Why would I not use the armour in SWA as earth ?

[mcquitmp]

Hi,

I'm involved in using some SWA cable over a longish run. As its quite expensive I think I'll be utilising the armour as the earth (as I know you have to earth it anyway). As a result I only need to puchase two core cable and it got me thinking - why would I ever puchase 3 core cable in this circumstance? and are their any problems with my approach that I should be considering?

Thanks...
Maurice

 

[bernardgreen]

Hi,
why would I ever puchase 3 core cable in this circumstance? and are their any problems with my approach that I should be considering?

Thanks...
Maurice

Steel rusts and corroded joints can become high resistance leaving the earthing system non effective if it relies on the armouring of SWA cable.

Bit the bullet and pay for three core and reduce the risk of biting the dust from a failed earthing system letting you get the shock of your death,

 

[dynomo]

hmmm...
intersting one

 

[Lectrician]

Steel also has a higher resistance than copper, and over a long run may not prove low enough for Zdb/Zs - Whack some copper in parallel and the figure will drop - you should of course be calcing this when selecting the cable - be a bit of a bugger if you only realised the EFL was exceeded during the testing!

There are also some sizes of SWA where the CSA of the armour is not great enough to be used as the CPC.