1、4862591 Ohb4 TBT m THE INTERNATIONAL TELEGRAPH AND TELEPHONE CONSULTATIVE COMMITTEE (C.C.I.T.T.) THE PROTECTION OF TELECOMMUNICATION LINES AND EQUIPMENT AGAINST LIGHTNING DISCHARGES (CHAPTERS 6,7 AND 8) Published by THE INTERNATIONAL TELECOMMUNICATION UNION 1978 4862573 b77b5 9Lb THE INTERNATIONAL T
2、ELEGRAPH AND TELEPHONE CONSULTATIVE COMMITTEE (C. C. 1.T.T. THE PROTECTION OF TELECOMMUNICATION LINES AND EQUIPMENT AGAINST LIGHTNING DISCHARGES (CHAPTERS 6, 7 AND 8) Published by THE INTERNATIONAL TELECOMMUNICATION UNION 1978 RE. V1.80 - 48b259L 06790bb 852 c O I.T.U. W 4Bb2591 0679067 797 TABLE OF
3、 CONTENTS Pape Edition CHAPTER 6 . Proteciive practices for speculc parts qftelecommunication networks . 1 . General . 2 . Protectionoflines . 2.1 Overheadlines 2.1.1 Open-wire lines 2.1.2 Aerial cables 2.1.3 Transitions from overhead lines 2.2 Underground cables 2.2.1 General . 2.2.2 Metallic-sheat
4、hed cables in contact with earth . 2.2.2.1 General considerations concerning such cables . 2.2.2.2 Cables of special design Cables with a metal sheath and a plastic covering 2.2.3.1 Use of shield wires 2.2.3.2 Other methods . 2.2.4 Laying cables in iron pipes . 2.2.3 3 . Protection of installations
5、against over-voltages due to atmospheric discharges 3.1 Dielectric strength of equipment . 3.2 Dielectric strength of cable accessories 3.3 Protection of line equipment 3.4 Protection of switching equipment . 3.5 Protection of subscriber equipment . 3.6 Protection of persons . 4.1 Voltage equalizati
6、on and earthing 4.2 Protection of equipment . Equipment installed inside transmitting towers . 4.2.2 Equipment installed in buildings near towers 4.3 Protection of lines to stations . 4 . Protection of radio stations in exposed positions 4.2.1 5 1978 5 1978 6 1978 6 1978 6 1978 6 1978 6 1978 7 1978
7、7 1978 7 1978 7 1978 8 1978 8 1978 9 1978 9 1978 9 1978 10 1978 10 1978 10 1978 11 1978 11 1978 11 1978 11 1978 11 1978 12 1978 12 1978 12 1978 12 1978 14 1978 CHAPTER 7 . Frequency of breakdowns in telecommunication systems as a result of lightning discharges . 15 1978 1 . General . 15 1978 i . 1 G
8、eneral data on the frequency of earth flashes 15 1978 2 . Anticipated frequency of breakdowns 18 1978 2.1 General . 18 1978 Ed . 1978 4862591 06790b8 625 Page Edition 2.2 Telecommunications towers 2.2.1 Frequency of lightning strokes to telecommunications towers . 2.2.2 Frequency of breakdowns to be
9、 anticipated in telecommunication-tower equipment 2.2.3 Frequency of breakdowns to be anticipated in buried cables with an outer conductive 2.3 Buried cable with insulating outer covering (type MP. P. A. B. C) 2.3.1 Anticipated breakdowns in buried cable with insulating outer covering without shield
10、 wires . 2.3.2 Efficiency of a shield wire buried parallel to a cable with an insulating outer covering 2.4 Buried cables with an outer metal sheath or armouring which is in contact with earth (type MPA.MA. M) 2.4.1 Total number of strokes to the cable 2.4.2 Anticipated breakdown values for undergro
11、und cables with an outer conductive sheath 2.5 Open-wire lines and aerial cable . 2.5.1 Anticipated frequency of strokes to open-wire lines and aerial cable . 2.5.2 Breakdowns to be anticipated in aerial cable with conductive outer sheath 2.5.3 Damage in open-wire lines . 2.6 Flashovers in the conne
12、cted equipment in case of absence or nonfunctioning of protectors 2.7 Conclusions concerning the degradation of cables sheath (type MA. M*) leading away from a telecommunications tower 18 1978 18 1978 19 1978 20 1978 21 1978 21 1978 24 1978 25 1978 25 1978 25 1978 25 1978 25 1978 26 1978 29 1978 29
13、1978 30 1978 Appendix 1: List of symbols and units relevant to Chapter 7 31 1978 Appendix 2: Explanations of the symbols H and C as used in formulae (3). (4). (8) and (10) 33 1978 Appendix 3: Examples for values of the transfer impedance of telecommunications towers 35 1978 36 1978 Appendix 5: Calcu
14、lation of anticipated breakdowns in buried cables with insulating outer covering (referred to in paragraph 2.3.1) . 37 1978 Appendix 6: Calculation of the anticipated number of strokes which will impact against an under- ground cable with an outer conductive sheath (referred to in paragraph 2.4.1) 3
15、8 1978 Appendix 7: calculation of the number of breakdowns anticipated in an underground cable with an outer conductive sheath (referred to in paragraph 2.4.2) . 39 1978 Appendix 8: Calculation of the number of strokes which may lead to faults in an aerial cable with Appendix 4: Arbitrary symbols of
16、 the types of cables (provisional table) . outer conductive sheath (referred to in paragraph 2.5.2) . 40 1978 CHAPTER 8 . Bibiiogruphy . 4 1 1978 *) See Appendix 4 for the provisional designation of various types of cables . Ed . 1978 = 4b2Cl Ob790b 5bl CHAPTER 6 PROTECTIVE PRACTICES FOR SPECIFIC PA
17、RTS OF TELECOMMUNICATION NETWORKS 1. General The purpose of the protection measures against lightning is to reach a balance between the cost of protec- ting lines and plant from lightning damage and the cost of repair and the tolerable number of faults (see Chapter i), in a given area. These costs w
