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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:
    If the answer is "it doesn't matter" (from the DNO perspective), and the TT earthing arrangement is not properly separated, then we are in a tricky situation in cases where the PME earthing system is not connected to the HV earthing system at the supply transformer, as the statement Uf = 0 in Table 44.1 of BS 7671 is no longer always true !

    Do you mean when the PME system is connected to the HV earth? (the situation then becoming similar to the first of the TN rows of 44.1 & Uf might be as high as RE x IE during a HV fault). Even so is there a practical difficulty? We generally construct TT installation using the same materials as a TN system and if a TN system can withstand RE x IE why should we expect a  problem with a TT system?

        - Andy.


    Yes, I did - apologies.


    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), the effectiveness of the chosen method of connection of surge protection, and potentially the insulation coordination of  switchgear etc


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  • 0
    AJJewsbury:
    If the answer is "it doesn't matter" (from the DNO perspective), and the TT earthing arrangement is not properly separated, then we are in a tricky situation in cases where the PME earthing system is not connected to the HV earthing system at the supply transformer, as the statement Uf = 0 in Table 44.1 of BS 7671 is no longer always true !

    Do you mean when the PME system is connected to the HV earth? (the situation then becoming similar to the first of the TN rows of 44.1 & Uf might be as high as RE x IE during a HV fault). Even so is there a practical difficulty? We generally construct TT installation using the same materials as a TN system and if a TN system can withstand RE x IE why should we expect a  problem with a TT system?

        - Andy.


    Yes, I did - apologies.


    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), the effectiveness of the chosen method of connection of surge protection, and potentially the insulation coordination of  switchgear etc


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