Extending earth rods making minimal difference?

[timhoward]

Today I found a 466 ohm Ze. That's actually a new upper record for me. I assume the rod was completely corroded through.
So started to bang in some new 5/8ths rods.
1 rod - 180 ohms.
2 rods - 95 ohms.
3 rods - 93 ohms. Hmm.
4 rods - 92 ohms! Grrrrr.

I gave up then as I'd run out of rods. I also wished I'd saved the last two rods to install in parallel!
Has anyone ever had similar, and have I accidentally discovered the house is built on a potential sink hole?! Or maybe an old well?!

 

[pc1966]

If you run the theoretical calculations for uniform soil you get less gain from length than the ideal R/l sort of expectation, but nothing that significant:

Here 'Row 12' was a rod calculation for 16mm diameter and soil resistivity of 120.492 (fudge to get 1m = 100ohm), and 'Row 13' just 100/length.

Formulae from here:

Measurements and Calculations Of Earth Electrode Systems (BS 7430) | EEP

System earthing involves the provision of a connection to the general mass of earth. This connection should have a resistance not greater than that required to operate safety mechanisms to isolate the electricity supply from a fault situation.

electrical-engineering-portal.com

 

[Robson689]

I had two pet hates with rods. One was that each lower coupler opened the entry hole and made the upper rods looser in the ground - although in time the ground would close up.
Pity they wasn't designed to screw into each other rather than use a coupler.
The other was the couplers always appeared to loosen and only by twisting the rods clockwise you could be sure that the lower rods below ground were as tight as possible. In my last year before retirement I started soldering the rods and couplers together just before disappearing below ground level. This certainly felt more solid when thumping them into the ground.

Solid copper rods use a threaded dowel as a coupler

Even better is the steel rods made by Dehn they have a pin on one end and a hole in the other so the pin compresses and locks the rods together as you drive them in the ground

Edit - a link if anyone is interested as it's an unusual product

Earth Rods | DEHN

For setting up earthing systems for down conductors or transformer stations.Features: No thickening of the cross-section at the coupling joint Self-locking coupling Corrosion resistance Easier storage and transport Universal use according to local soil conditions Constant resistance values...

www.dehn-international.com

 

[stevethesparks]

Spot on (plus my extra characters to post)

 

[pc1966]

My recent earth rod adventures used M16 stainless studding in 2m lengths and I used the stun nut runner to basically screw the thing in to the ground (dense clay at our site).

This sort of thing with an SDS-1/2" drive adaptor:

Stud Runner Socket 1/2" Drive – Metric Coarse Thread | Impact Socket

impactsocket.co.uk

 

[timhoward]

My recent earth rod adventures used M16 stainless studding in 2m lengths and I used the stun nut runner to basically screw the thing in to the ground (dense clay at our site).

This sort of thing with an SDS-1/2" drive adaptor:

Stud Runner Socket 1/2" Drive – Metric Coarse Thread | Impact Socket

impactsocket.co.uk

By the way, Linian also make something that looks similar:

 

[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?)