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Discuss Earth rod query in the Electrical Wiring, Theories and Regulations area at ElectriciansForums.net

D Skelton

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So we all know that the regs say that a TT systems Ze should be below 200 Ohms as anything above could be unstable.

My question is when installing an earth rod throws up results higher than 200 ohms, say 300 or 400 ohms, what is the point really in installing secondary/tertiary earth rods?

I recently rewired a TT installation and was called out by my Elecsa assessor on my assessment visit because the Ze was 212 ohms. He asked if I was planning on whacking another rod in to which I replied "no".

He spent some time questioning me over the issue whilst I basically explained to him that I couldn't see the point. In short, the earth rod was in the best possible position to reduce the likleyhood of fluctuating resistance values however the ground conditions surrounding the property were so bad that I'd have had to probably whacked in another two or three throughout the customers rather small garden to have even stood a chance at bringing it down to a more reasonable 100 ohms and even then, what would be the point. The fact is I'd have needed a Zs of 1.44 ohms for ADS to operate within 0.4 seconds during earth fault conditions and that is just never going to happen. We rely on RCD's in TT installations (rightly or wrongly) which allow us a permittable Zs of 1667 ohms anyway.

Anyway, after our conversartion, the assessor seemed more than happy with my reasoning and was happy with the installation but my question is, who came up with this 200 ohm figure in the regs and why? I mean I know it's a guide, but why 200 ohms? 300 ohms would be just as meaningless, as would 1 kohm or alternatively 20 ohms.

Tin hat on for the whole 212 ohm thing :D
 
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Richard Burns

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You hope that the IEE have done extensive assessment of the earth conditions around the UK and used the worst case scenario to assess the likely contact that the rod would have with the earth and that if the rod is in good contact even with poor earth conditions that the resistance would be less than 200ohms. Therefore a value above 200ohms may indicate that there was not good contact with the rod and that the value may then fluctuate with varying conditions.

More probably they have had complaints over time when rods had >200ohm resistance and therefore assumed 200ohms may be a good value.

This is all conjecture and it could well be a guess by someone ages ago and now enshrined in the regulations!
 
18.2.4.3 of BS 7430 recommends not exceeding 100 ohms for a 30mA RCD due to potential instability, so not quite sure where or why the IET seem to bandy this 200 ohm value from

The reason we want to achieve stability rather than value is to ensure touch voltages do not exceed that magic 50v, why 2 different British Standards give 2 different values is open to conjecture
 
One of the reasons for the 100/200 Ohm value was the average degradation deviance caused by drying/freezing of soil conditions.
As always, depth is king with earth electrodes-once you pass through the first 0.5 Metres or so of soil, any freezing/drying effects should be minimal so the electrode Ra reading should be very stable.

So, if you manage to drive in a rod to say 2.4 Metres (good luck) and attain an Ra of around 300 Ohms, that 300 Ohm value will barely deviate due to depth.
Then you have your 1.2 Metre rod with an Ra of say 50 Ohms on the day of installation-that could increase dramatically during exceptionally dry/freezing conditions due to the lack of depth!

I'd take the deeply driven rod with higher Ra any day.
 

D Skelton

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  • #8
One of the reasons for the 100/200 Ohm value was the average degradation deviance caused by drying/freezing of soil conditions.
As always, depth is king with earth electrodes-once you pass through the first 0.5 Metres or so of soil, any freezing/drying effects should be minimal so the electrode Ra reading should be very stable.

So, if you manage to drive in a rod to say 2.4 Metres (good luck) and attain an Ra of around 300 Ohms, that 300 Ohm value will barely deviate due to depth.
Then you have your 1.2 Metre rod with an Ra of say 50 Ohms on the day of installation-that could increase dramatically during exceptionally dry/freezing conditions due to the lack of depth!

I'd take the deeply driven rod with higher Ra any day.
Even still, I think in the worst case scenario I cant see that rod fluctuating more than around 50 ohms, but lets say for arguments sake that it fluctuates within an 800 ohm range, it will still be just as effective as a stable 3 meter rod at 50 ohms. Thats why I can't understand the random number in the BGB.

