Electrical bonding is the practice of intentionally electrically connecting all exposed metal items not designed to carry electricity in a room or building as protection from
electric shock
Electrical injury is a physiological reaction caused by electric current passing through the body. The injury depends on the density of the current, tissue resistance and duration of contact. Very small currents may be imperceptible or produce ...
. If a failure of electrical insulation occurs, all bonded metal objects in the room will have substantially 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
Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surfa ...
is lost, the occupant will be protected from dangerous
potential difference
Voltage, also known as electric pressure, electric tension, or (electric) potential difference, is the difference in electric potential between two points. In a static electric field, it corresponds to the work needed per unit of charge to m ...
s.
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
In mathematics and physics, an equipotential or isopotential refers to a region in space where every point is at the same potential. This usually refers to a scalar potential (in that case it is a level set of the potential), although it can al ...
zone. This is done in the
UK because many buildings are supplied with a TN−C−S
earthing system
An earthing system (UK and IEC) or grounding system (US) connects specific parts of an electric power system with the ground, typically the Earth's conductive surface, for safety and functional purposes. The choice of earthing system can affec ...
where the neutral and earth conductors are combined. Close to the
electricity meter
North American domestic analog electricity meter.
Electricity meter with transparent plastic case (Israel)
North American domestic electronic electricity meter
An electricity meter, electric meter, electrical meter, energy meter, or kilowa ...
this conductor is divided into two, the earth and the
neutral
Neutral or neutrality may refer to:
Mathematics and natural science Biology
* Neutral organisms, in ecology, those that obey the unified neutral theory of biodiversity
Chemistry and physics
* Neutralization (chemistry), a chemical reaction in ...
busbar
In electric power distribution, a busbar (also bus bar) is a metallic strip or bar, typically housed inside switchgear, panel boards, and busway enclosures for local high current power distribution. They are also used to connect high volt ...
in the
consumer unit
A consumer unit is a type of distribution board (a component of an electrical power system within which an electrical power feed provides supply to subsidiary circuits).
UK
The IET defines a consumer unit, also known as a consumer control ...
. If the ground connection to the neutral is lost, all wiring and other objects tied to the neutral will be energized at the line voltage. Examples of articles that may be bonded include metallic water piping systems, gas piping, ducts for central heating and air conditioning systems, and exposed metal parts of buildings such as handrails, 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
Electrical injury is a physiological reaction caused by electric current passing through the body. The injury depends on the density of the current, tissue resistance and duration of contact. Very small currents may be imperceptible or produce ...
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 metal object (other than conductors of the power circuit) over a certain size must be bonded to assure that all conductors are at the same potential. 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 an 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 metal 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 over-current protection (a
fuse
Fuse or FUSE may refer to:
Devices
* Fuse (electrical), a device used in electrical systems to protect against excessive current
** Fuse (automotive), a class of fuses for vehicles
* Fuse (hydraulic), a device used in hydraulic systems to protect ...
or
circuit breaker
A circuit breaker is an electrical safety device designed to protect an electrical circuit from damage caused by an overcurrent or short circuit. Its basic function is to interrupt current flow to protect equipment and to prevent the risk ...
) 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
A transformer is a passive component that transfers electrical energy from one electrical circuit to another circuit, or multiple circuits. A varying current in any coil of the transformer produces a varying magnetic flux in the transformer' ...
neutral, an
RCD 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 electrically connecting metalwork so that it is at the same voltage everywhere. Exact rules for electrical installations vary by country, locality, or supplying power company.
Equipotential bonding is done from where the distribution wiring enters the building to incoming
water
Water (chemical formula ) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living organisms (in which it acts as a ...
and
gas
Gas is one of the four fundamental states of matter (the others being solid, liquid, and plasma).
A pure gas may be made up of individual atoms (e.g. a noble gas like neon), elemental molecules made from one type of atom (e.g. oxygen), or ...
services. It is also done in
bathrooms where all exposed metal that leaves the bathroom including
metal
A metal (from Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, polished, or fractured, shows a lustrous appearance, and conducts electricity and heat relatively well. Metals are typicall ...
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, are not required to be bonded with the exception of construction utilizing metal stud work and framing seen in many taller condominium, apartment, and office/commercial construction.
Aircraft electrical bonding
In aircraft, electrical bonding prevents static electricity build-up that can interfere with radio and navigational equipment. Bonding also provides
lightning
Lightning is a naturally occurring electrostatic discharge during which two electric charge, electrically charged regions, both in the atmosphere or with one on the land, ground, temporarily neutralize themselves, causing the instantaneous ...
protection by allowing the current to pass through the airframe with minimum arcing. Bonding prevents dangerous static discharges in aircraft fuel tanks and hoses.
[Mechanic Support, "Aircraft hose and electrical bonding"](_blank)
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See also
* Earthing system
An earthing system (UK and IEC) or grounding system (US) connects specific parts of an electric power system with the ground, typically the Earth's conductive surface, for safety and functional purposes. The choice of earthing system can affec ...
* Earth potential rise
In electrical engineering, earth potential rise (EPR) also called ground potential rise (GPR) occurs when a large current flows to earth through an earth grid impedance. The potential relative to a distant point on the Earth is highest at the poi ...
* Stray voltage
Stray voltage is the occurrence of electrical potential between two objects that ideally should not have any voltage difference between them. Small voltages often exist between two grounded objects in separate locations, due to normal current flo ...
* Bonding jumper A bonding jumper is a reliable conductor to ensure the required electrical conductivity between metal raceways required to be electrically connected.
Bonding
"Bonding" is a method by which all electrically conductive materials and metallic surfac ...
Notes and references
Aircraft hose electrical bonding