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?!

 

[Yozza19]

Had a simslar issue a few years back doing a temp supply at Lancaster uni ended up is taking a 600x600 mat on a bed of marconite

 

[nicebutdim]

Amazing how little impact sinking below 8' had. I'd be tempted to experiment, even at my own expense, and drive another couple in parallel.

 

[123]

@timhoward 4 rods is impressive, unless they are only a foot long. Are you driving them into sand perhaps? Had a job recently where I got 4 rods in with relative ease, the 5th got about half way down. The area is built largely on sand I'm led to believe, and the reading wasn't anything great albeit about half of what you have.

 

[pc1966]

That is curious. Usually when you go deeper the conductivity increases as you have damper soil closer to the water table.

Your ability to get 4 rods, presumably 4' / 1.2m each, driven in is impressive! I wonder if they kept going straight down?

Putting in a mat, or even a trench around 0.5m deep and a run of several metres of bare 25mm or more copper wire, are other options to consider.

 

[pc1966]

Out of curiosity, did you need them to be better than around the 100 ohm from two rods?

 

[pc1966]

Has anyone ever had similar, and have I accidentally discovered the house is built on a potential sink hole?! Or maybe an old well?!

Is it an old mining area? Maybe a sewer or old rail/smuggler's tunnel?

 

[timhoward]

Thanks for the replies. Yes, they were 4 foot rods.
The 1st one wasn't that hard. The other were much harder, about 5mm movement for each clout. It felt a lot harder than sand.

I had no need for it to be better than 100 ohms, I just get personal satisfaction from low readings. After the 2nd one I thought "I'll do one more and maybe get it to around 50 ohms and then call it a day."
Then when it didn't I got curious and thought I might as well try the last one.

Amazing how little impact sinking below 8' had. I'd be tempted to experiment, even at my own expense, and drive another couple in parallel.

I do have to go back there, so I'll see how I feel. The 'garden' is like a jungle and clipping the cable to the wall on the right all the way along it to the boundary was not a nice job!
That said, the reading is now adequate and I've no reason to suspect the 1st 8 foot of a second electrode would be any different.

A couple of pictures....

Old and new rod. I originally wondered if I was measuring the conduit and the conductor wasn't connected to the rod any more. But I got the same high reading testing the old rod. (Getting the top off was fun!)

Is it an old mining area? Maybe a sewer or old rail/smuggler's tunnel?

In general, yes, it is an old mining area, but I'm not aware of any old coal fields in this bit. I would have expected some discernible difference in effort required if I'd broken into a void of some kind. I'll probably never know the answer. It just felt very strange.
At the end of the day the reading is at least acceptable.

 

[buzzlightyear]

I wonder what the moles and the rabbits said when you fencing them in. Ramming those rods in I bet it was like back to the future. Marty.

 

[pc1966]

In general, yes, it is an old mining area, but I'm not aware of any old coal fields in this bit. I would have expected some discernible difference in effort required if I'd broken into a void of some kind. I'll probably never know the answer. It just felt very strange.
At the end of the day the reading is at least acceptable.

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.

 

[timhoward]

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.

Interesting. I did high current loop test with wander lead, one probe going to new rod and other probe straight onto supply live (top of isolator)
I’m pretty sure I had old rod disconnected by now.

Funnily enough with bonding connected Zs drops to 0.19 ohms, via copper water pipe to next doors PME (I had sneaky look in their meter box!)
So if the old rod was still connected that would artificially improve matters not make them worse, right?

 

[pc1966]

Interesting. I did high current loop test with wander lead, one probe going to new rod and other probe straight onto supply live (top of isolator)
I’m pretty sure I had old rod disconnected by now.

Funnily enough with bonding connected Zs drops to 0.19 ohms, via copper water pipe to next doors PME (I had sneaky look in their meter box!)
So if the old rod was still connected that would artificially improve matters not make them worse, right?

If the supply is off a PME system, even if that property is TT, its supply Ze ought to be pretty low.

So back to the odd question of where the rods went and in to what material!

 

[stevethesparks]

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.

 

[timhoward]

I had two pet hates with rods.

I was conscious of both of these issues and know exactly what you mean.
In the past I've found the Niglon couplers are slightly lower profile and don't open the hole us as much as the ones I was using on that job (Screwfix convenience won the day as there was one very near)

 

[davesparks]

I've had diminishing returns from extending rods before, but never quite so extreme in nature.

The up side to this is that the final reading will, whilst higher than anyone would like, likely be very stable through the seasons.