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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!
 
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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
 
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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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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!
 
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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....
 

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