A green and yellow sleeved cable of at least 16mm2 connects the main earth at the incoming supply to the earth bar in the consumer unit. Earth conductors from the various circuits in the house are connected to this. There may also be a main earth terminal mounted near the consumer unit. This earthing system provides a low resistance path so that, in the event of a fault such as a live wire coming into contact with the metal casing of an appliance, a high current will begin to flow and this will blow the fuse or trip the circuit breaker at the consumer unit.
Bonding or Equipotential Bonding
The purpose of bonding is to ensure that all metalwork has an equal potential – hence the term ‘equipotential’. This reduces the possibility of dangerous potential differences if a fault should occur.
A 10mm2 green and yellow sleeved cable is used to provide the main bonding of the metal service pipes coming in to the building. These include water, gas and oil pipes. One conductor may be used to connect all these service pipes provided it is one continuous cable. Connection to the metal pipes is made with earth clamps which are labelled to clearly identify their importance. The position of these earth clamps will be close to the point of entry into the house.
Most commonly found in bathrooms, supplementary bonding is provided by 4mm2 yellow and green sleeved conductors. This connects together metal items including hot and cold water pipes, central heating pipes, towel radiators etc. Connection to the metal pipes is made with earth clamps which are labelled to clearly identify their importance. Other examples of supplementary bonding may be found in airing cupboards where the metal pipes are all bonded, and plastic water cisterns where the metal pipe work on either side is bonded.
Clamps, used to earth metal pipes, consist of a strap which is fastened around the pipe and a terminal to which the earth wire is connected. They include a label identifying what they are for.
The effectiveness of the connection is vital. So, if you are not certain how this should be carried out or have not done it before, have the work done by a professional and get them to show you how it is done.
A common error is to fit an earth clamp to pipe work without cleaning away oxidation or paint from the surface first. The result of this is that there may not be a good bond between the two. Positive contact between the pieces is essential.
Any bonding carried out should be inspected by a qualified professional.