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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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    That's an interesting one. On one hand any conductor should be expected to carry current, and working on it while it is doing so is generally considered to be a bad thing (not just from shock hazard from voltages, but arcing from current even if the voltage differences are small).

    That said, in the PME world, it's not really practical to "isolate" the incoming Earth (at least not without someone disconnecting something that's potentially carrying current) as even with L+N of the installation isolated, the risk of diverted N currents due loads in neighbouring installations almost certainly remains. I guess most of the time the currents will be small (<<16A) and if you're expecting the possibility of a bit of a crackle as you make or break the connection and behave accordingly (e.g. not jumping backwards out of fright and falling off the ladder or onto live bus-bars, or allow the arc to persist and damage conductor or terminal) the risks might well be acceptable.

    Isolating the installation would reduce the risks since at least any earth faults within the installation couldn't contribute, but won't eliminate them.

       - Andy.

  • 0 in reply to AJJewsbury
    AJJewsbury said:
    That said, in the PME world, it's not really practical to "isolate" the incoming Earth (at least not without someone disconnecting something that's potentially carrying current) as even with L+N of the installation isolated, the risk of diverted N currents due loads in neighbouring installations almost certainly remains.

    Currently, not every country that has a similar arrangement to what we term 'PME' subscribes to that view.

    I agree that is the UK view ... but it's not universally accepted (at least at the moment).

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  • 0 in reply to AJJewsbury
    AJJewsbury said:
    That said, in the PME world, it's not really practical to "isolate" the incoming Earth (at least not without someone disconnecting something that's potentially carrying current) as even with L+N of the installation isolated, the risk of diverted N currents due loads in neighbouring installations almost certainly remains.

    Currently, not every country that has a similar arrangement to what we term 'PME' subscribes to that view.

    I agree that is the UK view ... but it's not universally accepted (at least at the moment).

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