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Metal - clad buildings and the confused forum member.

UKPN
One of the members has queried ESQCR bonding/earthing PME requirements for the above. So its worth spending a little time putting across the companies requirements. Where metal-clad buildings incorporate a steel-frame that utilises steel vertical beams that are within the foundations, the steel frame will provide a good connection with the earth which will effectively limit the earth potential rise.

A PME service may be provided to a metal-clad building provided the following criteria are satisfied:


1. The metal cladding is bonded to the steel-frame.

2. The supply is either three-phase with less than 40% unbalance or the supply is single - phase and the frame to earth impedance is les than 20ohms.

Regards, UKPN?

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    AJJewsbury:
    The importance of this relates to Regulation 442.2.1 (in this case, if you are one of the TT consumers, you are having to rely on an additional earth electrode in someone else's installation - so how can the designer ensure BS 7671 is complied with)

    As I see it, if it was a pure TT system there wouldn't be an issue (Uf would be zero). The problem comes from the influence of the PME earthing system supplying another unit - but that PME earth itself presumably satisfies the requirements of 442.2.1 (presumably by some approximation to a 'global earthing system') - so I'm thinking that if the PME earth in the other unit is deemed safe, then it's influence in our TT unit can't be any more dangerous.


     


    If the building is metal-clad, but doesn't have any metal framework in good contact with the ground (e.g. metal frame on brick or block walls), then there could be an issue.


    If there are no other metallic services running down the side of the premises, the influence of the global earthing system is uncertain - if the next unit is a suitable distance away, it is "TT" proper, and yes no problem. The difficulty comes "in between" - and I guess the DNO will (or should) assess that in determining the approach.


    The difficulty is, it's the installation designer (who might be the installer) who has to comply with BS 7671, but it's not always easy to determine whether you've done so or not in some cases ...


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  • 0
    AJJewsbury:
    The importance of this relates to Regulation 442.2.1 (in this case, if you are one of the TT consumers, you are having to rely on an additional earth electrode in someone else's installation - so how can the designer ensure BS 7671 is complied with)

    As I see it, if it was a pure TT system there wouldn't be an issue (Uf would be zero). The problem comes from the influence of the PME earthing system supplying another unit - but that PME earth itself presumably satisfies the requirements of 442.2.1 (presumably by some approximation to a 'global earthing system') - so I'm thinking that if the PME earth in the other unit is deemed safe, then it's influence in our TT unit can't be any more dangerous.


     


    If the building is metal-clad, but doesn't have any metal framework in good contact with the ground (e.g. metal frame on brick or block walls), then there could be an issue.


    If there are no other metallic services running down the side of the premises, the influence of the global earthing system is uncertain - if the next unit is a suitable distance away, it is "TT" proper, and yes no problem. The difficulty comes "in between" - and I guess the DNO will (or should) assess that in determining the approach.


    The difficulty is, it's the installation designer (who might be the installer) who has to comply with BS 7671, but it's not always easy to determine whether you've done so or not in some cases ...


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