Adding a bonding conductor to an existing main earth terminal

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.

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.

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).