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TNS and PME connected]

lyledunn

I would appreciate a wee bit of advice on this one. Quarry with everything supplied by one transformer. There is an overhead bundle to the office block which DNO notice says PME. The quarry supply is TNS and the workshop is also supplied via the main panel in the quarry intake. There is a gen set that feeds into a changeover switch in the quarry intake that offers a back up to the quarry and to the workshop. A new SWA cable was installed from the panel in the workshop to a metal clad changeover switch in the office block to allow the office to benefit from the back up generator. The PME earth is now exported to the quarry. Issue?

one.

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  • 0
    lyledunn said:
    Issue?

    Clause 6 of BS 6907-2 Clause 6 doesn't include TN-C-S ... BUT i'd be happy to argue that, provided the changeover requirements are met, the supply to the quarry is TN-S (whether or not bonded or otherwise connected to PME).

    Nothing in BS 7671 or BS 6907 series, or ESQCR or the guidance on ESQCR, appears to prohibit connection of a PME earthing arrangement (whether or not via earthing or bonding).

    I guess there's the question (in terms of immediate risk) as to whether any part of the quarry is considered explosive atmosphere? Whilst again, the terminology in BS EN 60079-13 BS EN 60079-14 is unclear, there's a general professional understanding that electrical installations of hazardous areas with explosive atmospheres are not connected to PME (or, if it can be helped, exposed to diverted neutral currents).

  • 0 in reply to gkenyon

    'Whilst again, the terminology in BS EN 60079-13 is unclear . . .'

    Did you mean BS EN 60079-14?

    With reference to earthing systems, BS EN 60079-14 states that where a TN earthing system is used it shall be TN-S with separate protective conductor and neutral in the explosive atmosphere.

    - Ross

  • 0 in reply to Ross C

    Yes, apologies, typo edited previous post.

  • 0 in reply to gkenyon

     GK said;

    Nothing in BS 7671 or BS 6907 series, or ESQCR or the guidance on ESQCR, appears to prohibit connection of a PME earthing arrangement (whether or not via earthing or bonding)

    Yes, so it would seem. However, the safety manager for this quarry company also asked us to keep an eye on the "Management of Electrical Safety in Quarries...etc", issued by the Quarries National Joint advisory Committee. They recommend that PME is not used in a quarry situation. I guess its difficult to keep everyone happy these days! 

    My OP was really to get a sense of what the consequences might be and where in the risk spectrum it might sit. It is nice to be able to bounce things off the experts on this forum. Thanks to all!

  • +1 in reply to lyledunn

    The Energy Networks Association Engineering Recommendation G12 - Requirements for the Application of Protective
    Multiple Earthing to Low Voltage Networks appears to agree with (or may be the source of) the position of the Quarries National Joint Advisory Committee:

    '6.2.8 Mines and quarries

    Provided the building’s electrical installation wiring conforms to BS 7671, a PME earth terminal can be provided to mine/quarry permanent buildings (e.g. permanent offices and canteens). This excludes amenity shower blocks unless an earth grid is installed. Supplies to underground shafts, the production side of quarries or associated amenity shower blocks shall be from a TT system having a separate connection with earth, independent of the PME earth terminal. Where a mine or quarry requires a supply both to a permanent building and either an underground shaft or the production side of the quarry, precautions must be taken to ensure that these latter supplies have an earth system segregated from the PME earth system.

    If the site has a dedicated secondary substation that only supplies the consumer it will usually be possible to provide a TN-S earth terminal directly from the transformer neutral.

    NOTE: The requirements for supplies to the working areas are covered by specific statutory legislation (see
    BS 7671, section 110.1)'

    - Ross

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  • +1 in reply to lyledunn

    The Energy Networks Association Engineering Recommendation G12 - Requirements for the Application of Protective
    Multiple Earthing to Low Voltage Networks appears to agree with (or may be the source of) the position of the Quarries National Joint Advisory Committee:

    '6.2.8 Mines and quarries

    Provided the building’s electrical installation wiring conforms to BS 7671, a PME earth terminal can be provided to mine/quarry permanent buildings (e.g. permanent offices and canteens). This excludes amenity shower blocks unless an earth grid is installed. Supplies to underground shafts, the production side of quarries or associated amenity shower blocks shall be from a TT system having a separate connection with earth, independent of the PME earth terminal. Where a mine or quarry requires a supply both to a permanent building and either an underground shaft or the production side of the quarry, precautions must be taken to ensure that these latter supplies have an earth system segregated from the PME earth system.

    If the site has a dedicated secondary substation that only supplies the consumer it will usually be possible to provide a TN-S earth terminal directly from the transformer neutral.

    NOTE: The requirements for supplies to the working areas are covered by specific statutory legislation (see
    BS 7671, section 110.1)'

    - Ross

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