On 04/08/2020 18:53, No Name wrote:
On 04/08/2020 13:46, John Rumm wrote:
On 04/08/2020 13:04, No Name wrote:
On 04/08/2020 12:02, John Rumm wrote:
On 04/08/2020 10:59, No Name wrote:

Would Adam Wadsworth, John Rumm et. al. like to comment?

I think the first question is what earthing system does your house
have?

See:

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

Then have a look at the relevant sections he

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

As they cover these questions in some detail.



My earthing system at the meter cupboard is a TN-C-S system.

the wood shed is made of corrugated metal panels and the greenhouse
is aluminium framework with glazing. So I would be looking to earth
bond these in any case.
I have already got both an earth rod, a reel of 10mm single core
earth wire and of course the already buried 3 core 10mm2 SWA cable.
The length of this SWA is 20m.

The plan was to connect this to a 32A MCB in the house consumer unit
and have all RCBO sub-consumer unit in the wood store to supply both
the wood store and adjacent greenhouse.

With the house end on a non RCD protected feed?

(you don't want cascaded RCDs)


I plan to change the 32A RCBO in the main CU to a 32A MCB

Yup that would be ok.

As for the Sub CU, I am going to put in RCBOs with functional earth
which as I understand it gives a bit more protection against a missing
neutral or a missing earth?

It does - it allows it to trip on more fault conditions than just a
straight current imbalance.

Both are sited on a 100 mm sub base of MoT1, 50 mm of sharp sand and
50 mm x 600 mm x 600 mm paving slabs.

So it sounds like with your greenhouse, you should be able to include
all its extraneous conductive parts into the bonding should you want.

There is a slight risk still from the frame of the greenhouse itself
though since someone standing outside of it, has access to an
independent earth and the exported TN-C-S earth which I presume is
accessible without the need of a tool.


There is a concrete post and wood panel fence on two sides, then there
is 10m2 of slabbing on eon e side.

On the 4th side there is 90 mm wide slabbing before meeting the lawn.

90mm (just under a brick's width)?

Either way, it sounds like your setup is somewhat more substantial than
many a greenhouse sat directly on damp soil. So the risk of having the
exposed PME earth is going to be vanishingly small (and in reality there
are plenty of exported PME earths in garden sockets up and down the land
that don't typically kill any one. (or at least not due to the earthing!))


(personally I would use the TN-C-S earth to protect the submain as far
as the greenhouse (and other metal buildings), but isolate it there
and make those TT).

As I understood from John Ward, the exported earth has a lower impedance
than an earth rod....

It does (usually), although with TT you rely on the RCDs to handle earth
faults, so you don't actually need a particularly low earth impedance
for the system to work.

(all my greenhouse plants are in pots and there is an automatic
watering system which is fed by 25 mm MDPE from the house so no metal
pipe.

In that EV vehicle video, there was a discussion about putting in a
earth rod and connecting that to the MET.

If it is permitted, I have no personal objection to putting an earth
rod & exporting the MET earth for the eventuality that:

"that under fault conditions the exported TN-C-S earth could be
exporting a dangerous potential to the greenhouse."

putting an earth grid over 20 m2 of my garden does not appeal to me

If you add an earth rod to a PME earth, then it just becomes another
of the multiple earth connections.

I seem to recall reading somewhere ages ago that connecting a PME earth
and a TT earth together was not a good idea and that one should choose
between one or the other.

Well PME has multiple earth connections all along the LV side of the
distribution network (its one of the ways they try to reduce the chances
of there ever being an interruption in the PME conductor). So adding
additional ones does not combine TT and PME - it just makes the PME
network slightly bigger.

(there is an argument that if your property did get a PEN disconnection
along with a lump of the neighbourhood, you may not want your earth
spike serving the whole neighbourhood!)



--
Cheers,

John.

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