TT system what set up would you use

[Welchyboy1]

have to re wire an industrial unit and surrounding work shops but in two minds of how to install

currently there is NO EARTH for the whole site, the place consists of a large storage shed with the mains coming in, TPN 100a supply, the place at the moment is a death trap

no current RCD protection at all(not that it would help with there being no earth)

Loop test between L1/2/3 and N is 0.57ohms and has been confirmed no PME connection is available

4 surrounding buildings rented out, few workshops, 1 mechanics hut, mainly lighting and sockets with 2x tpn hydraulic ramps, no major loads

proposed new install will be

TPN dist boards in each building, 16mm swa submains from each outbuilding back to 63a switchfuses via 100A busbar to origin



my plan was to have a 300mA s type 4 pole RCD at scource with earth rod 2x large coupled together



either 30mA rcd main switches at all distribution boards, or mainswitch isolators and 30mA DP rcbos for additional protection as required

thing is im a bit nervous about this install as i know whatever set up i choose nusiance tripping is likely in the future and may take out the whole site in some cases, so im leaning towards the 30ma RCD mainswitch at each dis board so i can at least keep a fault from knocking out all the sites at once and keep it local to the building affected

most of the existing final circuits are staying for now(i am pushing for a full site re-wire but have to start at the distribution for now)and there are lots of bodges and add ons all over the place

i have made agreements with the landlord to correct C1 and C2 faults with the testing as i find them at extra cost

have a feeling this one will bite me on the bum in the future

any input? have i missed any thing that could improve the RCD set up or other suggestions?

 

[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!!

 

[Engineer54]

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!

I'll see what i can dig up and get back this evening!!


Off out on site now, ...some of us have got to work while you lot are far away in the land of Nod!! lol!!

 

[GLENNSPARK]

I'll see what i can dig up and get back this evening!!


Off out on site now, ...some of us have got to work while you lot are far away in the land of Nod!! lol!!

not going out shopping then?..lol

 

[Richard Burns]

This basic document on P7 says multiple rods should be installed at a spacing greater than their depth.
View attachment Earth Electrodes And Earth Electrode Testing (1) (2).pdf

This slightly more HV one, in section E5 seems to use a standard spacing of 3.6m for 2.4m rods.
View attachment East Earthing Manual Feb 2002.pdf

This one on pages 40/41 states that rods should be separated by at least their insertion depth
View attachment pub-119-earthing-practice.pdf

However this does not mean I fully understand everything they are saying!!
Just some references.

 

[Engineer54]

Grateful for R Burns to post those PDF's above, as i don't have a PDF of the data i typically use. But generally concludes with that of the ''East Earthing Manual'' above, except it goes into a little more detail of when you need to extend distances between connected rod/electrode positions. That will generally be down to soil conditions, eg the better (lower resistivity values) the soil conditions, the greater the distance required, so as not to overlap zones of influence, but i hasten to add, will also generally require less rod positions, if working to attain a specific Ra value.

I'm far more used to installing ground fields (ground nests) and based on soil resistivity and the depth of rods, we will generally space the rods to 1.5X to 2X the depth of rod, and occasionally up to 3X. On a grid system of say 30 rods (wow!! ....just thought about the amount of land needed if working to 10 times the driven depth...lol!!) each will be tested for both active individual Ra value, and for any deterioration in that plateau value caused by adjacent rods, which if present, will clearly show a dip in that plateau. Rarely have i ever seen that plateau dip during final testing. BTW all our rods are copper clad steel 3.2m X 3/4'' supplied by the industry leader in all things on Earthing!! ...lol

 

[D Skelton]

Cheers RB, I shall have a read through them later