Earth or Ground
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The whole planet earth is assumed to be at a potential of zero Volts.
Most electrical and electronic systems use this zero Volt reference level. This makes it easier to interconnect systems.
The outer metal case of domestic appliances and industrial machinery is connected to earth or ground for safety reasons. In the event of a fault occurring, the outer case might become live. Having it connected to ground will cause the power supply fuse to blow or trip. This disconnects the supply and leaves the equipment in a safe condition.
Domestic wiring often uses multi-point earthing. Instead of connecting to ground via a buried pole or plate, multiple connections are made to water supply, heating pipes and gas pipes (if present).
Theoretically the Earth is at zero volts. Locally this might not be true. Heavy industrial machinery and electric train tracks can vary the potential significantly. Thunder storms and lightning strikes can cause big problems too.
Electricity companies minimise the problem by using a balanced supply and transformers that use the local earth potential.
Data connections between buildings usually use fibre optics or radio links. Neither of these are affected by earthing problems.
Low frequency (below 30 MHz) radio antennas often rely on the conductivity of the earth's surface to function properly.
An earth or ground connection can be made by connecting to a copper pole driven into the ground. Alternatively a metal plate buried underground can be used. Multiple radial wires laid flat on the ground can be used too.
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