TT system what set up would you use

Skelton, good question. I was told whilst doing 2395 a bit back exactly that with detached steel buildings. I don't know the reason for this though. I was trying to take in everything else!

im not overly concerned at this point about the rods, the soil around most of the area i live is dense clay, i hardly ever get reading above 25ohms with a decent length rod even in the mid summer months, the moisture in the clay is pretty constant once you penetrate to a certain depth, also the structural steel will obviously be main bonded too so a stable RA will likely not be an issue, and i have left a margin in the price for any problems

its the rcd configuration i was more concerned about

driverman said:

Skelton, good question. I was told whilst doing 2395 a bit back exactly that with detached steel buildings. I don't know the reason for this though. I was trying to take in everything else!

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Whoever was telling you that is talking rubbish. The only reason I could see the need to separate the earthing system for a detached steel outbuilding and provide it with its own TT is if the main supply is PME.

Welchyboy1 said:

im not overly concerned at this point about the rods, the soil around most of the area i live is dense clay, i hardly ever get reading above 25ohms with a decent length rod even in the mid summer months, the moisture in the clay is pretty constant once you penetrate to a certain depth, also the structural steel will obviously be main bonded too so a stable RA will likely not be an issue, and i have left a margin in the price for any problems

its the rcd configuration i was more concerned about

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Get TN values then your RCD configuration becomes less of a worry.

Me personally, I'd be banging in rods until I got a good 0.5 ohms, lower if possible and scrapping the idea of an RCD altogether on the main distribution arrangement, depending on how happy I was with the stability of the Ra value would determine whether or not I would protect each sum main with a type-s, then I'd just be worrying about the 30mA RCDs needed on the finals for additional protection if needed. If like you say the area is dense clay then a sub one ohm Ra value is a piece of cake. I reckon three or four starlinked 16 footers would do the job perfectly!

Once done I'd be monitoring the Ra monthly for about a year to be sure of its stability.

One thing is for sure, I'd rather have one strong earthing arrangement at the origin of supply and an exported earth to each outbuilding than a bunch of weak seperate TTs at each outbuilding relying on their own RCD protection to meet the requirements for ADS.

D Skelton said:

Whoever was telling you that is talking rubbish. The only reason I could see the need to separate the earthing system for a detached steel outbuilding and provide it with its own TT is if the main supply is PME.




Get TN values then your RCD configuration becomes less of a worry.

Me personally, I'd be banging in rods until I got a good 0.5 ohms, lower if possible and scrapping the idea of an RCD altogether on the main distribution arrangement, depending on how happy I was with the stability of the Ra value would determine whether or not I would protect each sum main with a type-s, then I'd just be worrying about the 30mA RCDs needed on the finals for additional protection if needed. If like you say the area is dense clay then a sub one ohm Ra value is a piece of cake. I reckon three or four starlinked 16 footers would do the job perfectly!

Once done I'd be monitoring the Ra monthly for about a year to be sure of its stability.

One thing is for sure, I'd rather have one strong earthing arrangement at the origin of supply and an exported earth to each outbuilding than a bunch of weak seperate TTs at each outbuilding relying on their own RCD protection to meet the requirements for ADS.

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I must admit i have never even tried to achieve TN levels of RA, not really considered it an option before

thing is it must be a bit risky, as if due to corrosion or whatever other reason, maybe in years to come, the RA was to raise a little, theres not much of a margin before all my ZS values are exceeded and the whole install will become a hazard

i dont think i would rest easy with that, its too tight, and the zs values for my submains BS88 fuses at 63A is only 0.82ohm with no Correction, ive got to include the runs of cable on top of that too

and i dont really have an option to protect each submain independantly either with an s type, as there will be four switch fuses from a busbar, so will need to get the s type before all that

Welchyboy1 said:

I must admit i have never even tried to achieve TN levels of RA, not really considered it an option before

thing is it must be a bit risky, as if due to corrosion or whatever other reason, maybe in years to come, the RA was to raise a little, theres not much of a margin before all my ZS values are exceeded and the whole install will become a hazard

i dont think i would rest easy with that, its too tight, and the zs values for my submains BS88 fuses at 63A is only 0.82ohm with no Correction, ive got to include the runs of cable on top of that too

and i dont really have an option to protect each submain independantly either with an s type, as there will be four switch fuses from a busbar, so will need to get the s type before all that

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The Ra will only decrease over time due to ground consolidation and will not increase, the corrosion you speak of, although highly improbable would be picked up upon correct periodic inspection too. A correctly installed TT system whereby the main aim is to keep the ohmic value of the earth fault loop path as low as those seen in TN systems will be in 99% of circumstances MORE stable and safer than either TN earthing arrangements. I have personally seen TT systems with Ra values as low as 0.2 - 0.3 ohms and that aint easy considering the ground where I live (solid chalk) and I'm sure there are plenty of us on here who have seen waay lower than that! If your install is on solid clay, you should expect to be getting these sort of results with relative ease. Remember also that if you are achieving TN values, if ADS is being met by overcurrent protective devices then you are permitted to apply TN disconnection times too.

I personally think that relying on one RCD, type-s or not, to protect the entire installation is just asking for trouble and in my opinion is bad design. I would be more than happy to rely on a stable TN Ra value to protect the main distribution arrangement.

If you're used to working in the domestic/light commercial field then trying to achieve TN Ra values is just simply not cost effective and I could understand perfectly in those circumstances why someone would not want to try to achieve such a low Ra value, but in the circumstances that you are talking about I would say it would be improper to do the job any other way than the way I have already mentioned (short of relying on a different protective measure altogether).

again depth is the key here...
an earth nest arrangement would be good but not at the expense of stability...