Can you honestly say you've ever seen/tested an earth rod with more than 1667 ohms?

As for the original earth rod at this installation, it was so poorly installed only about a foot of it was actually buried and the Ze then was still only 400 ohms!
 
Even still, I think in the worst case scenario I cant see that rod fluctuating more than around 50 ohms, but lets say for arguments sake that it fluctuates within an 800 ohm range, it will still be just as effective as a stable 3 meter rod at 50 ohms. Thats why I can't understand the random number in the BGB.

Can you honestly say you've ever seen/tested an earth rod with more than 1667 ohms?

As for the original earth rod at this installation, it was so poorly installed only about a foot of it was actually buried and the Ze then was still only 400 ohms!
I totally agree with you and I've tested (literally) hundreds of earth electrodes over the years and rarely found a problematic Ra value.
The difficult part is measuring an Ra value of say 300 Ohms on an inspection but not knowing what depth of rod has been used-you then have to make an assessment based on likely soil conditions as to whether this reading is acceptable and likely to be stable.
 
E

Engineer54

Even still, I think in the worst case scenario I cant see that rod fluctuating more than around 50 ohms, but lets say for arguments sake that it fluctuates within an 800 ohm range, it will still be just as effective as a stable 3 meter rod at 50 ohms. Thats why I can't understand the random number in the BGB.

Can you honestly say you've ever seen/tested an earth rod with more than 1667 ohms?

As for the original earth rod at this installation, it was so poorly installed only about a foot of it was actually buried and the Ze then was still only 400 ohms!
So..... What size and length of earth rod did you install at this TT installation then??

Can't tell you anything about on how these numbtey Ra values managed to get into the Regulations. The whole sorry section needs a complete revision as far as i'm concerned... When i came into the industry 10 ohms (and often bettered) was the figure electricians aimed for, but then they were using 8 to 10 foot earth rods in those days, NOT the 1m twig things they are now fooling themselves into thinking are fit for purpose!!

I notice that once again, no-one has mentioned anything about chemical soil conditioning, when faced with high Ra values? The art of making and maintaining TT systems, seems to be yet another area in our industry that has taken a nose dive into the realms of obscurity, or left to specialist companies....

Rather than go through the whole 9 yards yet again, read some of the many threads here, as to why you should be aiming for good stability and the lowest Ra value you can reasonably obtain, and why you shouldn't be totally relying on a single RCD protective device on a TT system....
 

D Skelton

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So..... What size and length of earth rod did you install at this TT installation then??
5/8" x 4ft

Rather than go through the whole 9 yards yet again, read some of the many threads here, as to why you should be aiming for good stability and the lowest Ra value you can reasonably obtain, and why you shouldn't be totally relying on a single RCD protective device on a TT system....
I've looked but couldn't find any. I'm guessing that you're referring to posts within threads, if so, could you point me in the direction of the right ones :)
 
E

Engineer54

5/8" x 4ft
So you've only driven in one rod, you really need to extend that rod with another. The gain will be stability of your TT system and an improvement of your final Ra value....

I've looked but couldn't find any. I'm guessing that you're referring to posts within threads, if so, could you point me in the direction of the right ones :)
I'm off to bed now, it's late (or should i say early ...lol!! ) but you shouldn't have too much trouble finding the threads on earth rods and TT systems where i have posted. Maybe someone here will link you up. If not i'll have a look through tomorrow for you....
 

D Skelton

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So you've only driven in one rod, you really need to extend that rod with another. The gain will be stability of your TT system and an improvement of your final Ra value....
I go back to my original question... What is the point? There's no difference between 200 and 1000 ohms when it comes to an earth rods effectiveness??? This is the part I struggle to understand.
 
