Earth electrode tester or not?

[D Skelton]

The problem is that water here tends to just sit on the chalk it is so thick set which is why we have such soggy ground. My experience of driving rods into south east coast chalk is that you end up with much higher values than soil, clay or loose lay chalk found towards the south west. Like I said, with 2400mm worth of copper I can almost guarantee a reading higher than 150ohms, in fact, I'd bet money on it. It just isn't economically viable to try and get lower values down here. As long as the value obtained is stable, RCDs are the only way.

The best values I have ever obtained on TT systems have always been in the clay bed about 20 miles in land.

 

[Deleted member 9648]

The problem is that water here tends to just sit on the chalk it is so thick set which is why we have such soggy ground. My experience of driving rods into south east coast chalk is that you end up with much higher values than soil, clay or loose lay chalk found towards the south west. Like I said, with 2400mm worth of copper I can almost guarantee a reading higher than 150ohms, in fact, I'd bet money on it. It just isn't economically viable to try and get lower values down here. As long as the value obtained is stable, RCDs are the only way.

The best values I have ever obtained on TT systems have always been in the clay bed about 20 miles in land.

Which is where I am...a recent TT with two 5/8 coupled rods as a replacement for an existing 700 ohm reading got 12 ohms.Some would argue I should have gone on,I would refer them to post 70!

 

[terminator]

Apologies if I have missed something, but I've often wondered what is the point of trying to achieve TN figures when the distributor's earth electrode at the supply transformer could be as high as 21 ohms (usual stated maximum)?

 

[Marvo]

I was rather enjoying the debate with Eng, now I can't be bothered.

Thanks to everyone for toning it down a bit, much appreciated. This is always an emotive issue but I also enjoy the debate and I also think it's a worthy topic.

I'll have to accept your argument there as such things are outside of my experience. Can I ask if you have any evidence of actual situations where a high Ra has contributed to the scenario you describe?

Please don't think I'm some kind of earthing expert because I'm not, there's others on this thread who probably have far more experience and theory than me. I've had two separate HVACR plant manufactures failure reports in the past, different sites, different manufacturers and years apart. Both failures were due to corrosion and both occasions convincing evidence was presented proving it was due to various earthing issues but high impedance was the main contributing factor.

This topic has always polarised opinion and ended in tears....no need to get antagonistic matey,but apologies if your thread has been hijacked somewhat!

Yeah, I'm afraid this has gone somewhat off-topic from your original question Justin, apologies for that. Hopefully your original question was answered with enough info for you to decide.

no-one is ever going to go to the effort of trying to get sub 10 ohm readings for a domestic TT
its just not cost effective for the deminishing improvements that it brings for the average home-owner.
engineering practice takes back seat to value for money in the domestic market , simple econonmics im afraid.

I think this is a big part of the problem. By allowing a rediculously high Ra in the regs it means that there's no motivation for installers to even attempt to get better readings. In fact those who are conscientious about getting the lowest Ra possible will actually be pricing themselves out of the work so the regs could actually be a deterrent to good practice.

 

[Engineer54]

1ohm? Get off lol

It's not always as difficult as you may think. You will know, or should i say a good indication as to if the possibility is there, is from the values obtained after driving in 2 X 5/8'' rods. Again this will obviously depend on soil/ground make-up, but in general terms it can be achieved, or very close to it!!

I know several areas/villages in Essex where you would have no problem obtaining better TT values with just two rods or one 2.4m rod, (and as stable as a rock) than those in near-by towns with DNO TN-S (or was) earth provision, ...i jest not!! lol!!

Originally Posted by D Skelton You have to bear in mind that most of where I live is about 18"-24" of top soil then straight into solid chalk. Absolute CRAP for any sort of decent TT system! Hence why the typical domestic I spec always includes type-s RCDs.

I have no problem at all with what you have said here, There are always going to be situations where the conditions you are facing makes little sense financially, to continue past a reasonable point.

 

[Engineer54]

i've done hundreds of earth rod installs ( no exaguration ) whilst working for a county council highways dept.
we had entire roadworks teams with mini diggers and pneumatic jack hammers to install these earth rod installations at the end of every principle street lighting network.
our goal , set by the client engineer , was 20 ohms , a good solid & stable figure in my opinion.
99% of the time we achieved it , but sometimes with extra rods , sometimes by abandoning one location and trying another spot.
i can count on 1 hand the number of times we got less than 8 ohms.
so anyone insisting that a 1 ohm reading is realistic target for a domestic TT is clearly talking out of their arse.
;-)

I can understand those figures if were talking about highway lighting and the like!! Why they don't install the earth rods prior to the sometimes very deep stoned hardcore bases i don't know. It's what we do on any on-site roads, and our bases are no-where near the depth of some of the motorway bases i've seen, ...notably the M2 upgrades in Kent.

