Earth Loop Impedance on a TT system

[trev]

my old man used to grow lattice for salads. they were always above ground, never buried. :stupid:

My old man said follow the van. Why, I have no idea.

 

[Jock565]

Hi,....if you've got a problem like this, a nest of rods pulls the readings down...but I find peeing in the hole works very well also,.....

 

[Engineer54]

Only you can determine the condition of the ground and therefore make best judgement as to whether going deeper is better or more rods will be as effective. If the ground is poor conductivity for three metres and your are only putting rods to a depth of two meters then I doubt the reading will improve vastly, but put an additional Rod in slightly further afield then you may have much better quality ground therefore better earth reading.
The general rule is to get the largest surface area in as good ground as possible.

If your talking about a single point location, your going to ALWAYS achieve a better and and far more stable Ra value with deep driven (or bored rod electrode surrounded with a conductive compound or mortar) rather than a lattice mat buried at 1m!! And you can take that to the bank!! (just not a Cyprus bank though!!) lol!!

If your going for a 2 position earth electrode system, they will both still need to be at least 2.4m in depth, and be outside of each others zone of influence!!

Lattice mats are a substitute for a deep driven rod, where ground conditions aren't favourable to drive rod electrodes. eg, rocky outcrops at shallow depths etc!! Actually copper tape laid in extensive Bentonite lined trenches for the same rocky conditions, would also give superior values compared with a single lattice mat....

 

[Sharpend]

I'm not going to dispute that the deeper you go the better ground you're going to get,but its not always the most practical without machinery? And before you say it, yes I know that you'll need a machine to get a lattice mat into the ground, but alternatives are available.!!!!!!!

 

[Deleted member 9648]

I fully agree that the Rcd will trip on both 33 to 80 ohms and also 215 ohms but do not understand the arrogant remark ' dogs danglies'. Maybe better to give this a little further thought. Why is 33 to 80 better than 215 ohms ?. Maybe this will give an increased assurance that electrode will not rise above acceptable levels. Do not wish to get in to slagging match over this. Designing and installing these on a regular basis. Earth lattice matt over earth electrode will win hands down every time.

I reserve my slagging matches for E54...:wink5:....I've given this loads of thought,believe me,and nobody,including you, have ever given a satisfactory reason why 80 ohms is better than 200. (I dont accept your suggestion of 'increased assurance that it wont rise over acceptable levels'...it would have to rise over 1400-1500 ohms before it became ineffective,and if it was rising that much something is far wrong which a hundred ohms or so at the starting point isnt going to change.
....

 

[Buckfast]

Thanks for your more measured response. I think both yourself and another poster took my original post out of context. I stated that i normally get a reading of between 33 and 80 ohms. That is just a fact i am afraid with the conditions that i generally have to install in. I did not state that this is a reading that anyone else has to emulate, quite obviously lower the better. I also stated that i generally bury at a depth of 1 meter again another poster has taken this stick and run with it without knowing, area terrain or anything else regarding the earth pad installations i am involved with. After using both rods and lattice mats for more than 30 years i still maintain that the surface area contact with substrate is better with lattice mats. (Again all sites are not the same and rightly it can be argued tape could be equally as good in different enviromental conditions).

 

[Engineer54]

I reserve my slagging matches for E54...:wink5:....I've given this loads of thought,believe me,and nobody,including you, have ever given a satisfactory reason why 80 ohms is better than 200. (I dont accept your suggestion of 'increased assurance that it wont rise over acceptable levels'...it would have to rise over 1400-1500 ohms before it became ineffective,and if it was rising that much something is far wrong which a hundred ohms or so at the starting point isnt going to change.
....

Yep, stick with leaving that 700 ohm TT system (you mentioned previously) in place mate and Do Nothing, rely on that existing RCD to look after all the problems!!

Originally Posted by Buckfast.... Thanks for your more measured response. I think both yourself and another poster took my original post out of context. I stated that i normally get a reading of between 33 and 80 ohms. That is just a fact i am afraid with the conditions that i generally have to install in. I did not state that this is a reading that anyone else has to emulate, quite obviously lower the better. I also stated that i generally bury at a depth of 1 meter again another poster has taken this stick and run with it without knowing, area terrain or anything else regarding the earth pad installations i am involved with. After using both rods and lattice mats for more than 30 years i still maintain that the surface area contact with substrate is better with lattice mats. (Again all sites are not the same and rightly it can be argued tape could be equally as good in different enviromental conditions).​

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Then don't make blanket statements, because in cases where your not restricted, a deep driven rod will always out perform a 1m X 1m lattice buried at 1m. (burying beyond 1m will always involve the use of excavating machinery, unless you want to be there all day!!) Also as the ground needs to be fully excavated before you place the lattice mat, the back fill will need to be well and truly compacted (preferably in 30cm layers, with a thumper machine) to gain any meaningful value!! By the way, what is the cost of a Furse 1m X 1m copper lattice these days?? I would expect, several times the cost of a couple of 5/8'' copper-clad rods and coupler!!

 

[Deleted member 9648]

Yep, stick with leaving that 700 ohm TT system (you mentioned previously) in place mate and Do Nothing, rely on that existing RCD to look after all the problems!!