I go back to my original question... What is the point? There's no difference between 200 and 1000 ohms when it comes to an earth rods effectiveness??? This is the part I struggle to understand.
I can't be bothered to open up the 'let's get TN values of Ra' debate again but you're right, there's no difference.
Even in the grim, damp North of England, if we get an Ra of less than 15 Ohms we're doing well. I'd like to shadow the 'I always go for less than 1 Ohm' brigade just to see where we're going 'wrong' ;)
 
E

Engineer54

I can't be bothered to open up the 'let's get TN values of Ra' debate again but you're right, there's no difference.
Even in the grim, damp North of England, if we get an Ra of less than 15 Ohms we're doing well. I'd like to shadow the 'I always go for less than 1 Ohm' brigade just to see where we're going 'wrong' ;)
You mean to tell me, that all these periodicals, reports and statics, and god knows what else etc, that surround you, hasn't been able to tell you how?? I'm shocked!! :shocked3: ;)

As for that statement ''you're right, there's no difference. (200 and 1000 ohms)'' doesn't even warrant an answer. But why bother with installing a TT system at all , kinda springs to mind!!!

If were talking just domestic, then in all likelihood a couple of 5/8'' coupled rods, should be the minimum requirement. (Even that seems to be a problem for you, going by a previous post on this thread). But if were talking commercial premises or industrial of any size, why would you ''Not'' aim for a sub 1ohm Ra value?? ....Oh yes i forgot, your thinking about that cure all RCD that never fails...
 
You mean to tell me, that all these periodicals, reports and statics, and god knows what else etc, that surround you, hasn't been able to tell you how?? I'm shocked!! :shocked3: ;)

As for that statement ''you're right, there's no difference. (200 and 1000 ohms)'' doesn't even warrant an answer. But why bother with installing a TT system at all , kinda springs to mind!!!

If were talking just domestic, then in all likelihood a couple of 5/8'' coupled rods, should be the minimum requirement. (Even that seems to be a problem for you, going by a previous post on this thread). But if were talking commercial premises or industrial of any size, why would you ''Not'' aim for a sub 1ohm Ra value?? ....Oh yes i forgot, your thinking about that cure all RCD that never fails...
It's often the case with obsessions that logic goes out of the window!
In terms of overcurrent protective device operation, there is no difference between 200 and 1000 Ohms, that was the point being made!
Of course you aim for the lowest value of Ra but with the usual constraints of time, money and most importantly, compliance with the regulations that we work to in the UK!

As for 'installing rods being a problem for me', it's quite the reverse, no problems here whatsoever, that's the point I'm making!

As usual, I'll continue to install my BS7430 and BS7671 compliant earth electrodes on TT installations and as usual, I'll ask again for 1 single documented case of a death or injury on a BS7671 compliant TT installation, knowing full well that there isn't any because like it or not, the system works....
 
E

Engineer54

like it or not, the system works....
Only in cloud cookcoo land!!

This all stems yet again, from ''You'' not having read of any occurrence, so clearly it's never happened!! ...I'll say no more.

So, if you manage to drive in a rod to say 2.4 Metres (good luck)
Indicates to me, that driving 2 rods in the ground is going to be a problem??...

Seems to me, there is almost a total lack of training and knowledge given towards TT systems in the UK these days. That includes both installation and maintenance. How many new electricians knows how to maintain or improve a rod installation with a poor or not so good Ra value, let alone even give it a thought?? Not many....

I don't deal in obsessions, and my logic is sound, unlike the current attitude towards TT systems in the UK. Bring back the 10 foot rods, and the 10 ohm max value ( that was almost always bettered). Either that or make all domestic TT CU/DB's all RCBO protection (even better DP RCBO's) with an upfront S type 100mA RCD. Which i think they should all be anyway. At the very least, that will give a more than decent earth fault protection to most domestic TT installations and get past (to a good degree), a failing 30mA RCD device...







 

Top Cat

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Arms
Only in cloud cookcoo land!!

This all stems yet again, from ''You'' not having read of any occurrence, so clearly it's never happened!! ...I'll say no more.



Indicates to me, that driving 2 rods in the ground is going to be a problem??...

