25mm swa armour for cpc

[Rockomuir]

Hi all,
Just planning a sub main to an outbuilding .
It will be 80 metre run, buried direct on a 63amp fuse .

So I am thinking of running 25mm swa but just been doing my calcs on whether I can use armour as Cpc. The outbuilding is going to be TT so no need of a bonding conductor on the submain.

After doing calcs in 2 core swa I get
99/51 x16 = 31.02

What I cannot find is csa of 25mm armoured to see if it is big enough .

If anyone could give me the answer it would be most appreciated

 

[oscar21]

Why do you need an earth/cpc if its powering a TT installation.

 

[Rockomuir]

Why do you need an earth/cpc if it’s powering a TT installation.

Sorry I might being completely daft. The swa is going to need a Cpc from the supply end but disconnexted at the outbuilding end as this will become TT. Just wanted to work out if 25mm armoured is big enough to use for cpc

 

[westward10]

The csa of the armour is around 42.0 for a two core 25.0 cable.

 

[oscar21]

Sorry I might being completely daft. The swa is going to need a Cpc from the supply end but disconnexted at the outbuilding end as this will become TT. Just wanted to work out if 25mm armoured is big enough to use for cpc

You mean for if the cable is damaged by say a spade or something and you want to ensure the MCB/fuse blows in the required time? Well I've not done any calculations or anything but I would imagine that they wouldn't make the SWA in long lengths if that was a possibility, that's just common sense to me.

Also I thought problems around the size of the cable armouring only came in to play if 10mm bonding was required. If you are that concerned why not put a time delayed RCD upfront, I'd do that as opposed to up the cable size purely due to cost.

 

[westward10]

The size of the armour comes into play as it must be able to withstand fault current without suffering thermal damage the bonding issue is a separate situation.

 

[oscar21]

I get that but why would they manufacture a cable that couldn't cope with a fault it could cause.

 

[Marvo]

Price.
SWA comes in many armour variations, there's aluminium armoured (which isn't SWA in the purest sense), steel armoured and also combination steel armoured with varying numbers of copper armour wires to decrease the armour impedance. You just specify according to your application. If you're creating a TT at the remote end you could also run an external bare copper earthing conductor alongside the SWA cable and connect it to the rod to bolster the TT.


This is an example of SWA cable with some copper armour strands to reduce impedance if the adibiatic rule isn't met by purely steel strand armour.

 

[buzzlightyear]

Instead of running in 2 core use a 3 core
Two lives three has a earth, with out running in a separate earth.

 

[mainline]

Instead of running in 2 core use a 3 core
Two lives three has a earth, with out running in a separate earth.

No real point in doing that with a TT if the armour satisfies the requirement.

 

[buzzlightyear]

No real point in doing that with a TT if the armour satisfies the requirement.

Yes you are correct, but lose the armour
Then you are in trouble.

 

[mainline]

Yes you are correct, but lose the armour
Then you are in trouble.

That could be said for any cables' loss of cpc.
It's unlikely to lose the armour if the gland is fitted properly, Even then, it won't affect the TT end.

I've never seen a damaged/ dug up swa cable with the armour disconnected, leaving the cores intact.

 

[MDJ]

No real point in doing that with a TT if the armour satisfies the requirement.

totally agree