TT system bad earth help

[J25GTi]

"From what's been posted by the OP, this TT system is in trouble, it's certainly not showing signs of being a stable system... Probably as usual, just a single 1.2m 3/8 rod that can't be extended has been used.

By the way, any reading over a 100 ohms, especially in rain saturated ground is suspect and should be investigated, and where necessary improved..."


I could not agree more. It is definitely a bit of a mess. But thecompany are not willing to go back to the client unless it reaches the 200. I'm trying to explain to them that once it reaches 200 it is too late. And needs to be sorted out now!

 

[sjm]

"And that makes a 200 ohm Ra a good earth then does it??? lol!!! ...Do me a favour!!!"

What favour would you like me to do for you E54???
We all know your feelings about Ra values and the need to get it below 1 ohm or there abouts, which in some places here in Cornwall would be impossible without major digging, removal of Granite bedrock, soil conditioning and earth mats. I don't think many customers would be willing to pay for that when the regs give a max reccommended value of 200 ohms!!
A local electrician I know had 11 rods across a field the Ra was 210 ohms, another guy I know drove 4 rods deep to get JUST below 200 ohms.

I was merely pointing out the reasoning behind the max reccommended value of 200 ohms, if this value is measured under the right conditions then Ra could drop by up to 8 times. Giving a range of 25 to 200 ohms.
TT systems are by their very nature unstable, unless I'm missing something.

NOTE 2 To Table 41.5 of The BGB.

"The resistance of the installation earth electrode should be as low as practicable. A value exceeding 200 ohms MAY not be stable".

All that said I don't agree with the idea of banging 1 x twig into the ground and thinking that will do, a little more care and effort is called for, but we do have to live in the real world as well.

 

[J25GTi]

Again I agree. In this instance it is actally in lands end. Lol.

So yes I do understand that it isn't ever going to be 1ohm etc. But it is very easily going to be over 200 in dry conditions. And luckily this isn't a private home owner etc. Its a government agency who will just have to do what ever it takes to make it safe

 

[Engineer54]

"And that makes a 200 ohm Ra a good earth then does it??? lol!!! ...Do me a favour!!!"

What favour would you like me to do for you E54???
We all know your feelings about Ra values and the need to get it below 1 ohm or there abouts, which in some places here in Cornwall would be impossible without major digging, removal of Granite bedrock, soil conditioning and earth mats. I don't think many customers would be willing to pay for that when the regs give a max recommended value of 200 ohms!!
A local electrician I know had 11 rods across a field the Ra was 210 ohms, another guy I know drove 4 rods deep to get JUST below 200 ohms.

I was merely pointing out the reasoning behind the max recommended value of 200 ohms, if this value is measured under the right conditions then Ra could drop by up to 8 times. Giving a range of 25 to 200 ohms.
TT systems are by their very nature unstable, unless I'm missing something.

NOTE 2 To Table 41.5 of The BGB.

"The resistance of the installation earth electrode should be as low as practicable. A value exceeding 200 ohms MAY not be stable".

All that said I don't agree with the idea of banging 1 x twig into the ground and thinking that will do, a little more care and effort is called for, but we do have to live in the real world as well.

I don't think i've ever said here that you only go for sub 1 ohm values. Though i will say, that it is very achievable in many parts of the country!! As you say, there are certain area's in the UK that will be difficult to get decent Ra levels. Why anyone would bung rod after rod in, in such conditions is foolish to say the least. That's when you really start needing to make use of enhancing materials, such as conductive mortar and bentonite or even chemical enhancers etc if feasible. But then, to make full use of the better ones, you need to fill bore holes to drop your rods in, rather than drive rods in the ground!!

But your dead right, in that i have no time for the general consensus that a 200 ohm Ra is good for anything. Your aim should always be for the lowest value you can ''reasonably'' achieve... Also agree that these short twigs are totally useless as earth electrodes and it's about time they were withdrawn from the market once and for all....

 

[sjm]

I may have used the value of 1 ohm Ra for dramatic effect, lol, sorry E54.

I think we're both singing from roughly the same hymn sheet however!

 

[sjm]

Again I agree. In this instance it is actally in lands end. Lol.

So yes I do understand that it isn't ever going to be 1ohm etc. But it is very easily going to be over 200 in dry conditions. And luckily this isn't a private home owner etc. Its a government agency who will just have to do what ever it takes to make it safe

Good luck with that! Hope it goes well, but if your supervisor (or bloke who's going to sign off on it) is happy I wouldn't loose too much sleep if I were you. After all if it's a government agency/public building it should be tested every 5 years, so if they don't sort it now they'll have too sooner or later.

 

[J25GTi]

Yeah indeed, the problem is the company we have doing our testing are useless. (That's been kind)

 

[Deleted member 9648]

Years ago i had a problem with a high Ze on a TT plus rod not installed very well....drove in 3x heavy duty rods screwed together and got the reading down to 30 ohms.
Also another electrician told me that it helps to install a rod on the north facing side of a building.... if thats an option available to you.

Your 30 ohm Ra will give you a PEFC of 7.66 amps......which will not operate any normal overcurrent device. Can someone please explain to me any advantage an Ra of 30 ohms has over an Ra of <200 ohms where an RCD protects the system?

 

[Paul505]

Your 30 ohm Ra will give you a PEFC of 7.66 amps......which will not operate any normal overcurrent device. Can someone please explain to me any advantage an Ra of 30 ohms has over an Ra of <200 ohms where an RCD protects the system?

