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Topic Title: Getting a good earth on rocky ground
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Created On: 01 December 2012 11:44 PM
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 01 December 2012 11:44 PM
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mabx

Posts: 46
Joined: 11 February 2008

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.
 02 December 2012 09:52 AM
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daveparry1

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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.
 02 December 2012 10:17 AM
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leckie

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Ra above 200ohms is not considered stable
 02 December 2012 10:27 AM
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daveparry1

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

Dave.
 02 December 2012 04:03 PM
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mikejumper

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Originally posted by: leckie
Ra above 200ohms is not considered stable

Aren't TT earths inherently unstable?
 02 December 2012 04:45 PM
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UKPN

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"Ra above 200ohms is not considered stable"

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

Regards.
 02 December 2012 05:30 PM
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Legh

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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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 02 December 2012 06:05 PM
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Fm

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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,
 02 December 2012 06:20 PM
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Westonelectrical

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You could always take a ***** on it :-)
 02 December 2012 06:39 PM
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ebee

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Originally posted by: Westonelectrical

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


Only a temporary measure though

-------------------------
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Ebee (M I S P N)

Knotted cables cause Lumpy Lektrik
 02 December 2012 06:57 PM
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Jaymack

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Originally posted by: mabx
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.

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
 02 December 2012 07:21 PM
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Westonelectrical

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Temporary but highly effective :-D
 02 December 2012 07:43 PM
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mabx

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Originally posted by: daveparry1

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.


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 .
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,


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
 02 December 2012 11:32 PM
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alancapon

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Originally posted by: leckie
Ra above 200ohms is not considered stable

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.
 02 December 2012 11:36 PM
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leckie

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Originally posted by: UKPN

"Ra above 200ohms is not considered stable"



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



Regards.


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.
 03 December 2012 10:15 AM
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timothyboler

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Originally posted by: Westonelectrical

Temporary but highly effective :-D


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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 03 December 2012 10:24 AM
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Avatar for timothyboler                                      .
timothyboler

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Originally posted by: Westonelectrical

Temporary but highly effective :-D


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.
 04 December 2012 07:52 AM
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Al

Posts: 60
Joined: 02 February 2009

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.
 05 December 2012 04:31 AM
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Avatar for timothyboler                                      .
timothyboler

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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.
 05 December 2012 10:12 AM
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AJJewsbury

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Not sure why you would want the rod weather exposed?

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.
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