D Skelton said:

I reckon three or four starlinked 16 footers would do the job perfectly!

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Ive not installed many earth rods. The only ones ive installed was when I was an apprentice and it was just the single rod knocked into the ground using the small black conduit box cover

just interested in hearing how others install pits and rods.

also interested how you star link several rods I know you have to knock them in at certain distances apart otherwise they are pointless. What I'm interested in is how you link them together do you run conduit in between them and link all

[QUOTE. I don't know the reason for this though. I was trying to take in everything else![/QUOTE] Advice given but the reason behind the advice is not known. Priceless.

Gazthesparky said:

Ive not installed many earth rods. The only ones ive installed was when I was an apprentice and it was just the single rod knocked into the ground using the small black conduit box cover

just interested in hearing how others install pits and rods.

also interested how you star link several rods I know you have to knock them in at certain distances apart otherwise they are pointless. What I'm interested in is how you link them together do you run conduit in between them and link all

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as I and others have already said here:
depth is the ticket....
the only way to ensure stability in all weather conditions...

Gazthesparky said:

Ive not installed many earth rods. The only ones ive installed was when I was an apprentice and it was just the single rod knocked into the ground using the small black conduit box cover

just interested in hearing how others install pits and rods.

also interested how you star link several rods I know you have to knock them in at certain distances apart otherwise they are pointless. What I'm interested in is how you link them together do you run conduit in between them and link all

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This is a proper earth pit:



It is important to note that this isn't the usual chang wang piece of excrement peddled by some electrical wholesalers which is essentially a cheap plastic box with a sticker on it like the one you describe. For a TT system where you want to achieve TN Ra values, unfortunately the 3/8" 4ft rod just won't cut it I'm afraid (I'm not bad mouthing the 3/8" rod [or the twig as it is often described], I'm purely stating that it isn't suitable for this task). You would need to use a 5/8" rod, some are sold at lengths of 12ft but more likely you will find them 4 ft in length but with the ability to couple to other 5/8" rods by using an earth rod coupler. This way you can achieve great depth and very stable results.

This is how I would star link more than one rod (apologies for the rather rudimentary drawing):



I must note that there are many more methods available when it comes to linking rods together however in many cases the method used to link them together is often down to personal preference and often down to common sense. I prefer this method over just linking from one to the next in case the link between the first rod and the MET gets broken somehow leaving the installation unearthed! It also keeps the resistance of the main earthing conductors low as it uses four shorter lengths instead of one long linked length.

If a main earthing conductor is not mechanically protected then it should be buried at a depth of at least 600mm and must comply with 542.3.1, in particular it must be sized in accordance with table 54.1.

The key to achieving effective earthing in a star linked configuration is to ensure that the distance between each seperate rod is at least ten times the length of the longest rod so as to prevent any rod from intefering with another's zone of influence. For example, if you have coupled four 4ft rods (a total of 16ft length) and sunk them in and repeated this three more times (four seperate 16ft rods linked together) then the distance between that and any other rod should be at least 50m.

Hope this helps explain things a bit better.

trev said:

[QUOTE. I don't know the reason for this though. I was trying to take in everything else!

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Advice given but the reason behind the advice is not known. Priceless.[/QUOTE]i think what Damian is getting at Trevor by star connecting the earth nest is to introduce multiple parallel paths thus bringing down yet further the value of Ra...
as opposed to series connected...

D Skelton said:

This is a proper earth pit:

View attachment 20152

It is important to note that this isn't the usual chang wang piece of excrement peddled by some electrical wholesalers which is essentially a cheap plastic box with a sticker on it like the one you describe. For a TT system where you want to achieve TN Ra values, unfortunately the 3/8" 4ft rod just won't cut it I'm afraid (I'm not bad mouthing the 3/8" rod [or the twig as it is often described], I'm purely stating that it isn't suitable for this task). You would need to use a 5/8" rod, some are sold at lengths of 12ft but more likely you will find them 4 ft in length but with the ability to couple to other 5/8" rods by using an earth rod coupler. This way you can achieve great depth and very stable results.

This is how I would star link more than one rod (apologies for the rather rudimentary drawing):

View attachment 20153

I must note that there are many more methods available when it comes to linking rods together however in many cases the method used to link them together is often down to personal preference and often down to common sense. I prefer this method over just linking from one to the next in case the link between the first rod and the MET gets broken somehow leaving the installation unearthed! It also keeps the resistance of the main earthing conductors low as it uses four shorter lengths instead of one long linked length.

If a main earthing conductor is not mechanically protected then it should be buried at a depth of at least 600mm and must comply with 542.3.1, in particular it must be sized in accordance with table 54.1.

The key to achieving effective earthing in a star linked configuration is to ensure that the distance between each seperate rod is at least ten times the length of the longest rod so as to prevent any rod from intefering with another's zone of influence. For example, if you have used four linked 4ft rods (a total of 16ft depth) then the distance between that and any other rod should be at least 50m.

Hope this helps explain things a bit better.

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yep...its all about spheres of influence here...
overlapping spheres being an inefficient way of installing rods...

It wasn't Damian I was quoting, it was someone who he had pulled for giving silly advice. I've a lot of time for Damian because he talks a hell of a lot of sense.
By the way, I completely understand the concept of earth nests,multiple parallel paths and TT systems Glenn, I've been doing this a long time.

trev said:

It wasn't Damian I was quoting, it was someone who he had pulled for giving silly advice. I've a lot of time for Damian because he talks a hell of a lot of sense.
By the way, I completely understand the concept of earth nests,multiple parallel paths and TT systems Glenn, I've been doing this a long time.

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that was ignorant of me Trev.
sorry mate.

GLENNSPARK said:

that was ignorant of me Trev.
sorry mate.

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No bother, we cool.