Earthing

In electricity supply systems, an earthing system or grounding system is circuitry which connects parts of the electric circuit with the ground, thus defining the electric potential of the conductors relative to the Earth's conductive surface. The choice of earthing system can affect the safety and electromagnetic compatibility of the power supply. In particular, it affects the magnitude and distribution of short circuit currents through the system, and the effects it creates on equipment and people in the proximity of the circuit. If a fault within an electrical device connects a live supply conductor to an exposed conductive surface, anyone touching it while electrically connected to the earth will complete a circuit back to the earthed supply conductor and receive an electric shock.

People use an earthing because:
To protect a structure from lightning strike, directing the lightning through the earthing system and into the ground rod rather than passing through the structure.
Part of the safety system of mains electricity, preventing problems associated with floating ground and sky voltage.
The most common ground plane for large monopole antenna and some other kinds of radio antenna.

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