1、 I n t e r n a t i o n a l T e l e c o m m u n i c a t i o n U n i o n ITU-T K.101 TELECOMMUNICATION STANDARDIZATION SECTOR OF ITU (12/2014) SERIES K: PROTECTION AGAINST INTERFERENCE Shielding factors for lightning protection Recommendation ITU-T K.101 Rec. ITU-T K.101 (12/2014) i Recommendation ITU
2、-T K.101 Shielding factors for lightning protection Summary Recommendation ITU-T K.101 contains the calculation procedure for the shielding factors used in Recommendations ITU-T K.46, ITU-T K.47, ITU-T K.56 and ITU-T K.97, and it is intended to: 1) provide the rationale for the shielding factor appr
3、oximate formulas or values contained in the aforementioned Recommendations; 2) allow a more precise calculation of the shielding factors, by using the equations provided in this Recommendation. The shielding factors considered in this Recommendation refer to: that of the lightning current distributi
4、on in a telecommunication tower, i.e., that of the tower (T) as per Recommendations ITU-T K.56 and ITU-T K.97; that of the lightning current distribution among several conductors placed in a cable ladder, i.e., that of the feeder tray (F) as per Recommendations ITU-T K.56 and ITU-T K.97; that provid
5、ed by the cable tray () as per Recommendation ITU-T K.56; that of parallel conductors () as per Recommendation ITU-T K.47; that of refraction () due to the earthing connection of a telecommunication cable, as per Recommendation ITU-T K.46; that provided by a telecommunication cable having a metallic
6、 sheath (shield) () as per Recommendation ITU-T K.46. History Edition Recommendation Approval Study Group Unique ID* 1.0 ITU-T K.101 2014-12-07 5 11.1002/1000/12291 Keywords Earthing, lightning, shielding, telecommunication lines, telecommunication towers. _ * To access the Recommendation, type the
7、URL http:/handle.itu.int/ in the address field of your web browser, followed by the Recommendations unique ID. For example, http:/handle.itu.int/11.1002/1000/11830-en. ii Rec. ITU-T K.101 (12/2014) FOREWORD The International Telecommunication Union (ITU) is the United Nations specialized agency in t
8、he field of telecommunications, information and communication technologies (ICTs). The ITU Telecommunication Standardization Sector (ITU-T) is a permanent organ of ITU. ITU-T is responsible for studying technical, operating and tariff questions and issuing Recommendations on them with a view to stan
9、dardizing telecommunications on a worldwide basis. The World Telecommunication Standardization Assembly (WTSA), which meets every four years, establishes the topics for study by the ITU-T study groups which, in turn, produce Recommendations on these topics. The approval of ITU-T Recommendations is c
10、overed by the procedure laid down in WTSA Resolution 1. In some areas of information technology which fall within ITU-Ts purview, the necessary standards are prepared on a collaborative basis with ISO and IEC. NOTE In this Recommendation, the expression “Administration“ is used for conciseness to in
11、dicate both a telecommunication administration and a recognized operating agency. Compliance with this Recommendation is voluntary. However, the Recommendation may contain certain mandatory provisions (to ensure, e.g., interoperability or applicability) and compliance with the Recommendation is achi
12、eved when all of these mandatory provisions are met. The words “shall“ or some other obligatory language such as “must“ and the negative equivalents are used to express requirements. The use of such words does not suggest that compliance with the Recommendation is required of any party. INTELLECTUAL
13、 PROPERTY RIGHTSITU draws attention to the possibility that the practice or implementation of this Recommendation may involve the use of a claimed Intellectual Property Right. ITU takes no position concerning the evidence, validity or applicability of claimed Intellectual Property Rights, whether as
14、serted by ITU members or others outside of the Recommendation development process. As of the date of approval of this Recommendation, ITU had not received notice of intellectual property, protected by patents, which may be required to implement this Recommendation. However, implementers are cautione
15、d that this may not represent the latest information and are therefore strongly urged to consult the TSB patent database at http:/www.itu.int/ITU-T/ipr/. ITU 2015 All rights reserved. No part of this publication may be reproduced, by any means whatsoever, without the prior written permission of ITU.
16、 Rec. ITU-T K.101 (12/2014) iii Table of Contents Page 1 Scope . 1 2 References . 1 3 Definitions 1 3.1 Terms defined elsewhere 1 3.2 Terms defined in this Recommendation . 2 4 Abbreviations and acronyms 2 5 Conventions 2 6 Shielding factor due to conductors in parallel 2 6.1 Modelling the problem .
