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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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    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.
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  • 0
    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.
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