Dog days and Ra's

[Deleted member 9648]

A recent minor works on a small electrical supply on a playing field revealed a TNCS earthing system. Setup is a DNO head feeding a small DB with a 30ma RCD main switch and mcb's feeding a couple of 16/32a sockets and 13a sockets used for public functions and small fairground supplies. The local authority agreed to converting this to a TT (but declined the addition of a 100ma S type main switch and RCBO's on cost grounds).
So today in 30 degree heat I arrived with electrodes to do the conversion. First issue was that there was already a disconnected rod in the ground,in poor condition but presumably it had been TT at one time. Anyway a new 5/8 rod went in with a pit with some difficulty and I optimistically got the ELI tester out.....850 ohms! Coupled on a second rod and spent the best part of the next hour getting down 8ft....testing as I went it gradually came down and we ended up with 80 ohms.
My question is to everyone is would you have left it at that as I did or would you have gone further? If further how far?.....I guess a third rod and a lot more time might have got it lower....perhaps even as low as the magical 15 ohms, which seems to be the trigger for mutual congratulationary back-slapping from the 'you cant possibly rely on an RCD' clique on here.
This is not intended to be another full on Ra debate....I am just interested in what the good members of this forum would do with the installation.....Honest answers please!

 

[Bobster]

Time to invest in an sds bit, so you can use a hammer drill to knock them in.

Me personally would have put another one in.

 

[Deleted member 9648]

Time to invest in an sds bit, so you can use a hammer drill to knock them in.

Me personally would have put another one in.

What if another one only halved it?....where would you stop?....when you got to a TN value?,or would you accept 40 ohms and rely on the RCD?

 

[Bobster]

3 rods would produce a long term stable ra that will only decrease over time as the ground compacts around it.

If I was doing it for my own house I'd put enough in/another pit until I reached TN values, but It's going to come down to customer budget and tendering a low enough price to get the job.

 

[Marvo]

First issue was that there was already a disconnected rod in the ground,in poor condition but presumably it had been TT at one time.

I wouldn't look at an old rod as being an issue, I'd see it as an early Christmas present. Did you do an impedance test on the old rod? If you achieved 80ohms with an 8ft rod then the old rod could be even as low as half that value if the ground has settled around it. Give the top of it a quick brush and file and use it alongside your new rod.

Also if you Ra dropped from 850ohms to 80ohms when you coupled another rod on top then you got massive rewards for a little more depth so I wouldn't hesitate to add a third rod on top of the first two but do it with an SDS jack-hammer attachment.

We generally use 2.4meter rods and I whack two in coupled together as a minimum and sometimes a lot more.

 

[GLENNSPARK]

the thread had Ra as its title....
so what the hell has TN got to do with owt?
you go for the best of the earthing arrangements on visual...and prove it of course...
if theres a rod still connected then leave it...connected.....if considered extranious......or in other words as a back up....

 

[Deleted member 9648]

I wouldn't look at an old rod as being an issue, I'd see it as an early Christmas present. Did you do an impedance test on the old rod? If you achieved 80ohms with an 8ft rod then the old rod could be even as low as half that value if the ground has settled around it. Give the top of it a quick brush and file and use it alongside your new rod.
The old rod was a so called 'twig'....and pulled out of the ground very easily,and was not connected.I suspect a very high Ra value previously had resulted in the system being converted to TNCS.

Also if you Ra dropped from 850ohms to 80ohms when you coupled another rod on top then you got massive rewards for a little more depth so I wouldn't hesitate to add a third rod on top of the first two but do it with an SDS jack-hammer attachment.

We generally use 2.4meter rods and I whack two in coupled together as a minimum and sometimes a lot more.

the thread had Ra as its title....
so what the hell has TN got to do with owt?
you go for the best of the earthing arrangements on visual...and prove it of course...
if theres a rod still connected then leave it...connected.....if considered extranious......or in other words as a back up.... A TNCS system is not permitted for this type of installation,which supplies mobile units and caravans etc.

.......

 

[Engineer54]

Why would you only provide one earth rod position for an installation of this type? Why not two or more positions or a ''Crows Foot'' , ''Delta'' or similar arrangement?? When all said and done, this is an earthing provision for a public function area, where the paying public is involved. Shame on the local authority on putting £30 to £50 before safety!!

From 850 ohms to 80 Ohms is better than a tenfold improvement, and using more than one position (such as suggested above) each with at least 2 X 5/8'' rods would/may have brought you to, or a near your first ever TN RA value on a TT system!! lol!!

Using a decent SDS drill with a suitable attachment, would have been far easier and quicker than you with a club hammer or the like. and with a lot less earth disturbance too...

 

[rantoftheday]

Why would you only provide one earth rod position for an installation of this type? Why not two or more positions or a ''Crows Foot'' , ''Delta'' or similar arrangement?? When all said and done, this is an earthing provision for a public function area, where the paying public is involved. Shame on the local authority on putting £30 to £50 before safety!!

From 850 ohms to 80 Ohms is better than a tenfold improvement, and using more than one position (such as suggested above) each with at least 2 X 5/8'' rods would/may have brought you to, or a near your first ever TN RA value on a TT system!! lol!!

Using a decent SDS drill with a suitable attachment, would have been far easier and quicker than you with a club hammer or the like. and with a lot less earth disturbance too...

