The electrician said he didn't do the RCD trip tests as he said it couldn't trip with that resistance.

I'd agree that wasn't an accurate explanation at all (although I've heard worse when an electrician is trying describe electrical matters to a customer who may or may not be technically minded), and he might even not fully understand all the implications accurately himself and be over-relying on rules of thumb. Sounds like -1 for customer relations.  But even if his thinking is flawed, it doesn't necessarily follow that his final answer is exactly the reverse of what it should be.  The bottom line is I wouldn't want to give a clean bill of heath to TT system where there is reasonable suspicion that the earthing facility was showing signs of failing. As others have suggested, FI may well have been more appropriate than C2 - but both will give an unsatisfactory until more work is done anyway. If it's gone from 400Ω to 534Ω in the space of time between you both doing your respective tests, what's is lit likely to be in a month, at the end of the summer or next year? Can you really be reasonably sure it'll still be below 1667Ω? If not, how can anyone to state that the installation is satisfactory for continued service?

   - Andy.

Hi,

Not a C2 unless the RCD is faulty, which would be a rare occurrence but not impossible. I the last 3 months I have  had to replace two that no longer worked. It certainly is preferable to have an electrode resistance of lower than 100 ohms if at all possible. Got mine at home down to around 67 with 2 pairs of 5/8" rods.

He said he did not test the RCD trip times. He put >300ms on the report. My worst case trip time was 9ms. I don't think it's gone from 400 to 534. He loop tested it, I used a a long wire and a ground probe. I don't think it is unstable, but I will test it again after the weekend.

In his defence, Megger (and probably all the others) won't run the RCD test if the touch voltage will exceed 50V, which it would if he tried a 5x test (or auto).  What meter are you using, and what RCD tests did you do?  Is there a front end 100mA RCD, time delayed or otherwise? 

Gomez Addams said:

The electrician measured the rod resistance at 534 ohms.

How did he do that?

Good question. He wouldn't engage with me at all when I questioned his report. I did it with a Megger CM500, a long wire and a ground probe. Measured a least 15M from the rod in more than one place.

Today I called by to speak to my tenants about something else. I asked about the electrician, who I have never met, he was referred to me by a letting agent. My tenant told me he said he could only half the checks because "he didn't have the right equipment with him". She didn't know what he meant, which is fair enough, why should she? She asked him if the house was safe and he told her yes. I am wondering if he had any idea of what he was doing at all.

I used a a long wire and a ground probe.

Just one wire? How did you account for the resistance around your test electrode? Was that on a d.c. Ohms range? The salts in the soil and metals of the electrodes can often make little batteries, generating a small voltage that can easily influence the result of a simple d.c. test (you might well see differences simply by reversing the meter connections). The resistance around a temporary electrode is usually considerable - a 4-wire (or sometime 3-wire) test method using separate test electrodes for voltage and current and an a.c. test current is usually needed for dead testing of electrodes to get sensible results. Loop testing (from the mains L) is usually simpler - although it does include the resistance of the source electrode (e.g. at the substation & for PME additional electrodes in the distribution system) - which can yield higher figures than your electrode alone, the difference is normally small.

   - Andy.

Sorry, my statement before was a bit brief.

The meter has a jack socket to plug the probe into. The 2 wire test lead is used to do this as per Megger's instructions. The equivalent circuit in the user manual shows how the resistance is measured. So one lead is on the rod, the other on live and the long lead is on the probe. Current is injected into the rod and the probe in parallel. The meter the current injected and the developed voltage are used to calculate the resistance. It is a loop measurement, but with a "true" ground reference as the manual puts it. I did try the probe in more than one spot and the results were comparable.

The local substation here is about 250M away. 

How to fix the situation ...

(1) Measure the rod resistance properly and replace if excessive.

(2) Just bang in another rod. If you cannot measure the resistance properly, ensure that Ze is satisfactory.

The instructions for your meter state that the temporary electrode should be 20 - 30m away, so that may account for some of the discrepancy, plus of course you're not measuring the supply side, maybe another 20 ohms? 

Personally I always loop test if the supply is connected as it's far less faffing about.

It does sound like he's inexperienced when it comes to TT, or perhaps EICRs in general.  I would have coded as C3, with no expectation that any improvement would ever be sanctioned by the client. 

I know of at least one DNO that insist on less than 200 ohms for new installations and they can, and have, issued demands for improvement within a 3 month time frame.

You sound like you are competent, so why not put in another rod (or 2) and move on knowing that your paying tenants are safe?