The 5/8 Earth rod debate

[huntos33]

Did a re wire on a village hall TT system , after reading the threads on rods i went for it, chucked away the 3/8 twig coupled together 3 5/8 rods and away i went driving away with my 4lb lump hammer 3.6m of depth and got my ZE down to 14 ohms so i guess thats a ok reading for a TT well bellow the 200 ohms , but was hoping it would be a lot lower...

 

[StuSpiers]

Ze seemed very low to me also (that's kinda why I used it as an example)

On reflection though when you take the ground conditions into account the recent weather it isn't so strange.

This house was inset into a hill - the hill had been excavated to build the house.
The hill seemed to be a deposit of pure clay.
It was and appeared to be perpetually wet - the area was completely shaded but still open to the rain and deep moss was everywhere.

I only use ELI method for Ra readings, parallel paths would have been removed for Ze yes.

I appreciate it isn't the most accurate method of testing Ra and that's probably why I got inconsistency between the readings.

 

[Engineer54]

[QUOTEI only use ELI method for Ra readings, parallel paths would have been removed for Ze yes. ][/QUOTE]

So what was you using the MFT at the rod position for then??? I'm still sceptical with your ultra low Ra values, especially with just 2 rod's, but no doubt that it will provide a stable TT system. Maybe you have hit the Ra jackpot, in achieving a better than TN value, and hope that you Have. But there is this feeling, that's telling me something is not quite right....

Perhaps, if it's at all possible, you could test this rod on a regular basis say every couple of months or so. at the very least it will show you just how stable your Ra value is...

Clay can be a good soil for TT systems, but can dry out very quickly too, all depends on what type of clay it is. If this hill is all clay, i doubt if they would have built a house on it, too much of a risk of subsidence.

 

[PEG]

I'm with you on the regular testing bit re;earth electrodes,did a post a while ago concerning this.If any lads had done or had knowledge of electrode install and then could give conditions and test results over time,it would give an idea of whats good and what varies,just a thought....But stop scaring my missus with the clay/subsidence stuff! Mines on clay (blue) and not moved since 1937.Took 45 tonnes out for rainwater tank in garden.It's like IRON.Get your point with other types,ground heave etc just off to give her a calmer......:wink5:

 

[edexlab]

A 240mm SWA cable is in all likelihood, going to be a DNO cable, unless on an industrial or large commercial site. So in that instance there should have been cable protection tiles (usually 10'' X 10'' concrete paver's) so he should have known that he had hit something quite solid. Bit foolish to carry on thumping away, without making some kind of investigation. It's not as if he'd gone down that far before hitting that protection tile....

Yes it was a DNO cable and there were no tiles or tape and the depth of the cable was 320mm below the surface cable was installed in 1995
and in regard to a different post, as I'm sure most of us will appreciate irrespective of the earthed sheathing if the cable is penetrated it goes bang!

A firm I have subbed to also hit a cable some time ago again the cable was just below the surface approx 380mm and again no markers or tape and when the DNO sent their repair bill they challenged it with the fact that the cable was not deep enough

The Dno replied that due to ground settlement and erosion that cables can end up closer to the surface ,this seems unlikely as this cable had been in the ground for about 15 years, its actually a sheltered housing type of development and the cable route was through the middle of the garden and it is obvious that there has been no abnormal, if any erosion and only normal settlement, they also said that the absence of cable markers or tiles could be down to cables being installed by the use of a Mole machine, but the Director of the trust an ex civil engineer said the ground was excavated and cable laid and he has photos to prove it, which he took as the project progressed.

Anyway the company involved had their H&S consultant look at it and he's reported it to the HSE who are investigating the Dno for similar occurences, but I would imagine that the HSE would be duty bound to investigate the company's safe working practice as they were using builders plans which were vague and they never used a cable tracer and were using a pick and shovel, which is pretty much what led me to ask the question whether others scanned the ground before knocking the rods in ie if you do put a rod through a cable then the owner of a business is going to be liable for repair costs and fines at the very least.

 

[Marvo]

Working in Africa it's probably different in many respects to the UK. The safest place to knock a rod in here is exactly where the drawing shows a supply cable, I've never seen a council drawing that's accurate in over a decade and that's assuming there actually is a drawing.

Seriously I would always excavate at least 600-700mm and often a full meter with a spade before starting a rod in the ground. Also if I hit something solid in the first meter of driving the rod in I will stop and try again about a foot away. It's not 100% foolproof and there's always a nagging doubt in my mind that there could be services but after 1.5 meters below ground level it's highly unlikely. I would certainly agree with their statement though that ground levels can change radically over time. We've been back to a couple of jobs years later where we installed cabling at 1 meter depth and it's become exposed on the surface.

 

[La Poste]

Why near the supply cable?

Is it because the supply cable is lead sheathed and earthed, is the supply cable underground?

 

[Engineer54]

In both the instances you gave in your post, the DNO responsible will be cited for gross negligence if the burial depth does not adhere to their own publicised depths, and additionally installed with no warning tape or protective tiles. This is ''Standard Practice'' when all said and done, where cable(s) pass or are installed in public area's
Cable burial depths are actually increased where they are run through gardens etc where there is a higher likelihood of soil disturbance due to digging etc...

Now anyone driving an earth rod in the ground that hit's something solid (cable tile etc) will if they had any sense stop an do a little investigating before thumping their way through it. If your unsure if any services are in that area, then excavate 500mm or so, and then drive your rod in....

 

[huntos33]

Interesting to read about digging pits first, i assume this is not always possible but seems sensible i must admit i spent some time checking and was happy i would be safe where i was overhead supplies etc. This forum has opened my eyes to TT systems as i normally work in the retail area so when i have done the odd domestic job it's always been a twig, now im SE and doing more so it's been of a great help finding this forum, keep up the good work mods and members.