Adding a bonding conductor to an existing main earth terminal

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.

The MET is, presumably, just a lump of uninsulated brass, so again, no special precautions required when handling it. What you do not want to do is disconnect the earthing conductor accidentally.

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.

If  adding the additional bit of green and yellow requires you to remove other connections that are already there - which might interrupt a fault current, and allow voltage to appear on things that are supposed to be earthed, then there is a serious risk. However if it is a wire into an spare terminal tunnel just adding one more bit of wire to a collection of stuff that is already earthed and exposed, like radiators and other plumbing, is not a significant extra risk.

 Note that turning the supply off won't greatly reduce the risks from  touching the earth terminal or  exposed metal already attached to it, and  if you have concerns, it is more useful to test for voltage with an insulated wander lead between the things you are proposing to connect and the MET, before you actually do.

regards Mike.

Chris Pearson said:

the earthing conductor and MET will remain connected to the DNO's earth.

How, the OP stated that the supply wasn't TN-S or TN-C-S.

Electron666 said:

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

It would help us if you said what it is, rather than what it isn't? Is it an IT earthing arrangement, for example?

Without this knowledge, it's not really possible to answer your question.

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).

mapj1 said:

adding the additional bit of green and yellow requires you to remove other connections that are already there - which might interrupt a fault current,

or intended protected conductor current ... or indeed 'diverted neutral current'.