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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?
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  • 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
Reply
  • 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
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