18、ili depend on the number of thunderstorm days per year (keraunic level), soil resistivity and the degree of shielding provided by buildings, water pipes, and metallic structures in general. A classification of zones according to the risk of lightning strikes appears to be useful. It is not possible,
19、 however, to make a universally valid classification solely in terms of the number of thunderstorm days per year, because the effect of lightning on telecommunication cables (which is the real criterion) differs according to the other factors involved, including the local earth resistivity and the t
20、opographical situation. A relatively low keraunic level may therefore give rise to a low fault rate in one region but to a high fault rate in another. Each country must make its own local observations to determine the relation between the keraunic level in their areas and the fault liability, and to
21、 classify their zones accordingly. Figure 6.1 indicates how a synoptic table may be drawn up showing lowdanger and higher-danger zones as a function of the number of thunderstorm days and of earth resistivity. 100 - D 50 - - al al u C - .- a t Y 20 10 10 10 firn CCITT-ZZCUI Earth resistivity (p) 0 L
22、owdanger zone Highdanger zone FIGURE 6.1 - Schematic distribution of danger zones in terms of the number of thunderstorm days per year (keraunic level), and the earth resistivity, p Ed. 1978 6 D 48b259L Ob79070 283 CHAPTER 6/2 2. Protection of lines 2.1 Overhead lines The route followed by an overhe
23、ad line is usually dictated by local conditions and can hardy be changed to take account of the requirements of protection against lightning. 2.1.1 Open-wire lines As a rule, it is difficult to protect open wire lines against direct lightning strikes. In sectors where conduc- tors are often broken b
24、y the electrodynamic forces of powerful discharges, the frequency of such damage can be reduced, for example, by choosing conductors of larger diameter. Some protection may also be provided by installing shield wires above the open wires. Such shielding may be provided by either electric power dis-
25、tribution conductors in cases where common poles are used, or conductors installed especially for the pur- pose. The latter should be earthed at intervals of about 100metres. 2.1.2 Aerial cables The protection of aerial cables depends on the use to which they are put. Cables containing a large numbe
26、r of conductors and linking important traffic centres justify higher protection costs than less important cables of simple design with a small number of conductors. The replacement of less important cables in the event of infrequent breakdown is cheaper than the application of expensive protective m
27、easures. Cables of short length and simple design fairly often have plastic insulation and sheath without any metal screen, though they may have a metallic water barrier (see Chapter 5, paragraph 12, type A). The protective effect of a metal sheath may be characterized by the method described in Cha
28、pter 5, paragraph 1 1. The fact must be borne in mind, however, that earthing the metallic sheath at intervals (for ex- ample at suspension points) is less effective than a uniformly distributed earthing of a buried cable with wire or tape armouring. In consequence, to attain the same fault rate, ae
29、rial cables need a higher quality factor than underground cables. Thisincrease in the quality factor should be obtained mainly by choosing an insula- tion between the cable core and the metallic sheath having a high dielectric strength (for example synthetic plastic materials), because using a metal
30、lic sheath with a low resistance would result in an undesirable in- crease in the weight and of the cost of the cable. The protectors, installed at the end of such a cable to protect the equipment connected to the conductors at that point, also protect the cable over a certain length against breakdo
31、wn of the insulation due to surges. In very stormy regions and in the case of long cables, it might be advantageous to install protectors at distribution points. The purpose of such protectors is to prevent dangerous differences of potential between the conductors of the main cable and they must the
32、refore be put on the main cable side of distribution and cross-connection points. In the case of thermoplastic cables without metallic sheath, protection can be provided by a shield wire or a suspender wire earthed at a large number of poles. Open-wire line insulators should not be mounted on, and a
33、erial cables should not be hung from, trees. 2.1.3 Transitions from overhead lines i) Some Administrations fit protectors at transition points only when the open wire line or aerial cable ex- ceeds a certain length, for example, 500 metres. Howe-fer, experience in other countries has shown that in i
