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???