Advice on Earthing

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We have some shipping containers and metal Polly tunnels on some land our family has just purchased... these all have power to them fed from the main building that has a 3 phase supply to it.

My question is this:

at the moment they all take their earth's from the main building via a core in the armoured cables but I'm not sure if this is correct ? do they not need to be TT'd

also there are no earth bonds to either the containers or the Polly tunnels frames... is this ok?
 
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Depends on several things, such as

type of earthing for the main supply
size and type of the armoured cables
what is actually installed in the containers (as in metal, plastic, class I, class II, lights, sockets, other) and how is it attached to the containers
type of wiring system used in the containers
 
Depends on several things, such as

type of earthing for the main supply
size and type of the armoured cables
what is actually installed in the containers (as in metal, plastic, class I, class II, lights, sockets, other) and how is it attached to the containers
type of wiring system used in the containers

Supply is TNCS with cables ranging from 6mm to 16mm. we have none corrosive floressent fittings and metal clad sockets (attached to wood back boards) cable is twin and earth in 20mm plastic conduit
 
That's pretty much as bad as it could be.

You need an electrician to resolve that mess, preferably one that is experienced in dealing with agricultural installations.
Far outside the scope of what can be resolved on this or any other forum.
 
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That's pretty much as bad as it could be.

You need an electrician to resolve that mess, preferably one that is experienced in dealing with agricultural installations.
Far outside the scope of what can be resolved on this or any other forum.

There is actually a sticker saying PME on the head if this means anything
 
This is rather dated however it is a good start. The distance between outbuildings is important, from memory may be wrong, fire regulations means a caravan needs to be 2.7 meters at least from the main building, unlikely on a caravan site to be that close, so having main building TN-C-S and caravan TT is not really a problem.

However we have seen the problem with EV charge points, and caravans stored at home, my father-in-law had a charging point for his motor caravan which was stored around 1 meter from the electric and gas meter boxes, so one could very easy touch the caravan and meter together, it was on concrete, so safer to have caravan connected to TN-C-S same as house.

The EV charging points have a monitor and if the voltage is not in the range 207 to 253 then the lives are disconnected and then the earth, not sure I like that idea, but it seems it is accepted. Others use a reference earth spike to trigger supply disconnection.

Where I worked at one time at point of aye gas terminal there was a resistor the size of a small car between the DNO earth and site earth, the latter was very good, this was not the case with a radio ham I knew, his good earth was bonded to the DNO earth and when road works damaged the supply the 4 mm² cable was reduced to copper balls.

As @flameport says it is complex, not sure all electricians understand the problems, but to simply say get an expert to do the work sounds as if as a profession we are looking for work, so better to show how complex.

Personally I don't think TN-C-S should have ever been allowed, the idea of PME was as the M denotes to have multiple earth rods, where this is the case then no real problem, but to simply combine then separate C-S without those multiple earth rods, as the radio ham found out can result in silly currents down anything which is earthed causing gas pipes to melt (where earth bond wrong side of isolator) and very nasty fires.

Where I was an apprentice we had a problem with 110 volt traffic lights, the generator was not centre tapped, and the then new radar used the centre tap, so if the earth in the 7 core cable was lost, the traffic light head could become live at 55 volt approx, I was not worried, however the county legal department pointed out 25 volt was enough to kill a cow, so since used in a rural setting, they all needed converting so earth was only an earth not part of the supply to the radar.

Seems the old 50 volt limit is being raised to 70 volt for EV charging points, not sure I like that idea, having an EV charging point at end of my garden where I park my car should the PEN be lost, it is very unlikely anyone would touch the car while the supply was faulty, the same is not true for on street charging, or even when car parked at front of house with postman, milkman etc. walking past it most days.

So we are now looking at a risk assessment, my lawn mower is class II as are most garden things, but seen patio heaters class I which if actually on a patio made of concrete likely low risk, but on my wood decking which is not going to insulate very well, not really a good idea.

Many years ago we had ELCB-v which disconnected the supply if the earth voltage went above a limit to true earth, they seemed a good idea, however too often the bonded earth and real earth got linked, and it resulted in the ELCB-v not working, so they were banned and we moved to ELCB-c or better known as RCD. The same problem arises where you have TN-C-S and TT, the idea is they should never be closer than around 3 meters and not connected together, but far too easy in error for both systems to end up linked, and then high fault currents can result.

I have never with an EICR tested to ensure TT and TN-C-S not linked, clearly I should have tested, but will be honest and admit I did not check, I assumed ELI of 0.35 then TN-C-S and higher likely TT. And likely I would have highlighted the problem due measuring ELI, but clearly it can be missed.
 
Almost certainly it should be TT and not the "Earth" exported from the house.

But as Flameport said you need an electrician experienced in dealing with agricultural installations which will not be your average domestic electrician.
 
There is actually a sticker saying PME on the head if this means anything
It merely means that the supply is TN-C-S, which you already knew - and that is at least one of the 'problems'.

As has been said, this is a situation which requires an electrician with specialist knowledge/experience.

Kind Regards, John
 

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