1、W 48b2591 b83107 b5T 1111 THE INTERNATIONAL TELEGRAPH AND TELEPHONE CONSULTATIVE COMMllTEE (C.C.I.T.T.) THE PROTECTION OF TELECOMI1MUNI CATION LINES AND EQUIPMENT AGAINST LIGHTNING DISCHARGES Published by THE INTERNATIONAL TELECOMMUNICATION UNION 1974 COPYRIGHT International Telecommunications Union
2、/ITU TelecommunicationsLicensed by Information Handling ServicesW 4862593 0683108 596 = THE INTERNATIONAL TELEGRAPH AND TELEPHONE CONSULTATIYE CO- (C.C.I.T.T.) I THE PROTECTION OF TELECOMMUNI CATION LINES AND EQUIPMENT AGAINST LIGHTNING DISCHARGES Published by THE INTERNATIONAL TELECOMMUNICATION UNI
3、ON 1974 COPYRIGHT International Telecommunications Union/ITU TelecommunicationsLicensed by Information Handling Services= 4862591 Ob83109 422 W Page Edition TABLE OF CONTENTS CHAPTFX 1 . Introduction 5 1974 1.Gener al 5 2 . 5 3 . Definitions 6 Aims and limits of protection against iightning . 3.1 Th
4、understormday 6 3.2 Isokeraunic line 6 3.3 Impulsement 6 3.4 Impulse andsurge voltages . 7 3.5 Protective devices against surges 8 3.6 Earthing network . 8 3.7 Diffusion impedance of an earthing network . 8 3.8 Transfer (or coupling) impedance of a metal cable sheath . 8 3.9 Quality factor of a meta
5、l-sheathed cable . 8 3.10 Screening factor of a metal cable sheath . 8 1974 1974 1974 1974 1974 1974 1974 1974 1974 1974 1974 1974 1974 CHAPTER 2 . Armosphetic discharges and resultant physical phenomena 11 1974 1 . Electrical charge distribution and resultant physical phenomena . 11 1974 2 . Charec
6、teristics of discharge phenomena . 11 1974 3 . Frequency of lightning strokes . 15 1974 4 . Impulse breakdown of dielectrics 15 1974 Appendix . Waveform of impulses simulating lightning discharge currents 18 1974 CHAPTER 3 . The atmospheric electric fields associated with thunderstorms and lightning
7、 discharges und their efects 25 1974 1 . General Factors affecting the voltage in an overhead line conductor . 2 . Electric field strenght in the atmosphere Field when there is no lightning . 2.1.1 Charge due to charged particles . 2.2 Field when lightning occurs 3 . Voltages in a horizontal, overhe
8、ad conductor . conducting earth . 1.1 2.1 3.1 3.2 3.3 Lightning discharges to earth within about 3 km of a conductor and assuming a perfectly Lightning discharges to earth within about 3 km of a conductor, when the earth resistance is not zero Lightning discharges more than 3 km away from a conducto
9、r and cloud discharges . . 4 . Effects produced in the ground by lightning strokes . 5 . Other effects caused by lightning discharges 25 25 26 26 26 26 27 27 28 32 32 34 1974 1974 1974 1974 1974 1974 1974 1974 1974 1974 1974 1974 Ed. 1974 COPYRIGHT International Telecommunications Union/ITU Telecomm
10、unicationsLicensed by Information Handling Services4862591 0683330 144 Ediion 5.1 Effects on metal conductors 34 5.1.2 Mechanical effects . 35 5.2 Effects on nonconducting material . 35 Appendix . Critical distance from the point of impact for cables with an insulating covering . . 36 5.1.1 Heatinge
11、ffec . 34 CHApIER 4 . The effects of lightning discharges to overheadand underground teleconmunication lines 41 1 . Lightning strokes which terminate on an overhead route 1.1 The propagation and reflection of lightning surges and the effects on a multi-pair overhead route . 1.2 The effects on aerial
12、 cables 2 . Lightning strokes which terminate in the vicinity of a buried telecommunication cable . 2.1 Effects on cables with metal sheaths . 2.2 Effects on cables with an insulating covering over the metal sheath 2.3 Effects on cables with non-metallic sheaths 2.4 Effects on cables connected to ov
13、erhead lines . 2.5 Effects on cable to mountain top radio stations 2.6 Effects on cable conductors and associated insulation 2.6.1 Symmetric pairs . 2.6.2 Coaxial pairs . Appendix I : Propagation of lightning currents on an overhead line . Appendix 2: Simplified formula for the impulse voltage betwe
14、en conductors and metal sheath at the point of entry of the lightning current CHAPTER 5 . Protective devices 1.Generai . 2 . Types of lightning protectors 2.1 Air gap protectors . 2.2 Gas-filled protectors . 3 . Junctiondiodes 4.Fuses 5 . Coupling coils 6 . Choke coils in series 7 . Transverse prote
15、ctors . 8 . Forward protectors . 9. Shields or guards . 9.1 Shield wires placed above an open-wire line . 9.2 Shield wires above birried cables 10 . Iron pipes 11 . Cables of special design 11.1 Transfer impedance of the metallic sheath 11.2 Insulation of the conductors . 11.3 Qualityfactor 11.4 Equ
16、ipment connected to cables of special design . 12 . Code. construction and protection of plastic-sheathed or plastic-covered-cables 13 . Specification for transformers with high dielectric strength used in conjunction with cables of specialdesign Appendix . Calculation of cable characteristics with
