Have you tried testing the RCD using the upstream/downstream method (Test Method 2) in the 9th Edition (2022) of IET Guidance Note 3 Inspection & Testing ?

That will help identify if it's an issue with the earth fault path in some way ... or alternatively an issue with protective conductor currents, etc.

Assuming, of course, this is from a grid supply, not a generator less than 10 kVA?

Paul Berryman said:

Its a tt system earth is 205 ohms 

The earth electrode resistance is on the high side (Table 41.5, Note says that readings > 200 ohms may not be stable.?

As we are in the middle of the hottest June since 1880 odd and have had little to no rain for ages i would think it's fine.  :)

The 200 ohms is not a target for a wet autumn. Guidance would be to aim much lower under those conditions.

Plus, not all the country has a hoespipe ban ... it's not the same conditions everywhere.

There are 2 problems, one that prevents the RCD from operating, the other that is the electrode may or may not really be adequate for continued service. But lowering the electrode resistance may mask the problem that causes the RCD not to trip, as it should operate in the most onerous condition, which is a poor earth, so fix that first. Then go and look at the electrode and the wiring to it. Round here at least (sandy gravelly ground) a figure of 100 ohms or so is quite normal for a 4 ft rod,*

However, it is very possible that the rod or more often the clamp arrangement to connect it is corroded - really you want bright metal on all contacting surfaces, and perhaps a smear of vaseline or similar over the top once completed. (a quick way to do a large area of that is to part dissolve some in white spirit or petrol and paint it on with a brush as a cream/slurry. But be aware that it is not a 'fire safe' technique so outdoors only - even before you add the flammable liquid, a dob of vaseline makes a great emergency  fire lighter, especially on cotton tee shirt type material)

* Note that the RCD tester does not need the electrode to be perfect, and even a Zs of over 1000 ohms, the sort of thing that has a human in the loop, should still trip it - eventually some testers refuse to apply the fault current if they decide the earth voltage rise is excessive.

Mike

That wasn't what i meant. If the measurement was taken during a wet autumn i doubt we would be having this conversation. I take the point that not everywhere is bone dry at the moment though.

I'll add that i always presumed that it possibly being unstable meant if the ground were to get very dry as it is round here at the moment. Am i wrong about that?  Putting aside the obvious of checking it is connected properly. 

Gary

That wasn't what i meant. If the measurement was taken during a wet autumn i doubt we would be having this conversation. I take the point that not everywhere is bone dry at the moment though.

I'll add that i always presumed that it possibly being unstable meant if the ground were to get very dry as it is round here at the moment. Am i wrong about that?  Putting aside the obvious of checking it is connected properly. 

Gary

aligarjon said:

I'll add that i always presumed that it possibly being unstable meant if the ground were to get very dry as it is round here at the moment. Am i wrong about that?

Freezing etc also. It can also be an indicator that the electrode has corroded.