If you want to power the indoor installation from the generator, you would be recommended to keep the distributor's earthing terminal connected to the MET for the TN-C-S indoor installation. Which, if the outdoor installation is supposed to be separate from the PME, doesn't help connecting it back to it when the generator is powering the installation

Therefore, a second earth electrode would be needed connected to MET in any case.

Where is the generator physically located (in the "indoor" part, or "outdoor" part)?

thank you

yes as mentioned ... consider there is a DNO earth provided... but there is also a 'seperate'  TT installation part.

perhaps it was not clear,  when a generator is connected...there is already a earth electrode available (from the TT part)... so it was me just musing about switching the whole installation over to a complete TT setup (switching out the DNO earth) using the existing TT earth electrode, rather than having to put another in for the gen unit.... if that makes sense.

 it was a question about what is 'wrong' with that (if anything).

edit: the above is poorly worded. sorry. and this attempt to improve might be too :-) :    The gen when connected would make a TN-S  would it not, so the thought being then that it  only needs an earth reference  in effect and in that case, to use the existing TT installation (serving an outdoor part ) electrode ... or would that leave problems with the equipotential zone the outdoor TT was meant to address with the DNO supply ...

The idea of re-using the TT zone electrode as an electrode for a local genset TNS is fine - and there is no need to remove the connexion to the  DNO earth to call it that, it is just demoted to another extraneous earthed thingy, and one that may vanish if the DNO repair involves them cutting their  cable to replace a chunk.

That would mean a switch across the TT- TNCS gap, and a means to ensure that the DNO N and L are disconnected when it closes. That leaves the thorny question of sequencing, if the genset comes up to mains sync before tripping. If there is a 'dead time' of no supply and no genset, then the earth switch needs to do its thing in that time.
it may be easier to have 2 electrodes - an extra one  on the mains earth side and the one on the TT side. then In genset state, the TT area becomes TNS and the original TNC-s becomes TT with a rather ambiguous electrode that may include the suppliers' one.

Be careful as genset loop impedance are not usually as low as the mains, and depend on engine load.  You may find the ADS cannot meet its speed requirements without erlying on RCDs that were supplementary in the normal mode of operation.
Mike.

mapj1 said:

TNC-s becomes TT with a rather ambiguous electrode that may include the suppliers' one.

That wouldn't conform to BS 7671 (Regulation 551.4.3.2.1) ... you can't rely on the DNO earthing terminal for a switched alternative supply, so, unless you connect the TT to the TN-C-S (which then causes some headaches, including sizing of main bonding, and no separation from PME), an additional electrode connected to MET is required, regardless of whether you connect the generator to the TT portion or the TN-C-S portion, of the installation.

Include, not rely on - you need your own well defined electrode(s) too - no least because the suppliers connection may be absent in fault.
I think we agree actually, though perhaps I was not clear.

M.

"...the thorny question of sequencing...."   yes ...    that aspect had almost put me off when thinking about it the other day... i think I am put off now.