Bonding refers to the fact that 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, ducts for central heating and air conditioing systems, and exposed metal parts of buildings such as hand rails, stairs, ladders, platforms and floors.
If a person was to touch the metal (earthed casing) of an electrical device during the a fault condition and be in contact with a metal object connected to a remote earth then they would get an electric shock. If all metal objects are connected together, 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 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 is 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.
Equipotential bonding involves joining together metalwork that is or may be earthed so that it is at the same potential (i.e., voltage) everywhere. Such is commonly used under transformer banks by power companies and under large computer installations. Equipotential bonding is done from the Service Panel consumer unit (also known as a fuse box, breaker box, or distribution board) to incoming water and gas services. It is also done in bathrooms where all exposed metal that leaves the bathroom including metal pipes and the earths of electrical circuits must be bonded together to ensure that they are always at the same potential. Isolated metal objects including metal fittings fed by plastic pipe (water in a thin pipe is actually a very poor conductor) are not required to be bonded. European and North American practices differ here; equipotential bonding in bathrooms is not required by North American codes, although it is required around swimming_pools. In Australia and South Africa, a house's earth cables must be connected both to an earthing rod/stake driven into the ground and also to the plumbing. Exact rules for electrical installations vary by country, locality, or supplying power company.