2　Terms

2.0.1　Common earthing

The earthing mode means that all different forms of earthing for telecommunication installations in telecommunication bureau（station），including the operational earthing，protective earthing，shield earthing，anti-static earthing，logic earthing of IT devices，protective earthing of metal building members and lightning protection systems and surge protective devices，are connected together to the earth electrodes in the foundation of the building and the set earthing system，which are also shared by the lightning protection earthing of the building structure.

2.0.2　Air-termination system

The system is established to withstand the discharge of direct lightning flashes，so as to keep the object protected from being struck.It consists of the parts intercepting lightning flash:lightning conductor，air-termination strip（wire），air-termination net，metal roofing and metal member.

2.0.3　Down-conductor system

The metal conductors connecting air-termination system and earth-termination system.

2.0.4　Direct stroke protection system

The combination of air-termination system and down-conductor system.

2.0.5　Earthing electrode

One or a set of conductors which are buried into soil and used to provide electrical connection to soil（the earth）.

2.0.6　Earthing grid

A set of inter-connected bare conductors buried in earth to provide a common earthing for electrical equipment or metal structures.

2.0.7　Earthing lead

Conductors connecting main earthing omnibus and earthing grid.

2.0.8　Earth-termination system

The integration of earthing lead and earthing grid.

2.0.9　Equipotential bonding

The bonding of separated systems or conductive objects by using conductors or surge protective devices in order to reduce the potential difference induced by lightning current.

2.0.10　Earth conductor

Earth conductors are cables used for equipotential bonding，which connect large-size conductive objects in telecommunication bureau（station）like equipment，elevator rails，hoists，metal floors，metal door frames，metal pipelines，metal cable trays and on-outer wall metal railing to the nearest earthing busbar or main earthing omnibus.

2.0.11　Earthing busbar

Conductive bar to which all earth conductors are connected.

2.0.12　Main earthing omnibus

Main earthing omnibus refers to copper strip or flat steel used as earthing conductor.It generally acts as the trunk line of earthing system in telecommunication bureau（station）；it is classified into horizontal，vertical，circular and strip types in terms of laying method.

2.0.13　Earthing system

A complex system consists of all electrical connections and apparatuses for earthing of the systems，installations and equipment，including earthing electrodes buried into soil，earth conductors，cable shielding layers connected to earthing electrodes and other items connected to earthing electrodes like equipment cases，or exposed metal parts of equipment，steel bars of buildings，and frames of infrastructures.

2.0.14　Rolling sphere method

The simplified analytical method of electro-geometric model theory is applied in the analysis of lightning protection for buildings.The scope of lightning protection is calculated by rolling up and down an imaginary sphere along the surface of protected objects with the support of lightning conductors，lightning protecting wires，fences and other earthed metal objects.Any object under the protective curve surface formed by rolling the sphere will be protected；any object touching the sphere or piercing the sphere surface will not be protected.

2.0.15　Earth resistivity

A parameter characterizing the conductivity of earth，of which the value equals to the resistance between the two opposite sides of one unit cube of soil，the nominal unit is Ω·m.

2.0.16　Power frequency earthing resistance

The resistance between earthing electrode and remote earth while power frequency currents go through the earth-termination system.Its value equals to the ratio of the voltage of the earth-termination system with respect to the remote earth to the current flowed into the earth through earthing electrode.

2.0.17　Surge protective device（SPD）

A device protecting telecommunication system in case of lightning overvoltage or operational overvoltage.It protects the equipment by restraining transient overvoltage and by bypassing surge current，and it at least consists of one nonlinear element.