Megger do a much more in-depth guide to earth impedance testing, here is one of them (just follow the download button):
However, I remember seeing another Megger guide that covered earth red specifically and how to check situations like sub-station where you may not get far enough away for fall-of-potential to be accurate, etc.
[automerge]1590393361[/automerge]
Here it is. Well worth keeping a copy (and reading it) if you ever do need to put in earth rods and it really matters (e.g. if you need well below 200 ohms for a special installation with high current RCD on TT, or it has to be below 10 ohms for lightning guide lines, etc):
Earth electrode and looping, Theory and applications
Earth electrode and earth loop impedance testing Theory and applications - As two of the most common testing subjects debated amongst electricians.
uk.megger.com
However, I remember seeing another Megger guide that covered earth red specifically and how to check situations like sub-station where you may not get far enough away for fall-of-potential to be accurate, etc.
[automerge]1590393361[/automerge]
Here it is. Well worth keeping a copy (and reading it) if you ever do need to put in earth rods and it really matters (e.g. if you need well below 200 ohms for a special installation with high current RCD on TT, or it has to be below 10 ohms for lightning guide lines, etc):