System earthing

System earthing serves a purpose of electrical safety throughout the system that is not caused by an electrical fault. Its main purpose is to prevent static buildup and to protect against power surges caused by nearby lightning strikes or switching. Static buildup, as induced by friction for example, such as when wind blows onto a radio mast, is dissipated to the Earth.In the event of a surge, a lightning arrester, a surge arrester or a SPD will divert the excess current to the Earth before it reaches an appliance.

System earthing also allows for equipotential bonding to all metalworks to prevent potential differences between them.[5] Having Earth as a common reference point also keeps the electrical system's potential difference limited to the supply voltage.

 

Equipment earthing
Equipment earthing serves a purpose of electrical safety in an electrical fault. Its main purpose is to prevent equipment damage and the risk of an electric shock. This type of earthing is not an earth connection, technically speaking. When current flows from a line conductor to an earth wire, as is the case when a line conductor makes contact with an earthed surface in a Class I appliance, an automatic disconnection of supply (ADS) device such as a circuit breaker or a RCD will automatically open the circuit to clear the fault.

 

Functional earthing
Functional earthing serves a purpose other than electrical safety. Example purposes include electromagnetic interference (EMI) filtering in an EMI filter, and the use of the Earth as a return path in a single-wire earth return distribution system.