 

[MDJ]

It's not always as difficult as you may think. You will know, or should i say a good indication as to if the possibility is there, is from the values obtained after driving in 2 X 5/8'' rods. Again this will obviously depend on soil/ground make-up, but in general terms it can be achieved, or very close to it!!

I know several areas/villages in Essex where you would have no problem obtaining better TT values with just two rods or one 2.4m rod, (and as stable as a rock) than those in near-by towns with DNO TN-S (or was) earth provision, ...i jest not!! lol!!





I have no problem at all with what you have said here, There are always going to be situations where the conditions you are facing makes little sense financially, to continue past a reasonable point.

Appreciate that fella, I have had readings of around 3 ohms before, around here with the Cotswold stone in the ground 20-30 is the norm with 2 5ft rods screwed together though.

 

[Rockingit]

Sorry chaps, but STILL no-one has actually answered my point from earlier:

If we've got a typical BS-EN60898 distribution environment, and 245V as our actual incoming voltage, what the heck difference does it make to our disconnection times being met (or not) as to whether we've got 20 ohms or 200 ohms - each is no good!

Agreed, if we can attain 1, 2 or maybe 3 ohms then we stand a chance, but anything else it's just not a logical issue.

And I also think that people need to take a reality check about the domestic market place - Mrs Jones is just NOT going to either appreciate nor pay for a gang of blokes to make a mess of her garden/drive/patio whatever and charge her a fortune for it either, irrespective of the electrical ideals involved.

 

[Deleted member 9648]

Sorry chaps, but STILL no-one has actually answered my point from earlier:

If we've got a typical BS-EN60898 distribution environment, and 245V as our actual incoming voltage, what the heck difference does it make to our disconnection times being met (or not) as to whether we've got 20 ohms or 200 ohms - each is no good!

Agreed, if we can attain 1, 2 or maybe 3 ohms then we stand a chance, but anything else it's just not a logical issue.

And I also think that people need to take a reality check about the domestic market place - Mrs Jones is just NOT going to either appreciate nor pay for a gang of blokes to make a mess of her garden/drive/patio whatever and charge her a fortune for it either, irrespective of the electrical ideals involved.

Welcome to the 'It doesnt make sense whatever they say' club Matey!....Think there may be as many as 3 or 4 of us in it now?

 

[D Skelton]

Sorry chaps, but STILL no-one has actually answered my point from earlier

I keep asking people to prove the existence of God. I know full well that there is no such thing. I make very clear that reason and logic is what leads me to believe that the existence of God is highly improbable, but that still won't stop billions of people world wide believing in some sort of deity.

I think we're past the point of resolution, you just have to accept as I have that they believe what they believe and we believe what we believe. We're right of course hahaha, but they will never see it.

 

[Engineer54]

Sorry chaps, but STILL no-one has actually answered my point from earlier:

If we've got a typical BS-EN60898 distribution environment, and 245V as our actual incoming voltage, what the heck difference does it make to our disconnection times being met (or not) as to whether we've got 20 ohms or 200 ohms - each is no good!

Agreed, if we can attain 1, 2 or maybe 3 ohms then we stand a chance, but anything else it's just not a logical issue.

And I also think that people need to take a reality check about the domestic market place - Mrs Jones is just NOT going to either appreciate nor pay for a gang of blokes to make a mess of her garden/drive/patio whatever and charge her a fortune for it either, irrespective of the electrical ideals involved.

Perhaps if you read Marvo's posts # 55/57 you will see that's not quite the case at all, it's NOT all about disconnection times. These are points i brought up many many threads ago, maybe 18 months ago or more, but as always were poo, pooed by the one track mind brigade. This time, they seemed to have agreed on the argument (well for a few hours anyway), but have sadly fallen back to the crap given in the good old BS7671 along with, it doesn't matter if you attain 20 ohms or 200 ohms!! Well as the old saying goes you can lead the horse to water, but....


Where pray, has anyone suggested anything of the sort, about gangs of guy's and digging up gardens, drives and patio's?? This is just pure OTT nonsense, designed to substantiate a stand on a position that is fast crumbling under your feet!! And probably the reason why, they're only 3 or 4 of you!!

 

[Rockingit]

Sorry Eng, you've lost me now. If its NOT all about disconnection times, in an ADS design, then what IS it about?! I understand completely the inadequacies of BS7671 in certain areas, some of us have to work with the damn thing day in and out, but if we're now going so off piste as to start dismissing dis times, secondary protection and Ra values then please someone shoot me now!

