I'd have thought with the current state of electronics, it could be arranged
that the earth was mainly from the house and a circuit between the local
ground and the main earth could be monitored and the whole lot cut off if
the potential rose to outside tolerances.
In the murky past I've actually just bonded the incoming earth both to any
local mass and a ground rod, and nothing ever went bang or caught fire!
I can see however that there could be circumstances where a earth is
effectively from live to earth and it would be hoped that before the whole
lot failed or caught fire, the local fuse for that circuit be it breaker or
whatever would trip and isolate it all.
Of course there are other issues as has been mentioned before to do with
nasty noise on the mains etc, but that would probably only affect somebody
looking for electronically quiet supplies.
I do sometimes wonder as complexity goes up in this apparently simple
wiring, if we are not making things so awkward nobody will understand it and
just ignore it, which is also a danger.
Most folk around here I notice still just string some mains cable down
their fence and fit a 13 a socket in the shed.
Brian
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"Tim Watts" wrote in message
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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.
Of course, you'll need to test your earth rod - and that's beyond me -
never worked with them...
In my scenario, my garden supply goes to a wooden shed with a wooden
floor, so no extraneous earths in there and very little risk even if the
house earth did rise to some voltage above local ground.