The loop tester measures the full loop, i.e. The electrode to terra-firma mass of earth at the substation or in the street, in series with the transformer winding, the live cable between your building and the transformer,  and the wiring and then the resistance between your local electrode and mass of earth as well.

But the assumptipon is that resistance of the mass of soil and rock, dominates by at least a factor of ten, and maybe a factor of a hundred or more over the copper resistances.

(The substation electrodes may be no more than 20 ohms each and in practice single figure ohms are more likely at the DNO end, if your local electrode is a single rod it could be a hundred ohms, it maybe in the low tens. Do not expect much less than that unless you have buried a large grid or are  living in salt marsh.)

So although the loop tester measures the loop - well it would! - the error introduced by calling all of that reading your electrode resistance is small compared to the variation between dry and rainy days.

If you really want to verify the electrode resistance in isolation then you need a tester with additional electrodes - one to measure the potential of the earth far away from your electrode under test, and another to inject current, that way there is no error at the voltage measuring electrode as it is passing no current, and no error at the current injection electrode, as we do not measure the voltage at that point.

Such tests are quite fun and can be informative, depending as they do on soil types and many other parameters but doing the test that way is rather more faff than can be justified on a simple job where there is already a supply and the exam question is not one about the geology, but just if the RCD will trip when is is supposed to.

Mike/

Hi Mike thank you for your detailed explanation. It is very helpful.  

Hi Mike thank you for your detailed explanation. It is very helpful.