Various types of lightning protector are used for telecommunication lines based on the principle of a gaseous discharge between two electrodes which are either in free air or sealed in a container with a rarefied atmosphere. This type of device is the only one which satisfies two basic requirements for the protection of lines and the apparatus connected to them; it provides high impedance in normal operating conditions and very low impedance during discharges, the transition from one state to another being made very quickly. When inserted between a conductor on which there is a surge voltage and a potential reference point, which may be an earthing network or a metallic cable sheath, the lightning protector functions by short-circuiting the surge and limiting it to a residual voltage whose value depends on the electrical characteristics of the device. A detailed description is given below of the characteristics or protective devices establishing a connection between a telecommunication line and the ground when there is an over-voltage. Lightning protectors for telecommunication lines differ in several respects from those applied to power supply lines. Other devices discussed below play a complementary role but they cannot replace these surge-limiting devices which are connected to the conductors of the line to be protected.