On 11/05/2018 21:27, Tim Watts wrote:
On 11/05/18 21:15, Stephen wrote:
I am about to lay some 10mm2 CSA SWA from one end of the garden to the
other.

I need to decide whether to export the (Protective multiple earthed)
CPC from the garage via the SWA cable (where a supplementary CU is
being installed) or use an earth rod at the shed & greenhouse at the
bottom of the garden to form an independent earth.

Total seperation distance laterally is 13m but length of SWA cable is
nearer 20m.

And why is all SWA cable all 3 phase? I can't seem to get any 1 phase
cable so I am forced to shove some brown, blue, Green& yellow PVC tape
on both ends of the SWA cable?

You can if you try really hard, but to be honest, it's a good to have a
bit more CPC via a core as well as the armour - saves a lot of wibbling
trying to decide if the armour is sufficient to supply an earth for the
scenario concerned.

The main question I would ask:

Is there ready access to "local ground earth" in the location? You said
greenhouse, so that would be a yes. Even if it was a wooden affair, you
have open ground inside. More so if it has a metal frame and you bond
that (because you are mounting lights or sockets on it).

In the event of various conditions/faults on a PME (TN-C-S) supply, you
can get the supply earth going up to some potential above true earth -
which would mean a voltage between your imported earth and the likely
wet ground you are standing on. This is where the danger lies.

In this scenario, I would consider a local TT (earth rod) for safety,
obviously RCD on all circuits as per regs and do NOT connect the
incoming earth to the earth rod - separate it in a plastic enclosure
that terminates the SWA.

If your house supply is TN-S, it's less of an issue, but bear in mind
that TN-S is going our of fashion and the electricy co could convert you
to TN-C-S at any time.


The NICEIC are now saying that you should always treat TN-S as TN-C-S

--
Adam