hello all,

having recently moved to west wales from an East-Anglian flood plain, I'm confronted with an unfamiliar problem:-

I've gone to a customer property to quote for some work and found that the Ze (TT) is ~440ohm and the existing rod(?) is buried under concrete. The problem is the soil here is pretty thin (few inches) over shale/rock.

The supply is overhead cables - I haven't asked DNO yet about PME conversion, but in case they say no or ££££, I'm wondering how to get a good Ze here.

I can probably get a rod down 1m with the SDS but will it give me a good reading if surrounded by rock?

I've heard about tapes, meshes etc, but don't have any experience of them (& been rereading BS7430 just in case it offered any tips)

I was hoping someone might have experience of this area and could suggest what works well here? maybe?

cheers

mabx

PS. everything's on 30mA RCD so it's not unsafe, but I'd be very reluctant to do any work without a lower Ze & I cannot verify the existing rod.

It would of course be good to get a much lower Ra but remember the max. for a 30m/a rcd to keep touch voltage <50v is 1667 ohms,

Dave.

Ra above 200ohms is not considered stable

That's true Leckie but somtimes we can't do things quite the way we'd like can we,

Dave.

Aren't TT earths inherently unstable?

"Ra above 200ohms is not considered stable"

you have been reading those "guide books" again leckie.

Regards.

The ground has dried out under concrete slabs.
I had this problem when I installed a rod and got 34 ohms. Came back a couple of years later where a patio had been laid over the top and around the electrode and the Ra (Zs e) had crept up to 140 ohms.

You could dig a pit and bury a 15mm rod as far as you can get it in and then backfill with a conductive cement (marconite or Bentonite)

Legh

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Why do we need Vernier Calipers when we have container ships?

http://www.leghrichardson.co.uk

"Science has overcome time and space. Well, Harvey has overcome not only time and space - but any objections."

I do like my bare coppr tape, so you could find and area of soil, dig out a metre cubed, and drop a copper mesh arrangement in. Backfill accordingly, as Legh suggests additives help lower the ra,

You could always take a ***** on it :-)

Only a temporary measure though

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Regards,
Ebee (M I S P N)

Knotted cables cause Lumpy Lektrik

Try the supplier for a P.M.E. connection. If unsuccessful, drive a rod, measure the resistance, if too high, add another length and re-measure. If still too high, add another rod, or rods in parallel.

In general, doubling the length of rods decreases the resistance by 40%; the resistance of 2 identical parallel rods is again about 40%, 3 identical rods in parallel would give about 60% reduction and for 4 rods, about 66%.

Adding chemicals to reduce the resistance, is only recommended, where monitoring of the resistance takes place regularly.

Aim for an overall resistance of ±100 ohms

Search for "Getting down to Earth", a booklet by Megger.

Regards

Temporary but highly effective :-D

I confess I did think that as a fall back position in case I put in a rod and it doesn't make much of an improvement; but I too have been told >200 may be unstable - besides, I have a feeling this job may be the norm for this area so I need to develop my TT earthing skills anyway .


chisel out a meter cubed of rock??? Mmmm... no thanks I think I'll SDS a few rods in first. (i did look for soil, but 6" is the deepest I found .

I don't suppose anyone knows what I might expect from a rod i this area? I guess I should knock one in in my own garden and see...

I'll just have to hedge my quote to the customer to cover myself I suppose. :/

Thanks folks.

mabx

I agree. My employer will not allow us to connect a new supply to a customer if their Ra is greater than 200Ω. It is written in our "Service Requirements" document.

Regards,

Alan.

How smug. What are you supposed to do. Read nothing and not seek guidance? If you wish to ignore all guidance, all non statuary documents and think you know everything, crack on.

I have never tried this through experiment myself but I did read a presentation somewhere that was saying that unless the surrounding earth is extremely dry (water content < 10%) then adding water makes very little difference to the resistance of the rod. Maybe added urea makes all the difference

Couple of points here: Even if the existing earthing electrode is not accessible there should be a means of disconnecting it (with the power off) to make a measurement of it (see 542.4.1).

As stated already an Ze of > 200 Ohms is not the end of the world if it can be reasonably stabilized from the effects of extreme drying (<15% wc not recommended by BS 7430), corrosion and freezing. If you're in doubt then the easiest solution is to add multiple interconnected rods with a separation of more than twice the length of each rod until you get down to an overall acceptable value.

Regards, Tim

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Everyone loves a fireman - but hates the fire inspector.

I have never tried this through experiment myself but I did read a presentation somewhere that was saying that unless the surrounding earth is extremely dry (water content < 10%) then adding water makes very little difference to the resistance of the rod. Maybe added urea makes all the difference

Couple of points here: Even if the existing earthing electrode is not accessible there should be a means of disconnecting it (with the power off) to make a measurement of it (see 542.4.1).

As stated already an Ze of > 200 Ohms is not the end of the world if it can be reasonably stabilized from the effects of extreme drying (<15% wc not recommended by BS 7430), corrosion and freezing. If you're in doubt then the easiest solution is to add multiple interconnected rods with a separation of more than twice the length of each rod until you get down to an overall acceptable value.

Regards, Tim

-------------------------
Everyone loves a fireman - but hates the fire inspector.

Hi,

I would install a new earth rod in a weather exposed position and leave the existing rod connected. If the ground is rocky install the earth rod horizontal, there is not a lot to be gained drilling down in rock. Hopfully your top soil is good.

Maybe could work but you want to be careful that you get the rod significantly below the frost line otherwise it will become unstable. Not sure why you would want the rod weather exposed?

Regards, Tim

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Everyone loves a fireman - but hates the fire inspector.

I took it to mean drive the rod into a bit of ground that's not sheltered from the rain - i.e. where the soil will tend to be damper than in some sheltered positions.

- Andy.