Electrical bonding is the practice of intentionally electrically connecting all metallic non-current carrying items in a room or building as protection from electric shock. If a failure of electrical insulation occurs, all metal objects in the room will have the same electrical potential, so that an occupant of the room cannot touch two objects with significantly different potentials. Even if the connection to a distant earth ground is lost, the occupant will be protected from dangerous potential differences.
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.
How the earth protects
In a system with a grounded (earthed) neutral, connecting all non-current-carrying metallic parts of equipment to earth ground at the main service panel, will ensure that current due to faults (such as a "hot" wire touching the frame or chassis of the device) will be diverted to earth. In a TN system where there is a direct connection from the installation earth to the transformer neutral, earthing will allow the branch circuit overcurrent protection (a fuse or circuit breaker) to detect the fault rapidly and interrupt the circuit.
In the case of a TT system where the impedance is high due to the lack of direct connection to the transformer neutral, an RCD (Residual-Current Device, sometimes known as a Residual Current Circuit Breaker or Ground Fault Circuit Interrupter) must be used to provide disconnection. RCDs are also used in other situations where rapid disconnection of small earth faults (including a human touching a live wire by accident, or damage) is desired.
Equipotential bonding
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.
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