Supplementary bonding

[PLW]

I am a new trainee electrician and am I currently reading about supplementary bonding. I understand what it is and why it's used however I and confused as to why supplementary bonding conductors are not connected back to the main earthing terminal (met). Can someone please explain why this is. Thanks.

 

[Darkwood]

Welcome to the Forum PLW.

If you are actively in a training course it may be better for you to apply for access to our training section where we have a good selection of Mentors who can help you out with the basics.

Follow link...http://www.electriciansforums.co.uk...ttending-electrical-course-then-you-need.html

 

[PLW]

I understand what supplementary bonding is and why it's used however I do not understand why supplementary bonding conductors are not connected to the consumers main earthing terminal. Could someone please explain why this is?

 

[UKMeterman]

Ok,
It is just to ensure that any two or more points that you can touch at the same time are at the same potential. If they are at the same potential you can't get a shock off them. It is just to control the local potential. Main bonds are to ensure that different bits of metal from outside the property can't make bits of the inside of the property have different potentials.

Look up Faraday Cage.

 

[PLW]

Ok,
It is just to ensure that any two or more points that you can touch at the same time are at the same potential. If they are at the same potential you can't get a shock off them. It is just to control the local potential. Main bonds are to ensure that different bits of metal from outside the property can't make bits of the inside of the property have different potentials.

Look up Faraday Cage.

I understand that the potentials must be kept the same as a means of safety. I just don't understand why supplementary bonds are not connected to the main earthing terminal but the main equipotential bonding of the structural metal work is terminated at the main earthing terminal?

 

[davesparks]

I understand that the potentials must be kept the same as a means of safety. I just don't understand why supplementary bonds are not connected to the main earthing terminal but the main equipotential bonding of the structural metal work is terminated at the main earthing terminal?

A main bond is there to maintain the extraneous parts at an equal potential with that of the electrical earth. So one end is connected to electrical earth and the other end to the extraneous part.
Supplementary bonding is connecting other metallic parts together to maintain an equal potential between them, one of those parts will already be connected back to the met either via a main bond or via a cpc.
Remember a bonding conductor doesn't have to be a bit of copper with green and yellow insulation. conduits, trunking, ducts, pipes, structural steel can all be used as part of a supplementary bond

 

[UKMeterman]

Structural metal work will probably be in good contact with the ground outside, if there should be a broken combined neutral and earth on a PME supply the "main earth terminal" will try to rise in voltage compared to true earth. If the steel work was not deliberately connected to the main earth terminal then there would be either a potential difference between the two, or current though an alternative connection between the two possibly not designed to carry many many amps.

 

[Richard Burns]

As you know that supplementary bonding is used to reduce the touch voltage in the case of a fault in a local area of (usually) higher risk of electric shock.
Therefore it should be clear that since the supplementary bonding is only applicable to that area there is no requirement to connect it back to the MET.
The Supplementary bonding connections to electrical equipment will mean that the supplementary bonding is actually connected to the MET via the cpcs of the circuits in the area.

The intent of the wording is that there is not required to be a specific dedicated conductor going back to the MET as the effect of supplementary bodning is local and not covering the entire installation.