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Topic Title: TT installers - could I do better?
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Created On: 03 February 2019 07:14 AM
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 03 February 2019 07:14 AM
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alanblaby

Posts: 868
Joined: 09 March 2012

I generally work in the Midlands, where we have deep soil, and can get an earth rod in pretty easily, and even get 2 rods in joined together.
Yesterday was different, I have a family member in North Wales who wanted a CU and earthing upgrade.
Firstly I checked the Zs at a socket, which gave 582ohms.
OK, test the existing earth rod, which gave a Ze of 418ohms.Gas and water were plastic.
OK, new rod(s) needed. Which would be fine if you could get them in more than 450mm deep. Bedrock hit everywhere in the front garden at the most 450mm deep.
So I ended up with 2 shallow rods, and a Ze of 105ohms.
As the front garden was so small, I was a little unsure about adding a 4th rod (I kept the original connected), as I was under the impression that each rod must be its depth away from another rod.Add on the cable runs over the garden, and I didnt think another rod was wise.
What would you have done?
 03 February 2019 09:17 AM
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aligarjon

Posts: 4053
Joined: 09 September 2005

I would be happy with 105 ohms if it stays at that. I would want to test it in the summer though when things are a bit dryer.

Gary

-------------------------
Specialised Subject. The Bleedin Obvious. John Cleese
 03 February 2019 10:32 AM
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daveparry1

Posts: 8020
Joined: 04 July 2007

I agree with Gary, i'd be happy with 105 ohms in this situation, although as he said it would be good to check it again when drier conditions prevail.
 03 February 2019 02:37 PM
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Fm

Posts: 2032
Joined: 24 August 2011

We link our rods with 25 mm bare copper tape
And carry bentonite too but we do a fair few earth rods installs for lightning protection in sand.
A mixture of both would help lower that figure

Edited: 03 February 2019 at 03:16 PM by Fm
 03 February 2019 03:27 PM
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AJJewsbury

Posts: 17795
Joined: 13 August 2003

The French seem to have similar problems in some regions (along with a drier/warmer climate and TT being very common) - their solution seems to be a loop of buried copper wire or tape (in the foundation trench, under the concrete, for a new build) ideally following the entire building perimeter. (They also tend bring both ends out to the MET so one end can be disconnected and the whole length checked for continuity). So something like that approach might have been an option, especially if the location was such that the soil wasn't likely to dry out completely (or freeze down to the tape's depth).

I was under the impression that each rod must be its depth away from another rod.

A rod's length or two apart is good for minimising the effect of one rod on the other - necessary if the rods are connected to different systems - but where they're connected together anyway, the closeness only serves to reduce the effectiveness of the extra rods (i.e. the overall resistance would be higher than for two completely separate rods, but still lower than not having the extra rod there at all). If you can't get the rods further apart but you're still in a 'every little helps' situation, then more rods closer together is definitely better than fewer rods spaced over the same area.

Firstly I checked the Zs at a socket, which gave 582ohms.
OK, test the existing earth rod, which gave a Ze of 418ohms

R1+R2 of (at least) 164 Ohms??? I hope you found an explanation for that too!

- Andy.
 03 February 2019 04:17 PM
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alanblaby

Posts: 868
Joined: 09 March 2012

Originally posted by: AJJewsbury

Firstly I checked the Zs at a socket, which gave 582ohms.

OK, test the existing earth rod, which gave a Ze of 418ohms


R1+R2 of (at least) 164 Ohms??? I hope you found an explanation for that too!
.


No!
The original rod was doing very little, so I'm just presuming it was a flaky joint/rod causing the differences between the Zs/Ze results.
The installation was fully tested, all was ok, with a solid 105 ohms tested on each circuit.
 03 February 2019 08:49 PM
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sparkingchip

Posts: 11682
Joined: 18 January 2003

In lumpy, rocky ground or ground backfilled with builders rubble a metre long 10 mm drill can provide a reasonable pilot hole to get started.

Andy B.
 04 February 2019 02:01 AM
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mapj1

Posts: 12039
Joined: 22 July 2004

you can of course drive rods in sideways, or at least at a shallow angle. Rebar in a concrete slabs can be surprisingly effective, especially if the mixture is spiked with Marcionite before pouring . Floors of garages, sheds and radio shacks if you are lucky enough to have one make large area near-surface electrodes.
For that matter a mesh of the the chunkier re-bar would be permitted without the concrete.
Sometimes cunning is required. I have once buried an old hot water cylinder to great effect as a very fat electrode, and on another occasion a 'J' shape bent galvanised scaffolding pole was interred with a bit of help from a friendly digger driver.

Assuming the RCDs are 30mA, and you are not needing anything special, then 100-150 ohms is fine, as others have said. If that was verified by a loop test, it is worth noting that some of that is the supplier's electrode at the transformer end of things.

-------------------------
regards Mike
 09 February 2019 08:03 PM
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dustydazzler

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Joined: 19 January 2016

Scroll through to about the 15 min marker on this video

http://m.youtube.com/watch?v=-nz6dBIdTYM

They manage to drive home what looks to be about 7 or 8 metres worth of copper earthing rod
 09 February 2019 08:49 PM
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chrispearson

Posts: 1095
Joined: 15 February 2018

Originally posted by: dustydazzler

Scroll through to about the 15 min marker on this video

They manage to drive home what looks to be about 7 or 8 metres worth of copper earthing rod


Interesting use of an SDS drill!

Hereabouts we have a spade's depth of good top soil (where cultivated) and then another spade's depth of clay and sand, and then it gets flinty. That's where the digging stops. I wonder whether the SDS drill could wriggle an electrode between the flints, but I shall bear it in mind if necessary. (That TT EVCP Island?)
 10 February 2019 05:47 AM
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mapj1

Posts: 12039
Joined: 22 July 2004

An adaptor made from a dead SDS drill bit and a 3" length of 20mm steel conduit and about 15 mins with the welder and grinder has taken the arm ache out of conventional earth rods for me , assuming there is a supply already on for the drill.
The other tips are to grind or file a crude point or chisel tip onto the leading rod, which is definitely worth it and to get the area wet. In stony ground with a soft substrate, like very gravelly peat, water drilling can be used to great effect. Probably less useful with flint and clay.

-------------------------
regards Mike
 11 February 2019 08:28 AM
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lyledunn

Posts: 1228
Joined: 13 August 2003

Originally posted by: mapj1

Sometimes cunning is required. I have once buried an old hot water cylinder to great effect as a very fat electrode, and on another occasion a 'J' shape bent galvanised scaffolding pole was interred with a bit of help from a friendly digger driver.


Can do better than that Mike! An enterprising local farmer received a grant to convert some of his land in to a caravan site. He needed certification as he had done the electrical work himself so it was accepted by the government department that an EICR would suffice to determine if all was OK. What do you record when you find that the earth electrode is a buried Vauxall Viva? Does that fall under "other suitable metal work"?

-------------------------
Regards,

Lyle Dunn
 11 February 2019 08:51 AM
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Zoomup

Posts: 6117
Joined: 20 February 2014

Corrosion? 542.2.1. and is a buried car really "other suitable underground metalwork"? 542.2.2?

Z.
 11 February 2019 10:35 AM
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mapj1

Posts: 12039
Joined: 22 July 2004

Well so long as he has the SORN paperwork for it, or has it taxed and MOT.
Joking aside, I'm not sure how suitable it is, Like overhead T and E, I'd be marking it down as OK when tested but needs regular checking every few years for deterioratin, , and I presume much of it is painted so the contact area may not be that great.. Thin car metal can corrode surprsingly fast.

-------------------------
regards Mike
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