Your right... but the RCD is used for fault protection which will trip an RCD rated at 30mA with a PEFC at 7.66 amps, plus it will satisfy the (less than or equal to) 50 volt rule....30 ohms x 0.03 amps = 0.9 volts
The overcurrent device will only satisfy protection against overload in this case.
Ra of 30 ohms is more stable than 200 ohms which is an advantage, not exactly sure what you mean with the last bit in your post ?

 

[Deleted member 9648]

Your right... but the RCD is used for fault protection which will trip an RCD rated at 30mA with a PEFC at 7.66 amps, plus it will satisfy the (less than or equal to) 50 volt rule....30 ohms x 0.03 amps = 0.9 volts
The overcurrent device will only satisfy protection against overload in this case.....Exactly my point,you are totally reliant on an RCD,which will satisfy the <50v rule up to an Ra of 1667 ohms.....
Ra of 30 ohms is more stable than 200 ohms which is an advantage,Is it?....why is an Ra of <200 ohms not stable enough to provide earth fault protection with a 30ma RCD? not exactly sure what you mean with the last bit in your post ?

............

I can see the point in acheiving a sub 2 ohm Ra(as per E54) if you are not a believer in solely relying on an RCD for earth fault protection...I cant see the point in 30 ohms over 200 ohms as you are still totally reliant on the RCD for earth fault protection.

Either obtain an Ra which will operate an overcurrent device,or dont and rely on the RCD...in which Bs 7671 value of Ra are perfectly adequate.

 

[Paul505]

I definitely couldnt agree more with what you have pointed out Wirepuller....to achieve an Ra as low as possible, so both devices will protect against earth faults, so your not solely reliant on the RCD.
Also as normal, there was a higher rated RCD on the main and a 30mA RCD protecting the final circuits so you had a back up RCD anyway.
Your right an Ra of <200 ohms is stable enough according to the regs, but i would be very concerned at any high values upto 200 ohms if the ground conditions were perfect.
If i remember rightly 17yrs ago... the 20-30 ohms which was achieved was in the height of the summer, very dry conditions, not installed in the best location, known area for poor Ra readings and we got rid of the twig that was previously installed.
We did recommend that the earth rod would be more suitable in another location, or to add an additional earth rod... just before we removed the poorly installed twig...

 

[Acer]

I have posted recently on this sort of problem...(loop test problems)..and in my case it has proved impossible to sort out so the customer has had to get supply company involved....on another job I did ze on rod and got 210 ohms...the rod was not accessible as it had been built over, which appears common round here in my experience...so got another rod, banged into ground did test again and got 560 ohms!!!...this is ongoing situation but there are not many options to resite a further rod..but of course will have to try.....the ground is hard and rocky and situated on high ground so doesn't stay damp for long."however before I arrived the system had no rcd at so even though the original ze is 210ohms every circuit has at least 30ma protection....it may well be that I will have to use bigger rods etc..

 

[Engineer54]

............

I can see the point in achieving a sub 2 ohm Ra(as per E54) if you are not a believer in solely relying on an RCD for earth fault protection...I cant see the point in 30 ohms over 200 ohms as you are still totally reliant on the RCD for earth fault protection.

Either obtain an Ra which will operate an overcurrent device,or don't and rely on the RCD...in which Bs 7671 value of Ra are perfectly adequate.

As usual you can't see the point of having a low Ra and stable TT system because you've been brainwashed into this 200 ohm nonsense, because it's written in the Reg's. The idea is to obtain the best (lowest) Ra value you reasonably can, not bang a rod in the ground and if it complies with the good for nothing magical 200 ohms it's a good-un, it ain't and never will be!!! But as i remember, your advise to members, is not to even bother, great professional advise that!!! When i started in this game, 10 ohms was the value electricians were aiming for, and more often than not, surpassing that value... But then they were using 3m 5/8''-3/4'' rods in those days, and not the current daft 1 metre rods....

And as stated many times, reliance on a single RCD device, is not my idea of an overall safe installation!! At the very least, such installations with this high 200 ohm values should always be backed up by an up front S type rcd device...

 

[Deleted member 9648]

I agree totally with the single RCD issue,and have stated on here many times that I believe an up front 100ma s type should be a requirement on a TT.
My argument is simply that even at 10 ohms you are still reliant on an RCD....so what is the difference in practice between 10 ohms and 200 ohms?...the RCD will still operate exactly the same at both values. I also dispute the point that only lower Ra's are stable.....having installed maintained and tested many at 100-200 ohms I can assure you that they remain stable in my area regardless of season.
As far as I'm concerned unless you are going for TN values of Ra there is no point...and smugly stating 10-30 ohms is what should be aimed for and anything else is unprofessional is simply snobbery.

 

[Paul505]

I agree totally with the single RCD issue,and have stated on here many times that I believe an up front 100ma s type should be a requirement on a TT.
My argument is simply that even at 10 ohms you are still reliant on an RCD....so what is the difference in practice between 10 ohms and 200 ohms?...the RCD will still operate exactly the same at both values. I also dispute the point that only lower Ra's are stable.....having installed maintained and tested many at 100-200 ohms I can assure you that they remain stable in my area regardless of season.
As far as I'm concerned unless you are going for TN values of Ra there is no point...and smugly stating 10-30 ohms is what should be aimed for and anything else is unprofessional is simply snobbery.

Out of interest....if somebody was to install an earth rod on a TT and the Ra was bang on 200 ohms, then years later you tested the earth rod at an Ra of say 280 ohms with the install value unknown to you, what would your opinion be of the person that installed it.....?