TT system what set up would you use

[GLENNSPARK]

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

 

[Gazthesparky]

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

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

 

[trev]

[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.

 

[GLENNSPARK]

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

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

 

[D Skelton]

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

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.

 

[GLENNSPARK]

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

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...

 

[GLENNSPARK]

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.

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

 

[trev]

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.

 

[GLENNSPARK]

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.

that was ignorant of me Trev.
sorry mate.

 

[trev]

that was ignorant of me Trev.
sorry mate.

No bother, we cool.

 

[Gazthesparky]

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.

That Is really good and clear thanks for the info

As you state I have only come across the twig rods this was all that was supplied when I was training and carrying out domestic rewires

I can rember learning about earth rods at college and that they have to be installed at a distance of ten times the length of the rod to make sure that the zones don't interfere with each other

With regards to the earth pit I take it you dig a hole to sink the cover so it's level with the ground and then install the rod into this.

I also guess the reason for the pit is to provide a means to inspect the rod and connection when carrying out inspection and testing in the future

Thanks for the diagram of star linking rods it makes it clearer

I can see why it's better to run seperate cables to each rod rather than link all 4 with one cable

Can I just ask would you just install the cables direct in the ground at greater than 600mm or do you install mechanical protection using galv conduit or ducting ?

Thanks for the info and taking the time to share your wisdom it's really useful and good to know for future should I have to install a TT system

I won't be installing any more twigs

 

[Engineer54]

Agree with Mr Skelton to a large degree here, but i'd personally also have a rod position at each of the major buildings that would be linked back to the building containing the the incoming supply. I say this, because any of those sub-main cables to the other building can be interfered with at any time in the future, and through negligence or incompetence loose that substantial TT earthing arrangement at the main intake building. Those extra rod positions will also help lowering the sites overall Ra value.

Easy to check if these metal framed buildings are providing decent levels of Ra values. I'd bond them to the buildings MET/EMT at two diagonal locations as a matter of course anyway.

Any TT system that has been well thought out and installed will have a minimum of 30 years lifespan and more often than not considerably more. Depends very much on the soil make-up and whether these industrial units have been used for storage and affected by long term spillages of corrosive chemicals in the past.

As been stated, once you have attained stable TN values there is nothing ''risky'' about it. The RCD issues will indeed become far less of an issue. You basically treat the installation as a normal TN-S supply throughout!! If i were to include an up-front RCD, it would be more like a 500mA value than a 100/300mA, but if you attain a very low Ra value you really won't need anything, and you could actually be incorporating a nuisance tripping factor!!

On a final note, rather than not being ''Overly Concerned'' about the rods and earthing, this should initially be far more of a priority. Once you know what sort of Ra levels you have, then that is going to relate back to the whole installation, in terms of what you're going to need to supply in terms of protection etc!! I hope you have a decent SDS you can use to drive these rods into the ground?? lol!! Make up an adaptor by welding an old suitably sized socket that fits the rods driving bolt to an old cut off SDS chisel. It will cut rod driving time to a fraction of driving rods by sledge/lump hammer!! lol!!

 

[GLENNSPARK]

This should be interesting.

I've got me own thoughts, but I'm going to wait for E54 to get back.

well if its steel framed (with steel uprights) then surely multiple parallel paths already exist here?...

 

[GLENNSPARK]

Agree with Mr Skelton to a large degree here, but i'd personally also have a rod position at each of the major buildings that would be linked back to the building containing the the incoming supply. I say this, because any of those sub-main cables to the other building can be interfered with at any time in the future, and through negligence or incompetence loose that substantial TT earthing arrangement at the main intake building. Those extra rod positions will also help lowering the sites overall Ra value.

Easy to check if these metal framed buildings are providing decent levels of Ra values. I'd bond them to the buildings MET/MET at two diagonal locations as a matter of course anyway.

Any TT system that has been well thought out and installed will have a minimum of 30 years lifespan and more often than not considerably more. Depends very much on the soil make-up and whether these industrial units have been used for storage and affected by long term spillages of corrosive chemicals in the past.

