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Shipping container PME earthing arrangement.

Damo

There are a group of shipping containers on a landfill site which has a PME supply. They have been converted into offices, a workshop and a canteen for use by a contractor carrying out groundwork’s on part of the site. There is a supply to a  3 phase distribution board in one of the containers to which the other containers are connected by plug and socket. There is a fence running alongside the containers, less than a metre away on which there are floodlights which are supplied from the same distribution board as the one supplying the container distribution board and a length of cable tray with armoured cables on it which go on to a couple of control panels mounted a few metres beyond the containers. BS 7671 prohibits PME generally for transportable buildings building with an exception where they are under the control of skilled or instructed persons. However converting to TT would mean having two earthing systems in touching distance. What is the best way to approach this?

each of the containers has a small DB in it with a Hager fuse holder as a main switch. Obviously these are not household premises, is this acceptable.

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    Mike has some good points. I would add that the concern is likely more to be the fact that you have a metalclad building than that the buildings are transportable. The DNO will likely not be terribly impressed as a result if this becomes a permanent situation.

    It might be worth checking whether the site really is PME, or some other flavour of TN-C-S e.g. PNB, which might be more palatable.

    On the other hand if really only temporary might it be practicable to convert the to TT and install measures to reduce the risk of simultaneous contact? E.g. plastic Chapter 8 barriers preventing access to the "rear" of the containers where it's close to the fence for the duration of the work.

  • 0 in reply to Jam
    Jam said:
    It might be worth checking whether the site really is PME, or some other flavour of TN-C-S e.g. PNB, which might be more palatable.

    According to ENA Engineering Recommendation G12/5, PME conditions are considered to apply to consumers installations supplied by PNB (whatever the flavour).

    Advice to convert lighting to same TT system as containers is a good one. This really comes under Section 717 as it's written now - would recommend TT in the general case, unless the units are indoors, or outdoors on an upper storey of a building.

  • 0 in reply to gkenyon
    gkenyon said:
    According to ENA Engineering Recommendation G12/5, PME conditions are considered to apply to consumers installations supplied by PNB (whatever the flavour).

    Agreed for public LV supplies. I was thinking about a situation where there's an HV supply with private transformer to LV.

  • 0 in reply to Jam
    Jam said:
    Agreed for public LV supplies. I was thinking about a situation where there's an HV supply with private transformer to LV.

    Ah yes ... that's normally treated as "TN-S" proper ... regardless of where you sit on the "there must be at least a bit of CNE conductor" debate ... but regardless, PME conditions can only exist where there's a public supply. If the private transformer were considered "TN-C-S", this would not be PME, because there are other requirements for PME than simply a combined neutral and protective conductor.

  • 0 in reply to gkenyon

    I asked on-site whether our high-voltage/low-voltage private transformers were subject to PME conditions, and the DNO confirmed they were. However, can you confirm that for the type of system to be classed as a PME system, the supply neutral conductor is linked to several earthing points throughout the supply network, unlike the PNB variant, which connects only at a single point. This distinction was vital to address in the context of installing EV chargers.

  • 0 in reply to AMK
    AMK said:
    However, can you confirm that for the type of system to be classed as a PME system, the supply neutral conductor is linked to several earthing points throughout the supply network, unlike the PNB variant, which connects only at a single point. This distinction was vital to address in the context of installing EV chargers.

    If the LV side of the DNO transformer is owned by the DNO, even if it's TN-S today, they can convert to PME in the future, and/or supply PME feeds from it. If you have TN-S or TN-C-S, the DNO can "declare" it PME these days.

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  • 0 in reply to AMK
    AMK said:
    However, can you confirm that for the type of system to be classed as a PME system, the supply neutral conductor is linked to several earthing points throughout the supply network, unlike the PNB variant, which connects only at a single point. This distinction was vital to address in the context of installing EV chargers.

    If the LV side of the DNO transformer is owned by the DNO, even if it's TN-S today, they can convert to PME in the future, and/or supply PME feeds from it. If you have TN-S or TN-C-S, the DNO can "declare" it PME these days.

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