17、increased resistance to lightning currents and . withlayersheaths (Information furnished bythe Swiss Administration) 41 42 42 43 43 50 51 51 51 51 51 52 53 55 57 57 57 57 58 58 58 59 59 60 60 60 60 61 61 62 62 62 62 64 64 65 66 1974 1974 1974 1974 1974 1974 1974 1974 1974 1974 1974 1974 1974 1974 19
18、74 1974 1974 1974 1974 1974 1974 1974 1974 1974 1974 1974 1974 1974 1974 1974 1974 1974 1974 1974 1974 1974 1974 1974 1974 1974 1974 1974 1974 Ed. 1974 COPYRIGHT International Telecommunications Union/ITU TelecommunicationsLicensed by Information Handling ServicesCHAPTER 1 INTRODUClION 1. General St
19、atistics of the causes of damage to telephone installations in various countries show that many breakdowns and faults are due to atmospheric discharges. Both overhead lines and telecommunication cables and associated technical equipment are subject to these effects. Early in the development of tele-
20、 communication technique, great importance was attached to protection of overhead telephone lines (which predominated at the time) against lightning. With the growing use of telecommunication cables it became recognized that the latter are likewise subject to damage by lightning, although it was frs
21、t thought that a buried cable was sheltered from the harmful effects of atmospheric discharges. This is true only for some of the phenomena occurring at the time of the lightning discharge. The importance of cable telecommunications has obliged engineers to go deeply into the physical Characteristic
22、s of atmospheric discharges and to draw conclusions for the efficient protection of tele- communication lines and associated equipment. Furthermore, many installations contain solid-state elements (transistors, diodes, etc.) which are much more sensitive than classical elements and which are liable
23、to be damaged at fairly low voltages and currents. This Handbook contains a general survey of atmospheric discharge phenomena and of the protection devices in use. The theoretical explanations in the appendices to the various chapters are necessary for an under- standing of the phenomena as weli as
24、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 al
25、so 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 Hand- book does not deal with the protection of the stations themselves, since such problems concern the pro- tection of b
26、uildings 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. 2. Aims and limi
27、ts of protection against lightning The efficiency of devices for the protection of telecommunication equipment against lightning (atmo- spheric discharges) is governed by the expenditure that can be afforded according to the size of the instal- lation. A very big, expensive installation, such as a c
28、arrier cable including amplifiers for several thousand telephone channels and other transmission channels, requires-from the economic point of view-a protection that is as complete as possible in order to avoid service interruptions and the loss of revenue, while a radio installation located high up
29、 again requires the same complete protection since it ensures the safety of aviation and thus protects human life. On the other hand, it will suffice for a less important tele- phone line to use less expensive protection against lightning. Ed. 1974 COPYRIGHT International Telecommunications Union/IT
30、U TelecommunicationsLicensed by Information Handling Services4862573 Ob83112 TI7 6- cm 1/2 The distribution of lightning discharges according to place, magnitude and duration is subject to statistical variations which may differ for each country according to its geographical position and climatic an
31、d geological conditions. The extent of the tolerable incidence of damage being exceeded is affected by the number of thunderstorm days per year. Lightning discharges may range from a few hundred amperes in strength up to several hundred thousand amperes for an effective duration running from a few m
32、icroseconds to several milliseconds. It is obvious that expenditure for anti-lightning devices also varies according to the discharges that are to be expected. Observations made in many countries reveal that only a limited number of all lightning strokes exceed 50 kA (see Figure 2.2) and that for mo
33、st lightning strokes this current falls to half-value within some 65 microseconds. Hence, it seems useful and adequate to limit the devices for general protection against lightning described in this Handbook to the mean statistical values given in the preceding paragraph. Furthermore, if allowance i
34、s made for the local distribution of lightning discharges, adequate anti-lightning protection for telecommunication installations is obtained in a large number of instances. If frequent and relatively large discharges are to be expected, special protection is recommended in accordance with the relev