34、) This paragraph concerns those aerial cables having a plastic sheath, with or without a metal screen or water barrier. It does not concern aerial cables with a metallic sheath, which do not need protectors at such transition points. Ed. 1978 4862593 Ob79073 31T CHAPTER 612.1.3 7 areas where thunder
35、storms are frequent, there is a risk to subscribers and equipment even when the open wire line is short, and protectors must then be fitted. In any case, protectors provided at transition points must be connected between all conductors, including unused spare conductors, and the metal sheath of the
36、un- derground cable. In addition, the common point of the protectors should be connected to a special earth elec- trode (buried wire or spike) in order to reduce the discharge currents flowing in the cable sheath. Such an earth electrode should be laid in the transverse direction to the cable run in
37、 the horizontal plane. The resistance to earth of any additional earth electrode should be as low as possible. If a suficienty low resistance cannot be achieved, each conductor of the overhead line must be provided with additional ad- vanced protectors 200 to 500 metres from the transition point. Wh
38、en a protector strikes, an impulse voltage builds up along the cable between the conductors and the metal sheath (see Figure 4.6). It may therefore be advisable, particularly with paper-insulated cables, to provide additional protectors between the conductors and the metal sheath at intervals determ
39、ined by the extent to which the voltage exceeds the dielectric strength of the cable. All of the above considerations are also valid in the case of transition points between open wire lines and aerial cables. 2.2 Underground cables 2.2.1 General The considerations in Chapter 1, paragraph 2, concerni
40、ng the aims and limits of protection against light- ning are the frst things to bear in mind when planning cable protection. Cables laid in built-up areas, alongside rdways, near iron ducts, or near other installations which act as shields, have already a certain degree of protection. Even in open c
41、ountry, damage can be avoided to a certain extent by a careful choice of route. Cables should, as a rule, be laid well away from roots of trees, isolated or in rows, and buildings in an exposed situation. Similarly, when there are down-going conductors of lightning arrestors in the neighbourhood, su
42、fficient distance should be left between them and the telecommunication cable, unless special measures have been taken to protect the latter. 2.2.2 Metallic-sheathed cables in contact with earth 2.2.2.1 General considerations concerning such cables Cables of earlier classical design (for example, pl
43、astic-sheathed cables with an armouring of steel wire or tape or lead-sheathed cables) can be constructed with a resistance to atmospheric surges which should ensure satisfactory service reliability in areas where the soil conductivity is good to medium and which are moderate- ly exposed to thunders
44、torms (for example, flat country). In such cases, however, the following precautions should be taken : a) The cable armouring should be connected to the jointing sleeves and the metallic sheath by soldered b) The metallic sheaths of the cables should be connected to the earth conductors of exchange
45、and line c) All joints must be meticulously finished so as to obtain a dielectric strength as high as in the cable. d) In the vicinity of steel structures exposed to lightning stroke (for example, the earthing systems of pylons of electric power lines), the cable sheath must be insulated from the ea
46、rth (for example, by drawing the cable through a watertight thermoplastic tube). The area of risk is shown in Figure 3.7. connections. equipments. Ed. 1978 m 4662591 0679072 056 8 CHAPTER 6/2.2.2.1 Additional service reliability can be obtained by the following methods: Transformers may be inserted
47、to ensure the galvanic separation of the circuits in a cable from the equipments. They should have a dielectric strength adapted to that of the cable. This condition is nor- mally fulfded by cables with paper-insulated conductors, but not by those with thermoplastic insuia- tion. Protectors may be i
48、nserted with the shortest possible connections between all conductors (including spare conductors) and the metallic sheath at the ends of the line and at ali branching points, par- ticularly when the conductors are galvanically Connected to the equipments. One or more shield wires (see Chapter 5, pa
49、ragraph 9.2) may be laid above or alongside the cable. A suitable material for shield wires is galvanized iron wire, 6 to 8 rnm in diameter, which is more flexible than steel tape and is therefore easier to lay. When copper is used for shield wires, care must be taken that galvanic corrosion elements do not form in conjunction with other, nearby metals. 2.2.2.2 Cables of special design security against lightning currents is to be found in Chapter 5, paragraph 11, and in Figure 5.2. Guidance in the choice of sheath and insulation design for special cables so as to obtain a high
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