Incorrect 850 to 80 is not even a 4 fold improvement - think about it. The clue is in the name - FOLD - engineer??????

 

[GLENNSPARK]

.......

yep sorry....i didn`t read the post properly....

 

[Engineer54]

Incorrect 850 to 80 is not even a 4 fold improvement - think about it. The clue is in the name - FOLD - engineer??????

Yep, your right, thinking of something completely different!! But it's still a massive difference, what ever way you look at it!! lol!!

 

[Deleted member 9648]

Was hoping for more replies on this but never mind.....The main reason I was asking the question is because it has occured to me that in my area where TT's are pretty common I have NEVER seen the kind of multiple rod low Ra values which the majority on here seem to claim they are installing. At least on the kind of small scale (up to single dwelling) size TT's I encounter they are always single rod and nowhere near TN values.
Does make me wonder how many actually practice what they preach.....perhaps they are talking the talk rather than walking the walk.

 

[Silly Sausage]

Incorrect 850 to 80 is not even a 4 fold improvement - think about it. The clue is in the name - FOLD - engineer??????

Or even a single order of magnitude improvement!

 

[Engineer54]

I see you're living in Sussex, so i can't see how you haven't seen low Ra levels, especially anywhere around the broads or near the sal****er marshes etc... I have always tended to think of that part of the country as having good soil conditions and being very fertile!! Always thought you were based up North as they say, where i could've better understood unfavourable ground conditions.

 

[Deleted member 9648]

I see you're living in Sussex, so i can't see how you haven't seen low Ra levels, especially anywhere around the broads or near the sal****er marshes etc... I have always tended to think of that part of the country as having good soil conditions and being very fertile!! Always thought you were based up North as they say, where i could've better understood unfavourable ground conditions.

Maybe you've been abroad too long as Sussex is south coast....a long way from the broads!. My area has sandstone,chalk and clay regions.....the clay seems to be most consistant as far as Ra values go. But I am quite truthful when I state that none of the TT installs I encounter around here are anything like the TT installs which everyone claims to be installing.

 

[Engineer54]

Maybe you've been abroad too long as Sussex is south coast....a long way from the broads!. My area has sandstone,chalk and clay regions.....the clay seems to be most consistant as far as Ra values go. But I am quite truthful when I state that none of the TT installs I encounter around here are anything like the TT installs which everyone claims to be installing.

Your right, i must have been out of the country far too long, getting confused with Suffolk/Norfolk area ...lol!!

The closest i've ever driven a rod to you, was for an uncle in Broadstairs Kent (along the same coastline). Can't remember what the final Ra was now, but as i remember, it was more than acceptable.

Have you ever tried soil conditioning on any of you're TT installs?? A little while ago i posted what i thought was a good idea for any electricians that deal with a lot of TT installs. It centres around buying a relatively cheap fence post auger, extending the shaft on a 75 mm auger bit to around a metre or so and using this go down a metre into the soil Fill the excavated section with Bentonite slurry and then driving in two or more 1.2m rods (or one 2.4m rod etc). This will almost guarantee you a very low Ra...

 

[Deleted member 9648]

Your right, i must have been out of the country far too long, getting confused with Suffolk/Norfolk area ...lol!!

The closest i've ever driven a rod to you, was for an uncle in Broadstairs Kent (along the same coastline). Can't remember what the final Ra was now, but as i remember, it was more than acceptable.

Have you ever tried soil conditioning on any of you're TT installs?? A little while ago i posted what i thought was a good idea for any electricians that deal with a lot of TT installs. It centres around buying a relatively cheap fence post auger, extending the shaft on a 75 mm auger bit to around a metre or so and using this go down a metre into the soil Fill the excavated section with Bentonite slurry and then driving in two or more 1.2m rods (or one 2.4m rod etc). This will almost guarantee you a very low Ra...

No I havent tried soil conditioning....and nor have I encountered any other installs where this is apparent.
Oddly I had another Ra issue yesterday.....on a roof extension to a terraced property which was a TT. Final testing on completion showed an Ra of over 300 ohms. Two coupled rods got it to 24. At the moment there is an existing DB with RCBO's and a main switch,am currently trying to convince the owners to swap the M/S for a 100ma s type.

 

[Marvo]

Final testing on completion showed an Ra of over 300 ohms. Two coupled rods got it to 24.

Wow, you guys have no idea how lucky you are, I wsh we got those kinds of Ra's with just two coupled rods.

In six months or a years time your new rod will probably be down to 15ohms so it does beg the question why was the original rod such high impedance?

 

[Deleted member 9648]

Wow, you guys have no idea how lucky you are, I wsh we got those kinds of Ra's with just two coupled rods.

In six months or a years time your new rod will probably be down to 15ohms so it does beg the question why was the original rod such high impedance?

The original rod was located 6" from where two DNO cables dropped into a duct in the ground,the original installer had bottled it and only gone in about 2.5 ft! We shifted the electrode position well away from any services.

 

[Marvo]

the original installer had bottled it and only gone in about 2.5 ft!

You can't be serious!! They need to start including a minimum required depth in the regs me thinks. I also can't believe he got 300ohms at that pitiful depth. You must have superb ground for earthing and it makes me depressed just hearing things like that, I've had two separate spikes at over 8m deep and not got 300ohms on more than one occasion