Er.....if you read the post I didnt leave the 700 ohms in place and didnt 'Do Nothing'.....I got it down to a reading most on here would consider very low for a TT,including Buckfast from what he has stated as a target Ra,('on poor ground')and yourself as you have previously stated one should aim for 10-20 ohms on other threads. But it still relies on an RCD,a point you seem to overlook much of the time.

 

[Buckfast]

Thanks for your reply or should i say rant. The original poster wrote asking for ideas and advice. I think i gave a fair response. I do not think he asked for someone to continually re iterate nonesense, which you are doing. Calm down, move away from the keyboard and get some friends. Believe me i am fully aware of both rod , mat and also tape installations. This forum is for fair and measured advice not as you seem to think an ego trip. As i am not prepared to waste valued time on your futile responses by all means feel free to make any sarcastic comments or jibes. I will not be responding to your posts from here on in.

Yep, stick with leaving that 700 ohm TT system (you mentioned previously) in place mate and Do Nothing, rely on that existing RCD to look after all the problems!!



Then don't make blanket statements, because in cases where your not restricted, a deep driven rod will always out perform a 1m X 1m lattice buried at 1m. (burying beyond 1m will always involve the use of excavating machinery, unless you want to be there all day!!) Also as the ground needs to be fully excavated before you place the lattice mat, the back fill will need to be well and truly compacted (preferably in 30cm layers, with a thumper machine) to gain any meaningful value!! By the way, what is the cost of a Furse 1m X 1m copper lattice these days?? I would expect, several times the cost of a couple of 5/8'' copper-clad rods and coupler!!

 

[Engineer54]

Thanks for your reply or should i say rant. The original poster wrote asking for ideas and advice. I think i gave a fair response. I do not think he asked for someone to continually re iterate nonesense, which you are doing. Calm down, move away from the keyboard and get some friends. Believe me i am fully aware of both rod , mat and also tape installations. This forum is for fair and measured advice not as you seem to think an ego trip. As i am not prepared to waste valued time on your futile responses by all means feel free to make any sarcastic comments or jibes. I will not be responding to your posts from here on in.

I don't do rants, or futile responses and i certainly haven't talked any nonsense!! There is nothing that i can see, that can be considered or construed sarcastic or otherwise in the replies to your posts!! I'm afraid it's your problem if you have a thin skin.... I'm too old in the tooth and been around too many blocks to be on any ego trip, though i do apologise if i've dented yours.

I didn't say or imply your response wasn't a fair response or that using a lattice isn't an option, nor the values you are achieving, quite the opposite in fact. The only point being made to your posts, is that lattice mats have, dare i say it, have a limited use, and in general terms aren't a substitute for a well placed and installed deep driven rods. Limited in the fact that they are a far more expensive option to provide and install, and installing at just a metre depth, isn't going to overly guarantee it's stability over the seasons. They would only be called on, where installing deep driven rods isn't an option!! Now as far as i know, the OP hasn't stated that he has a problem with installing deep driven rods, only that the ground was water sodden...

Is this a rant?? Who knows and who really cares!!

 

[sparky79]

ok so for the people who seem not to know where the values come from. 21 ohms is the value that your local supply authority quote for the earth value at the their substation if you put a rod down there. this is not the reading you are trying to achieve. 1666ohms , this is the reading you need to achieve to stop a touch voltage of 50volts occurring when using a 30ma rcd. 50v/0.03A=1666ohms. also if you are using a 100ma then it would be 50/0.1=500ohms. the reading of 200ohms is the level that is deemed to be a stable reading, however a reading slightly above this as with the original post is neither here or there. you are not relying on the earth rod to operate an mcb, hence why you "have" to use rcds on all circuits on a tt system. the mcb is there to protect the circuit from overload only, in tt case this will be phase to neutral. and when the fault is to earth the rcd is the means of protection to the operator and not to the cable. so the reg of "do not use a rcd as sole means of protection" would mean you do not have a bank of rcds and no mcbs. this is where you use rcbos. so with all this in mind you as the electrician need to use some common sense and decide if you are happy with the reading. if you put in a brand new rod and get a reading of 220ohms then I would deem that as a stable reading. if it was an old bit of corroded water pipe giving a reading of 220ohms, then I would say that it should be replaced with a new rod. both surface area and depth are the factors to improve the reading. but there will be a minimum reading you will not get below as this is the resistance of the earth and substation earth rod.I hope this is of help. andy

 

[Engineer54]

Words just fail me!!

 

[Toonlad]

Words just fail me!!

Im not buying that......no way...never.....ever.... lol

 

[trev]

Words just fail me!!

I agree with your point but I doubt very much that you're dumbstruck mate. The killers have a song called Andy You're A Star. In this case, Andy you're not.

 

[Sparks1962]

lol ok but i have worked on a few tt systems and a longer rod buryied deeper usualy will bring the resistance down, i cant think of any other way that will

If it's not wet enough already then chuck a hose on it for an hour or two and then re-test! Seriously though, the only way you will reduce the impedance reading is to lengthen the electrode, but where do you stop?