35、ant chapters of this Handbook. As indicated above, the geological structure of the region plays an important part in lightning dis- charges, whether they occur directly to ground or to metal conductors that have been laid in it. Experience shows that lo. many regions it is unnecessary to arrange for
36、 special protection against lightning. As a general rule, these are non-mountainous regions where ground resistivity is relatively low, which is a favourable factor as regards natural protection against lightning. The influence of ground resistivity will be explained below. To place a further limit
37、on expenditure for anti-lightning protection, one could propose to consider regions having less than 15 thunderstorm days per year and a ground resistivity of less than 100 ohms * m only in particularly unfavourable cases and to dispense in these regions (particularly if the land is Bat) with any sp
38、ecial protection against lightning going beyond normal standards, such as the use of standard underground cable construction and overhead line surge limiters. From the point of view of economics the telephone density of the area concerned may also have to be taken into account. Every Administration
39、or recognized private operating agency is free to spend more or less than the sums indicated, according to the importance of the installation concerned. 3. Definitions The terms used in this Handbook have, in general, the same meaning as is given in the publications of the International Electrotechn
40、ical Commission, though there are some instances of terms used with a meaning that differs from that given by the IEC or by earlier C.C.I.T.T. documents. It has also been necessary, in this Handbook, to define certain terms having a special use in connection with the protection of telecommunications
41、 against lightning. 3.1 Thunderstorm day Day characterized by a meteorological phenomenon during which thunder is clearly audible. 3.2 Isokerawic line Line on a map connecting localities having the same mean number of thunderstorm days per year. 3.3 Impulse current An impulse current is ideally an a
42、periodic transient current which rises rapidly to a maximum value and falls usually less rapidly to zero. It is characterized by polarity, peak value, front time and time-to-half-value. A distinction is made between the ideal impulses used in theoretical investigations, for which these cha- racteris
43、tics may be precisely specified, and the impulses encountered in measurements where ambiguities may anse in the exact determination of peak value, front time and time-to-half-value. In the latter case, Ed. 1974 COPYRIGHT International Telecommunications Union/ITU TelecommunicationsLicensed by Inform
44、ation Handling ServicesCHAPTER 113.3 7 virtual values are determined by the procedure recommended by the International Electrotechnical Com- mission given below. Peak value, alternatively virtual peak value The peak value is normally the maximum value. In some measurements, overshoot or oscillations
45、 may be present near the current peak. The maximum value of the smooth curve drawn through the oscillations is defined as the virtual peak value. In what follows, the term “peak value” is to be understood as including the term “virtual peak value”. The truefront time T, of a double-exponential impul
46、se current is the time from the commencement of the impulse to its peak value. The true time to half-value Th of a double-exponential impulse current is the time from the commence- ment of the impulse to half the peak value on its back (tail). The virtual front time T, of any impulse current is defi
47、ned as 1.25 times the interval between the instants when the impulse current is 10% and 90% of the peak value (points A and B respectively in Figure 1.1). The virtual origin O, of an impulse current is defmed as the instant preceding that corresponding to the time at which the current is 10% of the
48、peak value by 0.1 Tl (IEC Publication 60, 7.1.4). The virtual time to half-value T, of an impulse current is the time interval between the virtual ongin and the instant on the back (tail), where the current has decreased to half the peak value. 3.4 Impulse and surge voltages i) Impulse voltage In ge
49、neral an impulse voltage is characterized with respect to time in the same way as an impulse current with the exception that the reference point A is chosen in a different manner, which results in the following changes : The virtual front time TI is defined as 1.67 times the time interval between the instants when the impulse is 30% and 90% of the peak value (points A and B of Figure 1.2). The virtual origin O, of an impulse voltage is defined as the instant preceding that corresponding to point A by a time 0.3 T,. The virtual time to half-value T, is the time interval between the virtua
