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 affect the safety and electromagnetic compatibility of the installation. Regulations for earthing systems vary among countries, though most follow the recommendations of the International Electrotechnical Commission (IEC). Regulations may identify special cases for earthing in mines, in patient care areas, or in hazardous areas of industrial plants. In addition to electric power systems, other systems may require grounding for safety or function. Tall structures may have lightning rods as part of a system to protect them from lightning strikes. Telegraph lines may use the Earth as one conductor of a circuit, saving the cost of installation of a return wire over a long circuit. Radio antennas may require particular grounding for operation, as well as to control static electricity and provide lightning protection. Purposes There are three main purposes for earthing: 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. 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.