Earth rod

[JohnW2]

All installations should have their own earth electrode, such things are now recommended in BS7671. Most other countries have required them for ever.

Quite. As I said, it was only at the 11th hour that it was dropped (as a 'requirement') from BS7671, so it's presumably only a matter of time before it 'happens'

However shoving in one of the usual thin copper clad steel rods about 1m long will achieve very little, the resistance will be too high to have any meaningful effect.

Exactly - as I said.

On the other hand, installing a substantial metallic grid under your driveway would have a very low resistance, possibly so low that failures

Indeed so - but that's simply not going to happen - at least, not for anything but possibly future new-builds.

such as a broken CNE conductor could be entirely concealed by the presence of it.

IF the local earth were ever that good, one could presumably forget regarding it as a 'CNE' and just regard it as a 'N' (and hence not connect it to the installation's earthing system), in which case a 'broken CNE' would no longer be something for people to worry and write reams about - since it would merely result in a 'loss of supply', just as would a 'broken L'.

 

[JohnW2]

A dedicated earth resistance tester is the best way. If you only have an electrician’s MFT, you could just carry out an EFLI test. Of course, you’d need to know what result is acceptable, you may need to extend the rod, or install additional ones to reach the necessary earth impedance.

You are presumably talking about 'what result would be acceptable' IF the rod were the sole 'earth' for the installation.

If one is merely (and 'unnecessarily') adding an extraneous-c-p to an installation that already has a DNO-supplied earth, I really don't think that there are any 'requirements' for the impedance of that extraneous-c-p, are there?

In any event, if I recall correctly BS7671 says that, if it is the sole earth for the installation, an earth electrode should have an impedance less than 200, and ideally less than 100. However, given what the OP is trying to achieve, even 100 would be far too high to make any noticeable difference to anything in the face of a broken PEN/CNE - so testing and interpreting the results in relation to BS7671 'requirements' would be of no value to him.

However, as flamport has effectively just said, if he really wanted a local earth that would substantially mitigate the effects opf a broken PEN/CNE, he would have to have his drive (and quite probably much more) dug up, and spend a lot of money!