Seems to me, there is almost a total lack of training and knowledge given towards TT systems in the UK these days. That includes both installation and maintenance. How many new electricians knows how to maintain or improve a rod installation with a poor or not so good Ra value, let alone even give it a thought?? Not many....

I don't deal in obsessions, and my logic is sound, unlike the current attitude towards TT systems in the UK. Bring back the 10 foot rods, and the 10 ohm max value ( that was almost always bettered). Either that or make all domestic TT CU/DB's all RCBO protection (even better DP RCBO's) with an upfront S type 100mA RCD. Which i think they should all be anyway. At the very least, that will give a more than decent earth fault protection to most domestic TT installations and get past (to a good degree), a failing 30mA RCD device...







Bored, bored, bored, bored, bored, bored!
 
E

Engineer54

And you keep banging on like you always do without knowing what i do!
Or knowing what your doing!!
 

Top Cat

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Arms
Best stick to your Meow jokes/humour...
OK, i thought i was the catty one, meow!
 
D

Deleted member 9648

I go back to my original question... What is the point? There's no difference between 200 and 1000 ohms when it comes to an earth rods effectiveness??? This is the part I struggle to understand.
I've been ploughing a relatively lone furrow on exactly this point on here for ages....and taken a lot of flack for it.
The logic of those who scoff at the Bs recommended max of 200 ohms,and then smugly state they wont settle for anything over ...say...40 ohms has always defeated me.
I would also point out that I have never accepted the single RCD on a TT....there should be a requirement for a 'master' 100ma T/D alongside the 30ma device unless it can be proved that an Ra value low enough to operate an OCPD can be maintained.
Bear in mind as well that most of the TT queries on this forum concern small installs....(not the kind of big stuff E54 deals with)....for which multiple super rods are overkill IMO
 
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E

Engineer54

I've been ploughing a relatively lone furrow on exactly this point on here for ages....and taken a lot of flack for it.
The logic of those who scoff at the Bs recommended max of 200 ohms,and then smugly state they wont settle for anything over ...say...40 ohms has always defeated me.
I would also point out that I have never accepted the single RCD on a TT....there should be a requirement for a 'master' 100ma T/D alongside the 30ma device unless it can be proved that an Ra value low enough to operate an OCPD can be maintained.
Bear in mind as well that most of the TT queries on this forum concern small installs....(not the kind of big stuff E54 deals with)....for which multiple super rods are overkill IMO
The flack came and rightly so, from you advising junior members to bung a twig in the ground and if you saw 200 ohms or less, that's all you needed for a TT system, and not bothering any further...

I think that i have stated many times now that as far as domestic installations go, a couple of coupled 5/8'' rods (coupled) should be the standard installation, coupled with an up-front S type RCD device. Do you really think that's over the top, considering that the TT installation you have installed should be robust enough to last a min of 30 to 40+ years??

Now let's say that driving 2 rods in the ground on a domestic installation gave you an Ra value of say 10 ohms. You know that in all probability, the stability is going to be present. Would you approach the client and give them a choice of adding another rod, thus getting down, pretty much close to the 1 ohm mark (after settlement) or not?? Which is the other point i've been trying to make over the many threads on this subject... Wherever or whenever possible, go that extra distance to get the desired results, and not just rely on what it says in the Reg's...

As for commercial and industrial installations that have a number of employee's etc why wouldn't you be trying to aim for TN values?? A 200 ohm's max in such circumstances, is complete tosh!! Oh, ...and what you call ''Super Rods'' were once the standard sized rods used in the UK and everywhere else and STILL ARE, in most of the countries i've ever been.... I actually saw for the first time, a 1m 5/8'' rod while in an electrical wholesalers in Cyprus while i was home a short time ago. I asked if that was the standard size they were using now in Cyprus. ...What i got was a blank look, he then disappeared out back and came back with a what looked like a 2.4m rod, ''first one of these, ...then one of these'', pointing to the 1m rods!!! So even in Cyprus the one metre rod is classed as no more than an extension piece!! lol!!!
 
E

Engineer54

Maybe he's clairvoyant the same as you!
Yep, ...must be!! lol!! :D
 
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