1...A 'pure' TT installation (i.e. no paths to earth other than the TT electrode - e.g. plastic service supply pipes etc.). Given that a domestic TT electrode will almost certainly not carry enough current for a 5/6A OPD to operate, and certainly not for any higher-rated OPD to operate, if one takes the view that the only point of 'earthing' is to facilitate ADS, then there really is no point in having it in such an installation. Indeed, one could go further than that... not only does 'earthing' not achieve anything useful in that situation, but the connection of exposed metal to earth could be said to merely increase the chance of (potentially fatal) L-E shocks - although a TT rod will not carry enough current to operate an OPD, it can carry more than enough to kill.
That may be correct in one of your vanishingly-small-probability scenarios.
What are you considering to be of vanishingly small probability – a true (and compliant) TT installation?
As you know from your own situation it doesn't really work out like that.
Exactly – see below.
Were the earthing conductor proved to be 'pointless' perhaps you could create a non-conducting location.
If you mean an “earth-free” location, then that’s almost self-fulfilling. If one did have a location without any extraneous-c-ps, then by removing the ‘pointless’ connection to an earth rod, one would presumably turn it into an earth-free location (give or take damp walls/floors, telephone lines etc.), wouldn’t one?
Can you produce a safe and reliable VOELCB?
In the situation with no extraneous-c-ps, I don’t think there would be anything unreliable or unsafe about the VOELCBs we used to have. However, of course, in practice we now have (and rely on for ADS in TT systems) RCDs, so that’s a bit moot – and one (at least I!) wonders whether it is sensible to regard RCDs as providing only ‘additional protection’ in a TT installation, particularly if incidental earthing via extraneous-c-ps is not adequate to provide the required ADS.
...Whilst that G/Y obviously does provide the desired equipotential bonding, it is, in that situation, also facilitating ADS - so is it really reasonable to argue, perhaps 'passionately', that this cable is not providing useful 'earthing' as well as bonding?
It would seem to me that there isn't actually any 'confusion' in the situation but an anomaly. The problem lies in the regulations (can you believe it?). In your house you have something which is no longer allowed in that the water supply pipe definitely
is (still) your earth and the rod
is bonded for little or no purpose.
Agreed, but there certainly are some ‘anomolies’, whether one calls them ‘confusions’ or not – and I presume that, at least in concept (if not degree) the situation in my house is the same as that in a high proportion of those with TT installations. For a start, given what you correctly say in that last sentence, should not the cable connecting my MET and water supply pipe be regarded as the (albeit 'non-compliant') Earthing Conductor (since it is the one ‘ensuring that the OPD operates’) and the cable connecting the MET to my earth electrode as a Bonding one (since it equalises potential between the rod and my {‘earth’} water supply pipe)?
Whilst, household electrics are a compromise (because of cross connections that cannot be avoided), it is necessary to apply safety methods as best we can. It would be better to make all dwellings TNS but obviously this cannot happen.
I’m not sure that it’s all that ‘obvious’ that it couldn’t happen – but it certainly won’t – particular given that TN-C-S is cheaper to provide, and hence is gradually becoming the predominant system.
However, this does not detract from the facts which I keep stating in that cpcs and the earthing conductor are to ensure that the opd operates ....
There never has been any argument (at least, not from me) about that. However, as above, if one wishes to avoid ‘confusion’, it may be appropriate to reconsider how one describes functions and conductors in an installation like mine (which I’m sure is not that uncommon, at least qualitatively. As I said, the conductor I have which ensures’ that OPDs operate in response to L-E faults is the one which goes from my MET to the incoming water supply pipe.
....and bonding conductors on extraneous parts are to equalise potential in the event of a fault.
.. and, again, no argument there - just the question as to how I should regard/describe the conductor from my MET to my ‘earth electrode’.
That a scenario can be devised where this appears to not exactly fit does not alter the situation.
Agreed. However, as above, I wonder whether (to avoid ‘confusion’) we should not describe conductors correctly in terms of what function they are fulfilling in the house in question, rather than what function the corresponding conductors would have in a different house?
Kind Regards, John