However - I suppose the TT figures actually do not, as such, serve any 'useful' purpose other than they have to be recorded and without them you would have no idea what they were.
The "useful purpose" of a Zs figure is to determine whether the protection in place against faults to earth is adequate.
In a literal sense, that's true. However, we know that, in a TT installation, Zs determined as Ze+R1+R2 will aways be too high for MCBs to provide that protection, and the R1+R2 would have to be so ludicrously high not to provide the protection via an RCD that fault protection would be the least of one's worries (the circuiot simply 'would not work'!), and the cause of the very high R1+R2 would have to be determined and corrected.
There is also the confusion resulting from parallel paths. In my house (TT), the directly measured Zs of every single final circult is well low enough for the OPDs to provide satisfactory disconnection times with L-E faults, whereas that obvioulsy is not true of Ze+R1+R2. So, as I just asked EFLI, which of the two (very different) Zs figures would you record as an indicator of whether "the protection in place for faults to earth is adequate"?
Are you saying there is no point testing the rod? Obviously not.
I don't think he is saying that. What I think he is saying is that the R1+R2 value of the circuit is so small compared to the max allowed Zs in the circumstances that you really don't need to actually do the sums to know what the answer will be to the question of if it passed or not.
Quite - or, as I put it above, if the Zs was so high as to be inadequate even for RCD protection, there would be a fundamental problem that needed to be sorted - and there would be no risk of 'missing' this situation, since the circuit simply would not 'work' in any meaningful sense. I guess one of the things that worries me is that some measurement-obsessed joker might measure a Zs of, say 1,500Ω in a TT installation (in the presence of an acceoptable Ze) and 'be content' that this satisfied the (RCD) fault protection requirements!!
What I do wonder though is why there is so much focus on P-E fault loop impedances and yet noone seems to care about P-N fault loop impedances. IMO in an install where you are relying on RCDs to protect against earth faults you really should be doing seperate calculations to determine if the protection against P-N faults is adequate.
Yes, I agree. When relying on an RCD for L-E fault protection, one really should look at L-N loop impedance to ensure it is satisfactory for OPD operation. I always meaure L-N loop impedance in my TT installation.
Kind Regards, John