As been stated, once you have attained stable TN values there is nothing ''risky'' about it. The RCD issues will indeed become far less of an issue. You basically treat the installation as a normal TN-S supply throughout!! If i were to include an up-front RCD, it would be more like a 500mA value than a 100/300mA, but if you attain a very low Ra value you really won't need anything, and you could actually be incorporating a nuisance tripping factor!!

On a final note, rather than not being ''Overly Concerned'' about the rods and earthing, this should initially be far more of a priority. Once you know what sort of Ra levels you have, then that is going to relate back to the whole installation, in terms of what you're going to need to supply in terms of protection etc!! I hope you have a decent SDS you can use to drive these rods into the ground?? lol!! Make up an adaptor by welding an old suitably sized socket that fits the rods driving bolt to an old cut off SDS chisel. It will cut rod driving time to a fraction of driving rods by sledge/lump hammer!! lol!!

just got there first eng....lol...

 

[D Skelton]

With regards to the earth pit I take it you dig a hole to sink the cover so it's level with the ground and then install the rod into this.

Yep, pretty much.

I also guess the reason for the pit is to provide a means to inspect the rod and connection when carrying out inspection and testing in the future

And also to prevent corrosion at the point of connection to the rod.

Can I just ask would you just install the cables direct in the ground at greater than 600mm or do you install mechanical protection using galv conduit or ducting ?

It all depends on the scenario, for the short domestic run to one rod then I'd normally use galv conduit for protection underground or clip the cable above ground, for a larger job that requires a star linked configuration I'd more likely be burying the earthing conductors deeper but without protection.

I won't be installing any more twigs

Twigs have their uses. Some may disagree as this is a hot topic often debated on here but if you have decent enough ground and some back up protection from a type-s then I wouldn't normally object to the the use of a twig for some types of installation (99% of the time they will be domestic of some sort, ie sheds and garages etc..). I personally will always use two coupled rods to guarantee a further level of stability (solid chalk where I live), but if I'm honest, I do this because I can, I have a ground rod driver and a demo hammer drill so driving two rods takes about 30 seconds, if I tried this with a twig it would just bend.

 

[D Skelton]

agree with mr skelton to a large degree here

Never?!?!?!?!?

 

[D Skelton]

Agree with Mr Skelton to a large degree here, but i'd personally also have a rod position at each of the major buildings that would be linked back to the building containing the the incoming supply. I say this, because any of those sub-main cables to the other building can be interfered with at any time in the future, and through negligence or incompetence loose that substantial TT earthing arrangement at the main intake building. Those extra rod positions will also help lowering the sites overall Ra value.

A very valid point here if you're looking for that extra security factor. Cost allowing, I'd agree wholeheartedly.

Make up an adaptor by welding an old suitably sized socket that fits the rods driving bolt to an old cut off SDS chisel. It will cut rod driving time to a fraction of driving rods by sledge/lump hammer!! lol!!

Or just buy an earth rod driver lol, they're only thirty squids!

 

[Engineer54]

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.

Going to have to disagree with you here, 10 times the driven depth is way OTT. You will achieve little to nothing by going over 3 times the driven depth!! A star connected rod arrangement is normally called a ''crows' or ''birds'' foot, and will consist one central rod, with three rods going off at angles in the shape of a birds foot, keeping a suitable distance between each rod. The three outer rods connect to the central rod, which in turn is connected to the MET of the building.

EDIT.... Should also point out that ''star'' and other similar layout systems are generally only used when and where space is restricted...


On your drawing, you will probably be better off running a buried (600mm depth) bare cable around the house (at least 1m out) connected to an earth rod at each corner.

 

[D Skelton]

Going to have to disagree with you here, 10 times the driven depth is way OTT.

Not really considering we're only talking about a 30m run from the main intake to each rod. For a site this size I'd say it was fine. That's my opinion anyway, it's what I was taught as a lad, it's what I have always done and it's what I will continue to do.

Any links to further info about the subject of muliple earth rod installations and the most effective distance between them would be much appreciated though. I'm not too stubborn to learn why what I'm doing is OTT if indeed it is!

 

[Engineer54]

Or just buy an earth rod driver lol, they're only thirty squids!

Zip squids is a lot cheaper than 30 squids!! lol!! Had my home made rod drivers (3/4'' / 5/8'') close on 25 years now too!!