1. If he decides on the TT option all main equipotential bonding must be carried out at the shed end
Yes. All extraneous conductive parts must be main bonded.
and this precludes earthing the armour of the supply
You should earth the armour at the supply end to protect the cable to ensure it will automatically disconnect the supply in the event of mechanical damage.
This must not be connected the the earthing system in the shed.
but you would still use the cpc inside the armour cable to distribute to accessories?
No, you should install 2 core SWA. The CPC for the final circuits must be connected to the shed MET, along with the main earth from the earth stake, and any MEBs.
2.If he decides to extend the equipotential zone from his dwelling he has 2 choices. He could use 10mm^2 swa as the cpc will be of sufficient csa to provide MEB.
As long as the SWA is 3 core, then this will suffice as the MEB conductor
Or does he have to run a 10mm^2 earth regardless of the supply cable size (ie in addition)?
This would only be required it the supply cable to the garage was <10mm²
3. Also he talks about 6 0r 10mm^2 cable......surely vastly oversized for his requirements. Without even bothering to get the tables out I would have thought 3core 2.5mm^2 would be more than adequate with as separate 10mm^2 earth run together in a duct or even overhead.
a 2.5mm² SWA with a 10.0mm² seperate earth would be fine.
4. Would you consider the shed to be an extraneous conductive part and need to be earthed?
Thats a bit more difficult, and would need to be assessed on a shed by shed basis.
If there is structural metal work into the ground then I'd probably say yes.
5. What would you personally consider as the best solution to this proposed installation?
Personally if there was an extraneous conductive part(s) then I'd export an earth, and probably go with a 10mm² SWA. It is less messing about, and gives future capacity.
It also avoides the additional precautions required for a TT installation.
