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Adding a bonding conductor to an existing main earth terminal

Electron666

Hi do you need to isolate the supply to a commercial building to connect a 16mm2 bonding conductor to the MET in relation to BS 7671 or HSE?

It isn't something I can find explicit guidance on.

The MET is at the intake and separate to the main LV board.

It is easily accessible without coming into contact with L&N conductors connected to the main LV board.

The earthing system is either TN-S or TN-C-S.

Isolating the supply is possible if required but would be costly, require multiple companies attending out of hours and would need to be planned in well in advance due to it being a refrigerated / food storage facility.

I note all these reasons are inconvenience related which is secondary to health and safety. Therefore, I'm only asking if there is a method that is commonly used such as standing on rubber mats or wearing rated gloves etc that is considered safe and proportionate to the to the small task being carried out.

Thanks

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    Electron666 said:
    The earthing system is either TN-S or TN-C-S.

    Having re-read this, the point about diverted neutral current is correct. 

    However, a shock risk must exist already between exposed-conductive-parts of the installation, and the extraneous-conductive-part you are bonding.

    This can happen whether the power is on or not.

    Chris Pearson said:

    When you isolate the supply, the earthing conductor and MET will remain connected to the DNO's earth.

    I would suggest that you put a clamp meter around the earthing conductor to check for any current there. You could also check that the MET is at Earth potential by applying a volt meter between it and a screwdriver in some convenient exposed earth.

    This type of approach would be recommended - check first for diverted neutral currents. There is guidance on this in Guidance Note 3 Inspection and Testing.

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  • 0
    Electron666 said:
    The earthing system is either TN-S or TN-C-S.

    Having re-read this, the point about diverted neutral current is correct. 

    However, a shock risk must exist already between exposed-conductive-parts of the installation, and the extraneous-conductive-part you are bonding.

    This can happen whether the power is on or not.

    Chris Pearson said:

    When you isolate the supply, the earthing conductor and MET will remain connected to the DNO's earth.

    I would suggest that you put a clamp meter around the earthing conductor to check for any current there. You could also check that the MET is at Earth potential by applying a volt meter between it and a screwdriver in some convenient exposed earth.

    This type of approach would be recommended - check first for diverted neutral currents. There is guidance on this in Guidance Note 3 Inspection and Testing.

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