On 30/09/2017 13:25, John Rumm wrote:
On 30/09/2017 11:36, wrote:

I have a 2-wire overhead sub-main to a large shed 40-45m away. The house
earthing is TN-C-S (with an earth rod, ref other append) and the shed
earth comes from a local earth rod; there's an RCD in the shed CU. I
plan to replace the overhead feed with 3 core SWA in the ground (0.5m
deep, with warning tape above it at 0.3m deep), terminating the SWA
internally in IP65 adaptable boxes at each end and running the final
couple of metres in T&E.

ok. Although in reality with the current configuration, two core SWA
would be adequate in the circumstance.

The feed will come from a non-RCD protected MCB in the house CU. From
the TLC cable size calculator it seems that buried 2.5 SWA is OK up to
about 3.6kW for non-lighting circuits (about 2.2kW for lighting
circuits).

Your limitation here is down to voltage drop. 2.5mm^2 SWA gives around
18mV/m/A of voltage drop. So 45m of 2.5mm^2 is going to give 10.53V on a
13A load. i.e. Just out of spec for a supply that will run lights...
(how much you care again will depend on the use case for the building)

The building is mostly a store with a powered hacksaw, log splitter and
small welder. Electrical loads are modest and not going to exceed a
couple of kW.

I plan to either feed the SWA from a 16A MCB in the house and
to use a 10A MCB in the shed or or to use a 16A type C MCB in the house
and a 16A type B MCB at the shed. Any comments?

At that distance I would run a larger CSA SWA - the difference in cost
of cable is small compared to the effort in laying it in. So allow for
some future proofing and expansion unless you are sure that the current
demands are unlikely to ever change.
I've already got enough 2.5 SWA to do it.

As to MCB selection, that rather depends on what the power requirements
in the shed are going to be?
I chose 10A at the shed to ensure the installation stayed compliant and
16A at the head to protect the cable and avoid nuisance trips.

I believe the correct earthing in this situation is to export the house
earth down the SWA (hence 3-core, the armour will only be earthed at the
house end) and to remove the shed earth rod. Once again, have I missed
anything?

There is no "correct" answer as such, which option you choose will
depend a bit on circumstance. With a PME head end, if you elect to
export the earth, then you also need to export the main equipotential
zone. For that to make sense, the outbuilding should not have access to
any other sources of earth potential that can't readily be bonded into
the zone. (so for example, it would not be possible to extend the
equipotential zone into a greenhouse with a bare soil floor).
The point about the floor is new to me, and very relevant to another
task on the tuit list. I have a greenhouse with an overhead T&E feed
(without local earch rod) that was signed-off by someone before I bought
the house.
Is there really a difference between a soil floor and a concrete floor
that doesn't have a DPM? Where do I find the details?

Also if extending the equipotential zone, the earth connection of your
submain will also need to function as a main bonding conductor and hence
meet the (copper equivalent) CSA requirements for that (typically 10mm^2
with PME - although that assumes size of the neutral of the supply is of
35mm^2 CSA or less, and where the supplier has not specified another
size be used).
If you are using 2.5mm^2 SWA, then its armour will typically be around
17mm^2 CSA. Divide that by 2.255 to get a copper equivalent of 7.55mm^2.
So if you add a 2.5mm^2 core to that, you just squeak in to meet the
minimum CSA for a PME main equipotential bond.
Yes, and I would also need to add supplemental cable between the main
earth and the SWA termination.

It's starting to look like I'm better to keep the shed earth rod and not
to export the house earth.

Thanks for your comments.