SUPPLEMENTRY BONDING

[dingbat]

Snowplough, this is really important if you are ever going to understand earthing and bonding.

The incorrect use of terminology is at the heart of many misunderstandings in the electrical and associated trades about these practices.

The first thing you have to do is understand the difference between earthing (exposed conductive parts are connected to the means of earthing via the circuit protective conductor, the main earthing terminal and the earthing conductor) and bonding (exposed and extraneous conductive parts are connected together [equipotentially bonded] by main and/or supplementary protective bonding conductors).

Get all those definitions straight before you start to read or listen to any explanations people give you. This includes any advice from your lecturers.
If you have a copy of BS 7671, memorise the diagram at the top of page 32.

Earthing provides a fault path (earth fault loop) to disconnect the circuit in the event of an earth fault current.

Bonding is there to reduce to a minimum the potential differences between exposed and extraneous conductive parts during the time it takes to disconnect the circuit.

Confuse the two and you're back to square one. Get it right now and it will stand you in good stead for the rest of your studies. Good luck.

 

[snowplough]

So to answer my question then am i right to think that even if one piece of extran metal is live say and u touch another say with your other hand the fault potential will flow from the live metal, through the person , through the other giving the person an electric shock.

In the same situation if all the pipe work was all bonded together and remaining live (not connected to met causing fuse to blow) someone can still get an electric shock if the fault current could flow through the person back to true earth forming a circuit

so surely equipotential bonding wont prevent someone getting an electric shock if not bonded to earth.

 

[RF Lighting]

No it wont stop someone getting a shock, but that is not it's job.

It will help to reduce the severity of the shock.

Imagine this scenario:
There has been a fault under the floor. A cable has been crushed, and the hot pipe has become live.

You go to run a bath, and grab the hot and cold tap at the same time.

The cold tap is at earth potential as it is buried in the street outside your house.

You receive a high current 230V shock across your chest.

Now, if there was supplementary bonding in place, the hot tap and cold tap are the same potential. They have now both become live due to the fault, and the bonding.

You again touch both taps at the same time. As both taps are at the same potential, no current can flow.

You may still receive a smaller shock, but it will be no where near as severe due to the insulating properties of the floor you are stood on, and the general fabric of the building.

 

[snowplough]

Thanks for that last example, i think thats what i have been trying to say and how understand it , that supp bonding is useful to make all metal work at the same potential , but say even with all emetal work bonded if a fault current can get back to earth by person touching the two pipes u said bonded together if the metal work is still live then i would think there would be high resistance in the metal work or the fuse would blow ,
so the person could create a low resistance path to earth hence electric shock