what size swa to use

Because using the 3rd core will give you an R1+R2 of 0.15, using the armour will give you an R1+R2 of 0.42 IIRC the maximum permitted for a bs88-3 fuse is 0.4. SO Using the armour is definitely a no go as its already over without adding Ze. Using the 3rd core has an allowance 0.25 for Ze. Dropping it down to a 60/63A as suggested for the 35mm cable would give the OP even more of a margin.

 

It was a farm environment Lee, 2 barns, the DNO didn't provide earth facility so had to make it all TT. 100m/a S type rcd at the origin and a 17th compliant boards in each of the barns.

 

TBQH I don't understand.
The chap wants to run an armoured cable from TT supply position A to a make a new supply B for building/outhouse etc..I think this would be designated sub circuit and not sub main.
The only way to do it would be to use 2 core swa with the armour not connected at A position.
At position B then it would earth pitted with rod etc and supply obviously treated as TT.

I cannot see how in this chaps instance that a 3 core swa could be used? - not saying it is going to be used, but just wondering if I am correct in what I'm saying.

 

Just consider it as a submain, the designer has to meet with the requirements of 7671 in that Zs need to be within limits. If the OP uses 2 core the steel will have a higher resistance than the copper so the R2+R2 will be higher than using the copper conductor of the three core, hence the run can be longer. However, the OP has given no info on the existing earth setup at the origin so this is all based on conjecture rather than fact. The alternative would be to fit an RCD at the origin of the submain and then two core could be used as the Zs will no longer matter. In this case if the rcd pops it's gonna be a bit of a trek to reset it.

 

That's why it should have a 100m/a S type at the origin Col.

 

Could also use 2 core Sen, a bit cheaper.

 

That's what I used on the job I was talking about Col, it was still almost 2 grands worth of cable though!

 

Just for info...

Using Draka's tables:
For their 35mm 2-core XLPE SWA, the SWA resitance is 2.6 Ohm/km = for 140m 0.364 Ohm
The copper resistance of the 35mm copper core is 0.524 Ohm/km = for 140m 0.073 Ohm

So R1 + R2 for 140m of Draka 35mm 2-core XPLE SWA is 0.437 Ohm

So should be fine for 63A BS88 Switchfuse on TN-S or TN-C-S with 5s disconnection time, possibly OK for 80A BS88 Switchfuse, dependent on Ze.

 

100mA time delayed or higher. Not uncommon to see 300mA, 500mA or even more.

 

Thanks Mr C