Swa & cpc

[chloebear]

I have a 3 phase sub board to run in SWA - I am proposing to use 16mm 4 core with the armour as the main Cpc for the cable. I was also concidering installing a seperate 16mm as a seperate earth.

I am having a battle with myself as to the necesity of the additional 16mm earth - I can see the logic of using one of the cores of the cable as a CPC (in this case I would need to run 5 Core!!!!) as this would be an integral part of the cable protection should the armour fail for some reason.

But what additional protection would a separtate 16mm cable afford for the cable & sub board????????

 

[Sintra]

A 16mm 4 core SWA cable to BS6346 has a 72mm csa of steel armour which has a copper equivilant of 31.9mm.

A 16mm 4 core SWA cable to BS5467 or BS6724 has a 50mm csa of steel armour which has a copper equivilant of 16mm.

So either of these types of cable are fine to use the SWA as the CPC without any additional conductors.

 

[Guest 004]

@ sintra where are the conversion tables from steel to copper?

 

[bugsy]

Lenny's done a sticky!!

 

[pushrod]

Is this it http://www.earthingnuts.co.uk/pdf/pvc_xlpe.pdf

 

[bugsy]

http://www.electriciansforums.net/e...ulations/23288-can-we-use-armour-swa-cpc.html

This is

 

[pushrod]

http://www.electriciansforums.net/e...ulations/23288-can-we-use-armour-swa-cpc.html

This is

linked to the same table

 

[Chr!s]

Is it tnc-s?, are there any bonding requirements?, are there any extraneous conductive parts within the location of the DB?

 

[i=p/u]

im led to believe to use the swa as cpc unless you think it could be damaged by other elements.

 

[Deleted member 26818]

Please note that the term 'copper equivelance' used in the chart, does not refer to equivelance of conductance as refered to in the note for Table 54.8 of BS7671.

 

[chloebear]

Thanks - good point regarding the bonding aspect - there is a metal sink unit within the same area that has been bonded back to the main DB - would it be Ok to leave this bonded back to the main DB or should I bond it to the Sub DB - or maybe even cross bond it to both???????

 

[malcolmsanford]

Is the sink an extraneous conductive part within a special location?

 

[Deleted member 26818]

Thanks - good point regarding the bonding aspect - there is a metal sink unit within the same area that has been bonded back to the main DB - would it be Ok to leave this bonded back to the main DB or should I bond it to the Sub DB - or maybe even cross bond it to both???????

Are you stating that the metal sink is an extraneous-conductive-part, and is bonded back to the MET, as per the requirements of BS7671?
What would be the purpose of bonding it to the sub board?

 

[bsrfc10]

The additional protection that the seperate external earth is that you will not lose your earth to the sub board if for any reason anything should happen to the SWA, corrosion of the steel wires if exposed to the elements. I know it shouldn't happen if the cable is installed with the correct glands etc but it does happen!

Would the sink not fall into supplementary bonding which we dont have to do anymore?

 

[malcolmsanford]

The additional protection that the seperate external earth is that you will not lose your earth to the sub board if for any reason anything should happen to the SWA, corrosion of the steel wires if exposed to the elements. I know it shouldn't happen if the cable is installed with the correct glands etc but it does happen!

Would the sink not fall into supplementary bonding which we dont have to do anymore?

IMO the only time I would not consider a SWA to be sufficient as a CPC with regards to external influences would be in a very wet and corrosive enviroment, such as a boat yard or marina, and even then I would back myself to make it good. It's like all erection methods, if it isn't done correctly then anything can fail. the art is doing it right.

I await Chloebear to come back on that. If it's not in a special location then no supplementary bonding is required. If it is then he should be guided as per 701.415.2.