Extending earth rods making minimal difference?

[HappyHippyDad]

How were you measuring it?

Just wondering if using MFT's Zs/Ze and you are now seeing the impedance of the other rod at a TT supply end limiting the performance.

I was thinking this PC.
A number of years ago I had awful Ze readings on a TT. Extra rods made no difference. Next door happened to be having electrical work (from a far more experienced electrician than me) and he was having the same issues. He contacted the DNO and said they did something with regards their earth rod.
The results drastically improved.

Is there an earth rod at a transformer?

Would a test of the Ra by using a spike test kit be more reliable? If this result is much better than your Ze then it may indicate the above scenario.

I'm on pretty uncomfortable grown here though, so could be way off track.

 

[pc1966]

I was thinking this PC.
A number of years ago I had awful Ze readings on a TT. Extra rods made no difference. Next door happened to be having electrical work (from a far more experienced electrician than me) and he was having the same issues. He contacted the DNO and said they did something with regards their earth rod.
The results drastically improved.

Is there an earth rod at a transformer?

That was my thought but as it was low when bonded to (presumably TN-C-S on PME) then it would be very unlikely to have a supply 'Ze' of 90+ ohms.

Would a test of the Ra by using a spike test kit be more reliable? If this result is much better than your Ze then it may indicate the above scenario.

That should be the most accurate way to measure Ra, unfortunately my MFT lacks the option.

However, if the goal is fault disconnection then the Zs method is actually measuring what you might see under fault conditions so in a sense is a more useful real-world figure.

 

[timhoward]

My little Metrel tester can do Ra tests and I did entertain doing this, however in this case I'd have had to borrow bits of front garden from a couple of neighbouring properties to get enough distance away from the rod as most of the front was tarmac.

By coincidence I have a job in a house a few doors further up the same street next week, and it also appears to be TT. So there could be further fun ahead.

 

[davesparks]

Is there an earth rod at a transformer?

Yes, even on a TT system the substation will have a network of earth electrodes connecting the start point to earth.

Substations can be classified as either 'hot' or 'cold' depending on how good the connection to earth is. I believe anything over 2 ohms resistance to Earth is considered 'hot' and a situation to be avoided.

 

[pc1966]

There used to be some rule that allowed HV and LV earths to be combined if the substation Ra was below 1 ohm but I don't think that has been the case for a while, now they have to be sufficiently far apart (8m springs to mind?)