Explanation
In a building with electricity it is normal for safety reasons to connect all metal objects such as pipes together to the mains earth to form an equipotential zone. This is done in the UK because many buildings are supplied with a single phase supply cable where the neutral and earth conductors are combined. Close to the electricity meter this conductor is divided into two, the earth terminal and the wire going to the neutral busbar in the consumer unit. In the event of a break in a neutral connection this earth terminal provided by the supply company will be at a potential (relative to the true earth) which is the same as the live wire (phase wire) coming to the home.
Examples of articles that may be bonded include metallic water piping systems, gas piping, airplanes, ducts for central heating and air conditioning systems, and exposed metal parts of buildings such as hand rails, stairs, ladders, platforms and floors.
A person touching the un-earthed metal casing of an electrical device, while also in contact with a metal object connected to remote earth, is exposed to an electric shock hazard if the device has a fault. If all metal objects are connected, all the metal objects in the building will be at the same potential. It then will not be possible to get a shock by touching two 'earthed' objects at once.
Bonding is particularly important for bathrooms, swimming pools and fountains. In pools and fountains, any metallic object (other than conductors of the power circuit)over a certain size must be bonded to assure that all conductors are equipotential and do not provide a hazardous conductive path. Since it is buried in the ground, a pool can be a better ground than the electric panel ground. With all the conducting elements bonded, it is less likely that electric current will find a path through a swimmer. In concrete pools even the reinforcing bars of the concrete must be connected to the bonding system to ensure no dangerous potential gradients are produced during a fault.
Read more about this topic: Electrical Bonding
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