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.

However, we can't discount the fact that if it's another earthing arrangement (say TT or IT), then if the area has PME, diverted neutral currents can persist through other extraneous-conductive-parts where these are shared with PME installations, or come close to buried metalwork connected to the PME system !

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'.

gkenyon said:

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

Graham, you may have mis-read that. I understood Electron666 to mean that it is one or the other.

Hi, thanks for your responses.
A 16mm2 bonding conductor is required to be connected to a spare way on the MET on a number of separate sites.
Some sites have a TN-S earthing arrangement and some are TN-C-S.
As mentioned the earth bar is separate to the main LV board and easily accessible without having to temporarily disconnect any other existing earth or bonding connections and operatives can keep safely away from any L&N conductors.

The question was asked if there need to be full shut down in line with BS 7671 and HSE requirements to make this bonding connection. A shut down is possible but timely, costly and inconvenient due to chilled food refrigerated circuits. If there was a safe way to make the connection without this then great. If not then H&S first and a full shut down needs to happen.
Hopefully this provides more clarification.

Unfortunately you're unlikely t get a simple yes/no answer - as UK safety legislation is less 'thou shalt or thou shalt not' but is more along the lines of obliging you to recognise risks and mitigating them, While isolating is one traditional way of "making things safe" it's not the only possible approach (and in this particular case perhaps not even an adequate one on its own). It's all horses for courses and allowing flexibility to deal with a vast range of differing situations in sensible ways.

   - Andy.

Electron666 said:

to be connected to a spare way on the MET

So, IIUC, the real problem (if there is one) is that:

the remote 'earth/CPC' that is to be bonded, from the remote system, to this existing spare way, may introduce a potential fault current, in some way, by the provision of an additional parallel path? 

The problem is NOT the need to disconnect one of the existing 'earths' at the MET to gain access to, or create, a spare way for the connection.