Earthing to ground spike.

[Russell griffiths]

Looking at an American video, as the foundation reinforcement was being placed the electrician installed an earth wire joined on to the reo cage to provide a good earth. 

 

We are just at that stage and I wondered if it would be worth taking my earth into the footing and onto the reo, instead of an earth spike located under the CU. 

 

@JSHarris   @ProDave 

 

if the the answer is yes

what diameter earth wire would you put in. 

 

Cheers russ. 

[Jeremy Harris]

The concrete almost certainly isn't conductive enough to make this work and meet the regs.

 

I do have an earth rod (double length) that goes through a hold drilled inside my workshop slab and a couple of metres down into the ground beneath.  I did this really as a way of protecting the earth rod connection a bit better.  In my case it was easy, as I had a bit of duct coming up in the corner of the workshop with a run of 6mm² SWA bringing power in, and I wanted to box the incoming cable in for protection, extending up the wall to a wooden cabinet that houses the CU for the workshop.  I fitted the earth rods adjacent to the incoming cable duct so I could put the earth connection and cable inside the boxed section for protection.

 

Apart from protecting the earth rod connection, doing it this way also meant that the earth rod was down into soil that is probably always damp, because of the concrete slab above it.  Not sure if that helped or not, but I doubt it did any harm. 

[Onoff]

Earth rods are generally better external in "flower beds" etc than internally. As above the damper the soil the lower the resistance and under a slab can be dry. Soil resistance anyway varies massively; in Summer when the ground dries and shrink away from the rod, in Winter when the ground freezes. Most people bang in a 4' rod and think that's it. The thinner rods are obviously cheaper but go for the thicker ones and they're threaded so you can connect them together to go deeper. Or you can space multiple rods in a particular pattern. Another option is an earth plate. Again, if you have a "trench" you can lay a copper tape in horizontally even. Anything to get more copper in contact with the mass of earth. Soil acidity etc plays a part to in the life expectancy of the rod...most are copper clad steel.

 

I was given a reel of copper bus bar tape which I intend laying in my water pipe trench (about 50m) and connecting to my earth rod. Should be interesting to see how much it brings things down by. 

 

Btw the connecting cable size on TT can be as small as 2.5mm2 if insulated AND mechanically protected.

 

The only way to tell how good the earth connection is is by measurement with the proper kit.

[Onoff]

The bit about the proper kit is true but....

 

One old school way of testing earth efficiency is with a "light bulb". Connect one side to Live and the other side to your earth rod. The brighter the bulb glows the better the earth! At the end of the day it's a great practical demo of Ohm's Law.

 

DON'T TRY THIS AT HOME!

[Russell griffiths]

So while I am below floor level I could bang in a 1m spike and join another to it and carry on banging until the second one is at outside ground level 

ready for a nice cover and connection later. 

 

What you think. 

 This would put 2m in the ground but I will only have to bang it 1.5 if that makes sense 

[Jeremy Harris]

16 minutes ago, Onoff said:

The bit about the proper kit is true but....

 

One old school way of testing earth efficiency is with a "light bulb". Connect one side to Live and the other side to your earth rod. The brighter the bulb glows the better the earth! At the end of the day it's a great practical demo of Ohm's Law.

 

DON'T TRY THIS AT HOME!

 

I used the "four rod method" to test mine, really just as an exercise in refreshing my memory as to how to do it.  In our case the ground is very definitely wetter under the workshop slab than adjacent to it, because that slab sits over a spring that runs down to the stream.  Also, the ground next to it which is the only place I could put an external rod is a steep bank, which is bound to be dry for the top half a metre or so in summer.  I went for two rods screwed together just as belt and braces, really, but it does make sure that most of the rod is in contact with damp soil.

[Onoff]

3 minutes ago, Russell griffiths said:

So while I am below floor level I could bang in a 1m spike and join another to it and carry on banging until the second one is at outside ground level 

ready for a nice cover and connection later. 

 

What you think. 

 This would put 2m in the ground but I will only have to bang it 1.5 if that makes sense 

 

If you want. Two rods a coupler and a box:

 

https://www.tlc-direct.co.uk/Products/TLER58.html

 

EDIT: Rods are 4' btw. Only way to know how good it is is by testing then periodic testing.

