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?

 

[Clive P]

Hi,

Just a few thoughts to get things going.
To try and rectify something that is not satisfactory is far harder than trying to install from scratch. At least with a rewire a proper design and layout can be put in place which would be far superior to what you have now. Personally I don't like the idea of 30mA up front especially for Workshops and ramps, Yes for the circuits deemed necessary but not at the DB.
As for the TT Earthing, shout loud enough and E54 may hear you, he's your man for this.

Regards.

 

[Welchyboy1]

thanks for the input

eventually the site will be fully rewired as there has already been one serious fire due to overload on cables too small and fuses too big

i have to get some pics of this place its UNBELIEVABLE!

but we have to start on the submains and switchgear as this is the worst and work outwards



but regarding the new install, i really dont want to protect a whole board with 30mA up front, its a crap design but, not much options

but at the same time i dont want a small fault on one building to take out everyone

and the kind of stuff they are using, is prone for problems

its a bit of a ballache tbh

 

[GLENNSPARK]

get em some rods in early doors...
like tomorrow....
thats the first thing...

 

[Engineer54]

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

This isn't a Domestic installation, ....you'll be needing a bit more than a couple of coupled 5/8'' rods on this installation!!
You'll be needing that sort of arrangement at each of the buildings and those rod positions linked out. What sort of building construction type are we talking about here steel framed, concrete framed brick infill, brick built or other?? Whereabouts in Essex is this industrial site??

Anyway, i'll get back here later after work, no time right now...

 

[Welchyboy1]

I was intending to site rods also at each tpn dist board location in the other main buildings too sorry

2 of the other buildings are steel structure, one is brick built, one is wood

 

[Knobhead]

This should be interesting.

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

 

[D Skelton]

Just my two cents, I will say this and this alone; on a TT like the one you have described, I would be banging in linked rods at least 16ft until I was achieveing a very stable TN value Ra at the origin of supply. 2 coupled rods just won't cut it I'm afraid.

This thread is going to get rather long winded from now on as every man and his dog tries to pipe up with RCD this and RCD that. Enjoy!

 

[GLENNSPARK]

This isn't a Domestic installation, ....you'll be needing a bit more than a couple of coupled 5/8'' rods on this installation!!
You'll be needing that sort of arrangement at each of the buildings and those rod positions linked out. What sort of building construction type are we talking about here steel framed, concrete framed brick infill, brick built or other?? Whereabouts in Essex is this industrial site??

Anyway, i'll get back here later after work, no time right now...

an earth nest....

 

[Marvo]

I'd start by knocking in a sample rod, maybe 3 x 2.3meter rods coupled together then do an Ra test. Until you get some info about the ground you're working with it's difficult to come up with a final plan or price for that matter. I'd also be looking at ways to incorporate the building and foundation steel work.

 

[GLENNSPARK]

I'd start by knocking in a sample rod, maybe 3 x 2.3meter rods coupled together then do an Ra test. Until you get some info about the ground you're working with it's difficult to come up with a final plan or price for that matter. I'd also be looking at ways to incorporate the building and foundation steel work.

well....theres the other way as well...
its called `specific ground resistance testing`....here:

 

[Engineer54]

well....theres the other way as well...
its called `specific ground resistance testing`....here:

View attachment 20147

I think you're talking about measuring ''soil resistivity'', which will give you a good idea of what the surrounding soil is going to provide, as far as earth rods and the like. Unfortunately you're illustration is for a standard 3 point Ra measuring situation. Measuring soil resistivity is always via 4 points typically using the Wenner or Schlumberger method....

 

[GLENNSPARK]

I think you're talking about measuring ''soil resistivity'', which will give you a good idea of what the surrounding soil is going to provide, as far as earth rods and the like. Unfortunately you're illustration is for a standard 3 point Ra measuring situation. Measuring soil resistivity is always via 4 points typically using the Wenner or Schlumberger method....

yes sorry eng....the image i posted is indeed Ra testing....this is more like it:

 

[driverman]

I,m no expert on TT systems, but as two of the detached buildings are steel (I'm assuming steel beams exposed) they should be earthed 100% separately from the intake position?

 

[D Skelton]

Why???

 

[driverman]

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!

 

[Welchyboy1]

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

 

[D Skelton]

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!

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.

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

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.

 

[Welchyboy1]

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.

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

 

[D Skelton]

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

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).