On 12/05/2018 09:09, Stephen wrote:
On 12/05/2018 08:25, ARW wrote:
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


So for clafication:

THe SWA cable has 3 equally sized conductors plus steel wire armour so I
can use one of these as a CPC.

If you want...

(places like TLC Direct (and most other wholesalers) will do two core
though if you want)

My house has a PME supply supplied so the earth is supplied to me by the
electricity supplier. So there is no secondary earth provisioned within
the main house.

So which is better: should I then export this supplied earth to the
outside CU or provide my own local independent earth to the outbuildings?

If one of those is a greenhouse, it can be near impossible to export
the PME equipotential zone into it, so TT would become a much better
option.

Have a look at:

http://wiki.diyfaq.org.uk/index.php/...rting_an_earth

and the following bit about exporting equipotential zones.

The seperation distance laterally is 13m.

All of my current and new consumer units all or will use RCBOs.

The ones in the house are 30mA RCBOs. If I use a local independent earth
for the outbuildings, won't that mean I then have to consider using time
delayed or 100mA RCBOs to avoid nuisance trips?

You can use a normal MCB for the submain feed rather than a RCBO. You
isolate the PME earth at the point it connects to the outbuilding CU. So
you have overload and fault protection for the SWA provided by the MCB.
Earth fault protection for the outbuilding is then provided by the local
RCD/RCBO.



--
Cheers,

John.

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