Edited August 11, 2018 by Onoff

[Onoff]

I seem to recall that Dungeness power station had a huge steel earth mat laid down on the site because the ground is basically shingle. 

[Jeremy Harris]

One thing I noticed was that when the DNO ran the new underground cable through the duct we'd laid across to our neighbour's house, so that the old overhead cable could be taken down, they added another intermediate earth from the joint and didn't bother to replace the old intermediate earth on the new pole (there had been one on the old pole).  That was just a length of copper strap laid in the trench, and as the cable and joint was below the water table their new intermediate earth was sat in water.   No doubt that worked a bit better than the old dry earth plate we dug out of the ground after the old pole had been pulled out.  I doubt that earth plate was doing much at all, given that it wasn't buried deep enough (it was only about 200mm below the surface).

[Grendel]

We are getting a new 3 phase supply and from the mains joint in the trench there is a reel of thick copper wire exposed. Is this part of the PME system?

 

Also when you change from a TT earth (temporary power supply to building site) to PME earth is it compulsory to disconnect the copper earth rod. If it's not compulsory to remove the rod, is it helpful to leave it connected as additional earthing for the future?

 

Does anyone ever install solid expensive copper rods or are the copper plated steel rods OK?

[Ed Davies]

 

On 11/08/2018 at 09:31, Russell griffiths said:

Looking at an American video, as the foundation reinforcement was being placed the electrician installed an earth wire joined on to the reo cage to provide a good earth.

 

On 11/08/2018 at 09:54, JSHarris said:

The concrete almost certainly isn't conductive enough to make this work and meet the regs. 

 

John Ward, who seems to know about this sort of thing, would disagree. The 18th edition of the wiring regulations updates the rules on using the foundation rebar for earthing but not in any terribly significant way. See about 2 minutes of discussion starting here:

 

[ProDave]

48 minutes ago, Grendel said:

We are getting a new 3 phase supply and from the mains joint in the trench there is a reel of thick copper wire exposed. Is this part of the PME system?

 

Also when you change from a TT earth (temporary power supply to building site) to PME earth is it compulsory to disconnect the copper earth rod. If it's not compulsory to remove the rod, is it helpful to leave it connected as additional earthing for the future?

 

Does anyone ever install solid expensive copper rods or are the copper plated steel rods OK?

Yes that pigtail of wire is one of the multiple earths of a PME system.  If you ever see the work n a new pole where a new transformer is installed they run a new LV earth in one direction, and an HV earth in the other in the trench

 

You can leave your rod connected, I have one at out house, here is talk if this becoming a requirement for PME systems.

[Jeremy Harris]

17 hours ago, Ed Davies said:

 

John Ward, who seems to know about this sort of thing, would disagree. The 18th edition of the wiring regulations updates the rules on using the foundation rebar for earthing but not in any terribly significant way. See about 2 minutes of discussion starting here:

 

 

The current regulations refer to only to foundation earth electrodes, with one new mention in the 18th Ed regs (which are not in force yet - they don't apply until after January 2019) of buried reinforced concrete, which I suspect may be hard to get to comply with the requirements in a domestic scenario .  

 

Foundation earth electrodes are in direct contact with the underlying ground and have to comply with the current  regs, which don't seem to have changed much from my old copy of the 17th Ed pre amendment 2, which also allows the use of "underground structural metalwork embedded in foundations".  The new draft regs just add that they have to be selected to have suitable corrosion resistance and mechanical strength, which may impact on the use of things like steel piles as foundation earth electrodes, perhaps. 

 

Like the old regs when installed a foundation earth electrode they must provide a suitably low earth impedance when tested.  It’s rare to find foundation earth electrodes used in a domestic installation, AFAIK, I’ve only seen them used once, and that was in a new build machine shop, around 30 years ago.

 

The resistivity of concrete varies over a very wide range, roughly 100 Ω.m for damp concrete, around 10,000 Ω.m for air dried concrete, to around 100,000 Ω.m for oven dried concrete.  Compared with the resistivity of any normal earth electrode material, concrete is exceptionally poor – even a cheap copper coated steel earth rod will have a resistivity of around 0.00000014 Ω.m, so massively more conductive than even damp concrete.  Additionally, damp concrete acts as an electrolyte, so is likely to give variable impedance values when conducting an earth impedance test.