Would you recommend a C3 for no visibility of the earth spike/plate?

It is certainly a matter for further investigation.
Depending how you measured that 70 odd ohms the installation may be relying solely on something not intended to be an earthing facility such as an external  water or gas pipe, that may be interrupted at any time by some unlucky plumber or gas fitter. They could then find themselves painfully or even lethally in series with leakage currents from the house. There has been the odd plumber electrocuted while changing a stopcock.

If however that was a test result with bonding and final circuit CPCs removed, then presumably there is a wire to an electrode somewhere that may be totally buried, and that may be OK.

very hard to say without seeing it.

The 'if in doubt' option is probably to recommend  a new electrode in parallel.

Mike

Agreed with Mike. It could be hidden in a flower bed behind a wisteria or whatever.

Not C3 for me - either it exists (no code) or it does not (at least C2) so as Mike says, all depends upon functional testing.

We take a fairly stern approach. Example of recent observation on the issue;

I agree. The condition of the connection cannot be confirmed, risking undetected deterioration that could undermine the earthing system’s safety. (FI/C2)

I recall a TT system installation in a steel-frame building with a 10-ohm earth resistance. There was no access to the earth rod. It was later discovered that the earth rod had been stolen, leaving the conductor buried in the mud

AMK  raises a good point ... an earth fault loop impedance measurement, or even an earth electrode resistance measurement, doesn't guarantee an appropriate electrode is there.

However, what if all you can see is the termination (in a pit or otherwise) ? That could also be 300 mm of rod just pushed into the ground!

There are always going to be limitations ... but what is for certain, that if you haven't seen it for yourself, you can't say it's there.

gkenyon said:

if you haven't seen it for yourself, you can't say it's there

But philosophically, nor have you any evidence that it does not exist.

OK, I will shift my position and agree that a satisfactory resistance of the earthing conductor to the general mass of the earth does not prove that the means of earthing is satisfactory.

What if the earth rod is under the floorboards?

I was under the impression that if the cable going to the spike was 16mm it did not necessarily need to be able to be inspected. I have that in my head from a long time ago so may have changed. I think due to sufficient mechanical protection. That obviously doesn't guarantee how good the connection is but then you are testing that from the consumer unit anyway.   Gary

Edited to say looking at the on-site guide it seems to suggest that the connection can be buried if it's a 16mm cable and is protected against corrosion Table 4.4(111) page 56

The question comes back to what is included in that 70 ohm reading - if it was the sort of building where foundation steels were the electrode by design, that would be perfectly acceptable, but a wire vanishing under the floor to who knows where could just as easily be a bond to an incoming gas pipe or similar, that should not be relied upon,.  There really is no substitute for a knowledge of the original intention..

A short earth rod, that achieves an adequate resistance, even when short, is also OK - because it is intended to be an earth electrode, and it meets the resistance requirements of the earth fault relay or RCD.

There is a third way common in countries that require house electrodes on PME, like Germany, for example, where it is common to see two well-spaced connections made to  the same foundation steel or whatever used as electrodes  so that a very low resistance measurement (sub ohm), can be made that actually does  verify the hidden metal is complete. A separate electrode test, confirms the function of the electrode in terms of a reliable fault path back to the substation electrodes via the terra-firma sort of earth, but that may be near a hundred ohms or so , and may not find a rusty bolt problem.

Wire or ribbon electrodes run around the outside of the building can also be verified as complete in a similar way, so long as both ends are brought to the surface, by a similar ring-round test - which will detect if the middle has been mechanically damaged or rotted out.

Mike.

edited for clarity.

Chris Pearson said:

But philosophically, nor have you any evidence that it does not exist.

How can you certify a means of earthing was there when you couldn't ascertain it was? I'm not sure what philosophy is at play here ... if you say you've seen it, then it was, otherwise, well, you're surely only assuming?

Chris Pearson said:

OK, I will shift my position and agree that a satisfactory resistance of the earthing conductor to the general mass of the earth does not prove that the means of earthing is satisfactory.

What if the earth rod is under the floorboards?

Hence the requirement for connections to be visible ... but again, it's not me that's signing this "certificate" to say the means of earthing is there ... after all, what you certify you've seen or verified, that's what it is (according to you ...) and what you would have to stand up in court and justify ?

Better to say you can't see it and don't know, than assume it's there?