I understand Marvo's comments about functionality, but logic would suggest that if you get the rare event of high standing current throughs SMPs / whatever then as your socket outlet is 30mA RCD protected regardless, even if you have an Ra of .01 the final cct RCD is going to break regardless, so it actually doesn't matter how efficient the conductivity.

And the practical reality of Mrs Jones garden isn't all that removed from my hammed-up version....you go 1 to 2m away from footings to sink a rod and you're in the middle of someone's lawn!!

 

[GLENNSPARK]

Sorry Eng, you've lost me now. If its NOT all about disconnection times, in an ADS design, then what IS it about?! I understand completely the inadequacies of BS7671 in certain areas, some of us have to work with the damn thing day in and out, but if we're now going so off piste as to start dismissing dis times, secondary protection and Ra values then please someone shoot me now!

And the practical reality of Mrs Jones garden isn't all that removed from my hammed-up version....you go 1 to 2m away from footings to sink a rod and you're in the middle of someone's lawn!!

who said owt about someones lawn eh?
get a grip will ya....

 

[Rockingit]

who said owt about someones lawn eh?
get a grip will ya....

My grip is firmly where it ought to be! . My point is that if you draw a 1m boundary around the vast majority of domestic properties you will find an awful lot of places that no-one would want to sink a rod, cover, cable etc.

 

[Engineer54]

As i pointed out above, read Marvo's posts, he has given viable and factual reasons, to keep TT impedance's as low as reasonably possible.

I'll state again, if you consider 2 X 5/8'' rods as being the standard minimum size and depth for any domestic TT installation, your not going to go far wrong in the majority of situations in the UK. Though i can't see as an extra rod is going to be OTT on costs if your original Ra value, indicates that a very low single digit value will be achieved by doing so!!

You are going to have go to into someone lawn or whatever, no matter how many rods you may or may not install. That's the nature of any TT install.

 

[Deleted member 9648]

Perhaps if you read Marvo's posts # 55/57 you will see that's not quite the case at all, it's NOT all about disconnection times. These are points i brought up many many threads ago, maybe 18 months ago or more, but as always were poo, pooed by the one track mind brigade. This time, they seemed to have agreed on the argument (well for a few hours anyway), but have sadly fallen back to the crap given in the good old BS7671 along with, it doesn't matter if you attain 20 ohms or 200 ohms!! Well as the old saying goes you can lead the horse to water, but....


Where pray, has anyone suggested anything of the sort, about gangs of guy's and digging up gardens, drives and patio's?? This is just pure OTT nonsense, designed to substantiate a stand on a position that is fast crumbling under your feet!! And probably the reason why, they're only 3 or 4 of you!!

Because having thought about Marvo's comments on reasons other than disconnection times for an Ra value I am convinced that such things are not relevant to the small scale TT installs I and most others work on. I am also convinced that if the potential issues Marvo highlighted were realistic the IEE would have been on it way before now,they hardly need any excuse to bring out an ammendment and fill their coffers.

 

[GLENNSPARK]

]My grip is firmly where it ought to be![/COLOR] . My point is that if you draw a 1m boundary around the vast majority of domestic properties you will find an awful lot of places that no-one would want to sink a rod, cover, cable etc.

lol....
we dont need toooo much info on a pubic forum rockin.....

 

[Engineer54]

My grip is firmly where it ought to be! . My point is that if you draw a 1m boundary around the vast majority of domestic properties you will find an awful lot of places that no-one would want to sink a rod, cover, cable etc.

As i say, if the property needs a TT system installing, it has to go in somewhere no matter how many rods you intend to sink!! That's a bit of a non-starter, as far as the prevailing discussion is concerned...

 

[Rockingit]

As i say, if the property needs a TT system installing, it has to go in somewhere no matter how many rods you intend to sink!! That's a bit of a non-starter, as far as the prevailing discussion is concerned...

Well, that's agreed here, too, as far as logical arguments is concerned. All I was trying to hammer home was the point of how, whether we like it or not, in the domestic environment (which is the majority of this forum) the same practical rules just don't exist as for an industrial/commercial job. It's fine and dandy to be able to make grand statements about being able to achieve TN values when you have the resources, and budget, available to make it happen. When Sparky X has spent an hour trampling down the dahlias whilst banging two rods down at £25-30 /hr, plus £20+ on bits, Mrs Jones is tapping her feet and looking at the bill wondering what on earth does this have to do with the new light she's asked to be fitted in the front room.

 

[Engineer54]

Not sure what your saying here, if an installation ''requires'' a TT system to be installed, then your going to have to make a mess of Mrs Jones dahlias anyway, if that's the only place to drive your rods.

No-ones making grand statements either, i've made the point several, several times over, that any typical domestic house TT system, should be a minimum of 2 X 5/8'' rods or preferably a single 2.4m 5/8'' rod. Are you saying this is OTT??