However he reminded me that as the SWA is entering the building from outside, the armour, although insulated, is an extraneous conductive part, and therefore must be connected directly to the MET via 10mm bonding cable. To be honest I can't remember ever seeing main bonding attached to SWA armour before.

It does make sense to me, so is this correct?

 

[Graeme Harrold]

No, SWA han an insulated cover and is there to protect the cable. Just terminate with a gland and fly lead to the CPC..........simples

 

[Smugley]

Hi Graeme, that would have been the way I was going to do it, but he was insistant, and has many more years experience than I. But having thought about it, I think it is important to separate the functions of earthing for the purposes of disconnection, and bonding for the purposes of avoiding multiple earth potentials within the building. Linking the CPC to the armour at the box only satisfies the earthing side, even though the CPC provides a path back to the MET anyway... bit confused as I think he might be right, but I've never seen this in practise.

 

[ukpablo]

poppy cock..............just do the fly lead from gland into cpc as graeme said, that will comply! only need to do one end, i would do the one closer to the cu end!

you already have an earth using 3 core swa...........connecting the outer sheath is gold star and up to regs standard! thats what i would do.

 

[Smugley]

Ha ha. Okay, good to hear another vote for simplicity!!

Along the same lines, I've never seen anyone main bond the shield of incoming telecommunications cables either, despite the last paragraph of 411.3.1.2.

 

[ukpablo]

im sure that main bonding is anything extraneous entering the property, gas pipe which is conductive, water, soil pipe etc........all being conductive.............swa, telecoms cable etc should have some form of insulation barrier, ie outer cable..........basically, if the outer case is metal, bond it!

Thats only my opinion on it all!?................

 

[Smugley]

Yeah, I'd agree, but the moment there is a cut or split in the outer pvc sheathing, then there could be a potential difference. to the MET. Anyway, I'm going with you guys on this. After all, what chance is there that the householder will get acess to the SWA armour. It comes in through the wall, under floorboards, and terminates inside an IP rated box!

 

[ukpablo]

best of luck bud

 

[Sparky Ninja]

Extraneous conductive parts - Gas services, Water services, Lightning Protective Systems & Metal framework. We provide protective bonding to these as they introduce an earth potential local to the installation and are not currently connected to the supply earth.

External conductive parts - Steel Trunking, Steel Conduits, Steel Wire Armour (if you say so ), Ladder racking etc are simply supplementary bonded to the CPC's local (or via a seperate earth if necessary) that are included within thier system. This will result in the containment being of the same potential to the CPC's included. If you were working on the system, and the containment was directly earthed via a main protective bonding conductor, should a fault occur, the earth fault return path may choose the containment as the higher earth potential available. This, in theory, could result in yourself being used by the fault current as a quick access to the earthed containment, should there be a better potential present than the existing CPC's.

 

[ukpablo]

exactly!..........bit long winded though, but straight to the point!!!!!!!!!...lol

 

[Risteard]

The armour of an SWA cable is earthed, not bonded. It is an exposed conductive part.

 

[Sintra]

Ha ha. Okay, good to hear another vote for simplicity!!

Along the same lines, I've never seen anyone main bond the shield of incoming telecommunications cables either, despite the last paragraph of 411.3.1.2.

The reason that a metallic sheath of a telecoms cable is an extraneous conductive part is that it is terminated and connected to earth at the supply end were ever that may be. This could introduce another potential into your property.

An SWA used to supply power from your property to say a garage will not introduce another potential as it is connected to your supply.
So simply no need to main bond back to MET.