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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
    Certainly caravan-ish, or possibly a mobile/transportable unit or maybe just a separate TT'd outbuilding. Some of the details - e.g. all the double pole stuff for caravans is mostly because they might be taken abroad an connected to a non-polarised supply, or have foreign caravans with unpolarised wiring connected to our supplies - perhaps aren't critical in this case.


    Is there anything connected to the PME earthing system within reach of the containers? (e.g. lighting columns, EV charge points or air con/refrigeration units or perhaps the main building itself if of steel construction).


    Are there likely to be any services buried below the tarmac? (gas, water or cables?) - not just in terms of the risk of hitting them, but your rod picking up a PME voltage from the soil surrounding then.


    Also keep an eye on the need to ensure double/reinforced insulation for the live conductors as they enter the unit - so they can't cause a fault to the TT earthing system prior to the first RCD. (Especially if there's any risk of them being unplugged and plugged into a different socket that might not have an RCD upstream).


        - Andy.
  • 0
    The two could and should share an earth if they can both be touched by a person with long arms.

    With a 16A supply, strictly  you do not really need the MCBs inside and I am not sure how you make them discriminate with the ones outside, but a small CU is a very handy place to marshall earths and provide a main switch.


    Comments about other earths nearby are key, you need to be aware of that. It may be safer to use the PME earth if the ground is covered in cement or tarmac, and if there are already PME earthed street lights or similar  within touching distance.
  • 0
    mapj1:

    The two could and should share an earth if they can both be touched by a person with long arms.



    Agreed.
    With a 16A supply, strictly  you do not really need the MCBs inside and I am not sure how you make them discriminate with the ones outside, but a small CU is a very handy place to marshall earths and provide a main switch.



    An option may be to use a B10 for the socket-outlet circuit. However, it's worth remembering that selectivity has already been lost as a result of BS 7671 requirements - the 30 mA RCD in the mobile and transportable unit itself is required because of the socket-outlet circuit (and you can't guarantee it won't be hardwired in future) won't discriminate with the 30 mA RCD for the 16 A socket-outlet on the building. This is no different to a caravan.
    Comments about other earths nearby are key, you need to be aware of that. It may be safer to use the PME earth if the ground is covered in cement or tarmac, and if there are already PME earthed street lights or similar  within touching distance.

    Definitely. Key points for this:

    1. Ensure the TT earth electrode has effective separation from buried metalwork connected to the PME earthing system. GN7 recommends 10 m for caravan supplies, but (unless the DNO disagrees) you might consider using the guidance for separation for electric vehicles provided in Table H1 of the 4th Edition of the IET Code of Practice for Electric Vehicle Charging Equipment Installation.

    • Consider the risks associated with striking underground services when installing earth electrodes. Earth electrodes do not need to be a "driven rod". Some companies will not install earth electrodes without excavation in areas where there are no underground services drawings, or doubts exist about those drawings.

    • If there is buried metalwork connected to the PME earthing system under the units, or in their immediate vicinity, then depending on soil and any ground conditions, there may be a risk of returning the PME touch voltage. This risk may be lessened if the containers are sitting directly on wet ground and there's any metal-to-ground contact, but we can't really rely on that over time.


    Some other things to consider:

    • Section 717 of BS 7671 is perhaps the best fit for the installations within the container. Containers are specifically listed in item (ii)  of Regulation 717.1.

    • The outer shell of the unit is bonded in the same way as a metal caravan chassis, but the bonding conductor has to be finely stranded (717.411.3.1.2)

    • PME is not generally permitted - as you have already identified.

    • You need an identification notice according to Regulation 717.514 - effectively a "rating plate" type notice for the unit.

    • The wiring system within the unit should use flexible cables (Regulation 717.52.2). This has been in place since 2011 (BS 7671:2008+A1:2011 - 17th Edition Amendment 1) and is still the case



     Graham Kenyon
  • 0
    Thanks for all the helpful information everyone. In conclusion I'm going to do the following.


    1.Leave 16A circuits as they are with RCBO's 30mA.

    2. Small Consumer unit inside with B10 and B6 MCB's and RCD

    3. Earth rod(s) in yard as far from building as possible to avoid any metal work of the building connected to PME supply. Avoiding underground services of course.

    4. Bond containers together and onto Earth rod

    5. Bond outer shell of container with finely stranded cable back to small consumer unit.


    I think that covers all your helpful points.


    Thanks
  • 0
    These containers are normally just convenient sheds, never intended to be moved again. Personally I wouldn’t see the need to treat it as a special location and I certainly wouldn’t TT it. Sure they are sometimes set on wooden skids but assuming that you can invoke 114.1, any risk is, in my opinion minimal. I would, of course, comply with the regulations that do apply.
  • 0
    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


  • 0
    The correct answer is probably TT the entire installation.


    Andy B.
  • 0
    The correct answer is probably TT the entire installation.

    Even that can be dubious. Say the main installation had metallic gas or water supplies shared with a neighbouring property that used PME. The PME potential could then be imported into the main building via the bonding and so out to the container via the shared TT earth. Sometimes you just can't win.


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


     


    That is an interesting perspective. The ESQCR may have an impact on a customer but they do not apply to customers. As far as I can see the supplier is not to offer a PME terminal to a caravan or boat but I don’t see anything else mentioned. But that’s beside the point, once a supply is delivered to an installation then I doubt that the supplier will take a great deal of interest in what the customer connects to it thereafter. Maybe there is a UKPN scanner van that checks such things, if so there are a lot of caravans parked in driveways with orange leads going through walls and windows that should be investigated.! 


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