Adding a bonding conductor to an existing main earth terminal

Makes me wonder how anything actually gets done in this country, and when it eventually gets done it costs an arm and a leg if you excuse the pun.

Gary

Electron666 said:

A shut down is possible but timely, costly and inconvenient due to chilled food refrigerated circuits.

Even assuming that working dead is preferable, surely, making the connexion at the MET will take only a matter of seconds. Will the system not withstand such a short break in the supply?

Chris Pearson said:

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

you're right! Apologies

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.

Leaving aside environmental risks (and in that I include accessible live or neutral conductors adjacent to the main earth terminal) then there are the following main risks:

1. Disturbing the MET unintentionally interrupts existing bonding causing something somewhere which should be bonded to cease to be with the associated risk of shock or failure of overcurrent protection arrangements.

2. The MET itself is at a hazardous voltage compared to its surroundings and therefore touching it presents a risk.  This should never (except perhaps if the MET is outdoors?) be the case in a properly designed and maintained installation.

3. The bonding cable that you wish to connect to the MET is at a hazardous voltage with respect to the MET and its surroundings and consequently there is a risk of shock if both are touched simultaneously.

4. The installation of the new bonding cable provides a lower impedance path for an existing (or potential) current and there is arcing (or in the worst case your 16mm2 cable melts) causing damage / injury when you attempt to connect it to the MET.

Without knowing more details of the installation it is impossible to assess the likelihood of those hazards being present.  All of those hazards are amenable to being addressed through a suitable procedure containing measures to confirm whether they are present before actions presenting a risk of harm are carried out.  Such a procedure need not necessarily involve isolation of the supply in the "normal" case where everything is as expected.

The extent of safety measures required during the work needs to be identified based on your knowledge of the installation and the work to be carried out.  Unless working outside, specific safety measures should not be necessary in touching the MET but if you don't have the confidence that your installation is properly designed and maintained then you may wish to specify protective measures to further control the risk.  There are some protective measures which should always be selected - for example, the use of insulated tools generally has no down-side whilst others - for example the use of insulated gloves may make a job more difficult increasing the likelihood of error or lower quality workmanship.  Don't forget the risks associated with the new cable, for example, if the other end of the 16mm2 bonding cable you are adding is disconnected and visible in the same room then you might not need to, for example, test for voltage on the new cable or connect the new cable to the MET via a temporary 10A fuse to convince yourself that it isn't going to melt when connected permanently but if it runs for 50m through a factory and has already been terminated onto a piece of equipment then you might.

According to national-claims.co.uk/ the cost of an arm and leg (complete set of parts) would be 

One leg (above knee) £127,930 – £167,760

One arm (at shoulder) Not less than £167,380

I'd expect that it's more in the land of the free and easy lawsuits .

so ~ a third of a million pounds.

Worth a few minutes discussion on a forum perhaps . And some PPE, just like the DNOs use when checking for diverted Neutrals Neutral Current Diversion Demonstration.

All that's happening here is [essentially] the installation of that Neutral Current Diversion path!

DShutt said:

connect the new cable to the MET via a temporary 10A fuse to convince yourself that it isn't going to melt

Seems like a worthwhile step either way (and perhaps a smaller fuse initially, depending on context).