I disagree. Perhaps, even more pedantically, it can't be an extraneous-conductive-part by definition, as it's part of the electrical installation.

Fair point. Another example of where the definition doesn't exactly help with what the physics tells us we need to achieve. (Similarly with c.p.c.s to class 1 equipment outdoors that's in contact (directly or indirectly) with the general mass of the Earth).

   - Andy.

AJJewsbury said:

(Similarly with c.p.c.s to class 1 equipment outdoors that's in contact (directly or indirectly) with the general mass of the Earth).

I'm not with you 100 % on that ... surely the issue with the physics here is making sure the diverted neutral current goes where you want it to, because larger currents will take the path of least resistance (hopefully extraneous-conductive-parts), and therefore helping keep touch-voltages that might result at the outdoor equipment to a minimum by providing some local earthing within the installation via extraneous-conductive-parts? We'd definitely prefer the current to exit via good earthing offered by real extraneous-conductive-parts, than a cpc to an item of Class I equipment.

There is a gap, though ... what if there are no extraneous-conductive-parts in the installation (plastic water pipes, no gas, no metal structural parts in contact with the ground?

The recommended supplementary earth electrode in Regulation 411.4.2 would help to address that gap, but only if the protective conductor connecting it were sized in the same way as a main protective bonding conductor. (Although I don't want to give the impression here that the electrode is recommended solely for the reason of back-filling for the job that water and gas pipes, and PILC or hessian covered distribution cables, once did - as prosumer's electrical installations become more common, a supplementary earth electrode will become a necessity.

I'm not with you 100 % on that ... surely the issue with the physics here is making sure the diverted neutral current goes where you want it to, because larger currents will take the path of least resistance (hopefully extraneous-conductive-parts),

I'm thinking of of class 1 items (say a luminaire) attached to some very substantial metalwork outdoors (say steel perimeter fence, or a bridge) or even something that has a connection with another installation's earthing system (the "Downing St" issue - where outside lights are mounted on a wrought iron fence that's common to adjacent buildings). BS 7671 no longer requires main bonding to extraneous-conductive-parts outdoors, so we're potentially left with a simple c.p.c. trying to handle the currents (diverted PEN or otherwise) that normally would be shunted by a main bonding conductor).

  - Andy.

AJJewsbury said:

I'm thinking of of class 1 items (say a luminaire) attached to some very substantial metalwork outdoors (say steel perimeter fence, or a bridge) or even something that has a connection with another installation's earthing system (the "Downing St" issue - where outside lights are mounted on a wrought iron fence that's common to adjacent buildings). BS 7671 no longer requires main bonding to extraneous-conductive-parts outdoors, so we're potentially left with a simple c.p.c. trying to handle the currents (diverted PEN or otherwise) that normally would be shunted by a main bonding conductor).

I do see that - I'm thinking in my head about the fact that if we have low-impedance connection with Earth from extraneous-conductive-parts, plus low impedance connections (10 sq mm or greater, short-distance main protective bonding), this presents overall lower resistance path to Earth than  a longer run of larger cpc + connection to Earth of exposed-conductive-parts (with cpc's already connected back to MET).

AJJewsbury said:

I'm thinking of of class 1 items (say a luminaire) attached to some very substantial metalwork outdoors (say steel perimeter fence, or a bridge) or even something that has a connection with another installation's earthing system (the "Downing St" issue - where outside lights are mounted on a wrought iron fence that's common to adjacent buildings). BS 7671 no longer requires main bonding to extraneous-conductive-parts outdoors, so we're potentially left with a simple c.p.c. trying to handle the currents (diverted PEN or otherwise) that normally would be shunted by a main bonding conductor).

I do see that - I'm thinking in my head about the fact that if we have low-impedance connection with Earth from extraneous-conductive-parts, plus low impedance connections (10 sq mm or greater, short-distance main protective bonding), this presents overall lower resistance path to Earth than  a longer run of larger cpc + connection to Earth of exposed-conductive-parts (with cpc's already connected back to MET).

plus low impedance connections (10 sq mm or greater, short-distance main protective bonding),

Which is fine if the bond is there. The trouble for me is that BS 7671 seems to be saying it's no longer required (as the part is outside of a building).

   - Andy.

But BS 7671 has been updated to recommend that a supplementary earth electrode is provided for TN systems (Reg 411.4.2) so there ought to always be at least something connected to MET acting in this capacity - we have already agreed how the protective conductor connecting the electrode to MET ought to be sized ?

hello GK hello AJ   Thanks for hanging in there . Not sure I understand all the dialogue 

Im rather stuck here :  Take on board  belt and braces 10mm2 for the earth link met to rod. That done.

Can I hope for some kind of indication the earth proposal is in fact SAFE   Given all other parts equal and installed by qualified.

Answer to that is stopping everything.  

I still have a number of questions about the solar package   Is there a solar forum on this net ?

Solar installers seem to just have menus.. Certainly cant negotiate this.

ciao MsO  

But BS 7671 has been updated to recommend that a supplementary earth electrode is provided for TN systems

We might be at cross purposes here... a supplementary electrode - with no particular requirement for resistance - so let's say 200Ω - vs either a km long perimeter fence with steel uprights in the ground every 2.5m - let's say <1Ω overall to the general mass of the Earth- or a outside light on wrought iron railings that are continuous to next door's similar light which is similarly connected to their MET by a thin c.p.c. - say 1Ω or 2Ω to the supply PE - seems to me the bulk of the current is going to prefer the little c.p.c.s ....

   - Andy.

It's amazing how complicated a bunch of electricians can make a bit of bonding (I'm not actually an electrician, but a member of the IET).

Outside the IET, you migh want to look at the "Green and Ethical Moneysaving" section of the Money Saving Expert forums.  https://forums.moneysavingexpert.com/categories/green-ethical-moneysaving,  You won't get expert electrical advice, but you may get information from solar experts.