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Wiring a metal container

cydodimead
I have a job where the client wants some power and lighting in 2 containers in the yard of their unit. Conveniently there happen to be a couple of 16A single phase commando sockets (protected by RCBO's 16A 30mA) on the exterior wall of the unit a couple of metres from where the containers are located. Would I be right to treated the container as a Caravan? Supply to unit is TN-C-S (PME) so I don't want to export the PME earthing to the containers. I'm thinking the following to meet requirements.


1. Swap RCBO's for MCB's

2. Put 2 pole 30mA RCD in line (perfect bit of trunking on inside of building where I can put an enclosure with the RCD)

3. Small DB inside Container with MCB for twin 13A socket, and 2nd MCB for lights. All protected by double pole RCD. Earth from 16A socket on building not connected to container DB.

4. Earth stake from container DB through tarmac ground soil. Could I link bonding from one container to the other?


Having never wired a caravan pitch, caravan or container I just wanted to be sure I have read the regs correctly?
  • 0
    UKPN:

    Quite correctly, you choose not to use the PME supply, The ESQCR regulations make it clear that this kind of structure, with a steel covering needs an independent earth. The DNOs requirement is 2 meters separation from any metalwork connected to the PME network, which I am confident you can establish. 

    Regards, UKPN ?Zap


     


    The BS 7671 requirements for simultaneous contact between earthing systems is 2.5 m (arm's reach).


    As I said earlier, the separation of earth electrodes to buried metalwork underground is a separate requirement.


  • 0
    lyledunn:

    AJ, just to be clear, what PME potential are you referring to, is it a heavily loaded neutral or a broken one?


    Either is of course possible. The impact of that will depend on a number of factors.


  • 0
    Sparkingchip:

    The correct answer is probably TT the entire installation.


    Andy B.


    The latest guidance for that which has been published in Annex H of the IET Code of Practice for Electric Vehicle Charging Equipment Installation, 4th Edition, is 10 m separation underground between buried metalwork and earth electrodes connected to the converted TT system, and buried metalwork and earth electrodes connected to the PME system.


  • 0
    My thinking is that a yard where work activities may or will take place will probably have extension leads running all over the place.


    There could easily be an extension lead from the main building or the external sockets mounted on it which are connected to the PME earth system running into the steel cabins which are TT earthed, or maybe even vice versa.


    St the very least I would be looking at making the existing external sockets TT, but if they are mounted on a steel clad building or there’s air conditioning units or the like within reach it needs a bit more consideration. 


    I posted a picture of a TT EV charging point on a steel building the other week I was stood looking at , I will find it again.


    Andy B
  • 0
    I looked at this and thought although it's convenient the EV charge point would actually be better on a freestanding pillar at the other end of the car park with its rod under it.

    c7e85226544fcd939ba0a8027375cf02-original-20200517_133131.jpg
  • 0
    If you are being really picky it probably is possible to park a car inside that unit and pass the charging lead back in through the personal door to charge it, it’s definitely possible to park within reach of metalwork connected to the PME earth system.


    There is no particular risk from using that charging point under normal circumstances, but the separate earthing zones are not clearly defined and separated. It is actually quite a challenging installation, had the installer suggested digging a trench and running a cable up the car park to a free standing pillar then reinstating, everyone would probably think he was going completely over the top.


     Andy B.
  • 0
    AJ, just to be clear, what PME potential are you referring to, is it a heavily loaded neutral or a broken one?

    Both really. The biggest danger comes from a broken PEN, but if we're picking a TT system specifically to avoid the dangers of a PME system the the TT earthing systems needs to be permanently beyond the hazardous influence of the PEN conductor, so we'd naturally avoid 'normal' raised potential on the PEN as well.


    It would be less of a worry in the situation where PME would be acceptable but we're having to resort to TT only because of the lack of a supplier's facility (supplied by open 2-wire overheads for example) - where any fortuitous connections (say via bonding) to PME earth might be considered just a beneficial reduction in Zs. But situations like the OP's where PME is specifically to be avoided we need to be stricter I think.


       - Andy.
  • 0


    When I was at college doing the C&G we had a discussion one evening about locating electric fence earth rods on farms.


    Many farmers position the fencing unit in the workshop, some then run the earth cable from it to installation earth rod as it’s nice and handy. If there is a fault on the installation the voltage on the installation rod can be picked up and transmitted to the electric fence at National Grid voltages, even having a separate rod in close proximity can be lethal. Just connecting the fencer earth to the steel frame of the building can have all sorts of issues.

    https://www.fwi.co.uk/livestock/grassland-management/how-to-put-up-an-effective-electric-fence-for-livestock


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