1887

The Protection of Telecommunication Lines and Equipment Against Lightning Discharges (Chapters 1 to 5)

image of The Protection of Telecommunication Lines and Equipment Against Lightning Discharges (Chapters 1 to 5)

This Handbook contains a general survey of atmospheric discharge phenomena and of the protection devices in use. It gives the explanations in the appendices to the various chapters are necessary for an understanding of the phenomena as well as for calculating the design of protection devices. For a wider study of theory and physical phenomena, reference should be made to the numerous specialized publications. It seems best to limit the field of application of this Handbook to telecommunication lines and associated equipment. The latter also include cable and overhead lines which lead to transmitting installations, radio relay stations, etc. at high altitude, which are thus more than normally exposed. The present Handbook does not deal with the protection of the stations themselves, since such problems concern the protection of buildings against lightning and this, for transmitting stations and radio relay stations, falls within the domain of radio technique. The protection of the buildings in which the telecommunication equipment is installed is governed by the national regulations on lightning protection.

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Protective devices

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.

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