17、 2 6.2 Guard-wire 4 6.3 Telecommunication towers 4 6.4 Cable trays and bundle of cables 7 7 Earthing connection of cables . 8 8 Shielding factor of telecommunication cables 10 Annex A Geometric mean radius of conductors . 11 Rec. ITU-T K.101 (12/2014) 1 Recommendation ITU-T K.101 Shielding factors f
18、or lightning protection 1 Scope This Recommendation provides the rationale and formulas for calculating the shielding factors used in series K Recommendations dealing with lightning protection of telecommunication lines and structures. The shielding factors considered are related to: the use of a me
19、tallic tower to hold telecommunication feeder cables and antennas; the placement of several feeder cables in a metallic cable ladder in the tower; the placement of telecommunication cables in a metallic cable tray; the use of a guard-wire above a buried telecommunication cable; the connection of a t
20、elecommunication cable to earth; the use of a telecommunication cable with a metallic sheath. 2 References The following ITU-T Recommendations and other references contain provisions which, through reference in this text, constitute provisions of this Recommendation. At the time of publication, the
21、editions indicated were valid. All Recommendations and other references are subject to revision; users of this Recommendation are therefore encouraged to investigate the possibility of applying the most recent edition of the Recommendations and other references listed below. A list of the currently
22、valid ITU-T Recommendations is regularly published. The reference to a document within this Recommendation does not give it, as a stand-alone document, the status of a Recommendation. ITU-T K.46 Recommendation ITU-T K.46 (2012), Protection of telecommunication lines using metallic symmetric conducto
23、rs against lightning-induced surges. ITU-T K.47 Recommendation ITU-T K.47 (2012), Protection of telecommunication lines against direct lightning flashes. ITU-T K.56 Recommendation ITU-T K.56 (2010), Protection of radio base stations against lightning discharges. ITU-T K.97 Recommendation ITU-T K.97
24、(2014), Lightning protection of distributed base stations. 3 Definitions 3.1 Terms defined elsewhere This Recommendation uses the following terms defined elsewhere: 3.1.1 cable tray ITU-T K.56: Rigid structural system used to securely fasten or support cables. 3.1.2 feeder cable ITU-T K.56: Wave-gui
25、de or coaxial cable that conducts signals to an antenna. 3.1.3 guard-wire ITU-T K.47: Metallic wire buried above a cable in order to reduce physical damage due to direct lightning flashes to the cable. 3.1.4 refraction factor ITU-T K.46: Ratio between the common-mode surge voltage travelling in the
26、line after passing through a discontinuity in its surge impedance and the surge that would travel in the line if there was no discontinuity in its surge impedance. 2 Rec. ITU-T K.101 (12/2014) 3.1.5 shielding factor ITU-T K.56: Factor that represents the attenuation of the voltage or current in a co
27、nductor due to the presence of a nearby shielding conductor. 3.2 Terms defined in this Recommendation This Recommendation defines the following terms: 3.2.1 shielded cable: Group of one or more pairs of twisted wires balanced with respect to earth, assembled together and covered by a continuous meta
28、llic sheath. 3.2.2 unshielded cable: Group of one or more pairs of twisted wires balanced with respect to earth and assembled together without a metallic sheath. 4 Abbreviations and acronyms This Recommendation uses the following abbreviations and acronyms: DC Direct Current GDT Gas Discharge Tube G
29、MR Geometric Mean Radius SPC Surge Protective Component 5 Conventions None. 6 Shielding factor due to conductors in parallel The shielding factor, considered in this clause, is relevant to the following Recommendations: ITU-T K.56: tower shielding factor (T); feeder shielding factor (F); shielding f
30、actors of cable trays (). ITU-T K.47: guard-wire shielding factor (). 6.1 Modelling the problem In many practical situations, there is a transient current injected in a transmission system formed by multiple conductors in parallel. This is the case of a metallic tower struck by lightning, where the
31、current is distributed among the tower structure and the available conductors. As the time-derivative of the lightning current is high, the initial current distribution is determined by the magnetic flux linkages within the conductors. Therefore, if the low time-derivative wave-tail is not of concer
32、n, it is permissible to neglect the resistance of the conductors when assessing the current distribution. Under this assumption, a number of practical problems may be represented by the canonical structure shown in Figure 1, in which two perfectly conducting cylinders of radius r1 and r2 are placed
33、in parallel and submitted to a time-varying current I(t). The problem consists in determining the currents I1(t) and I2(t) of each conductor, based on their dimensions and separation b. The boundary condition required to calculate the current distribution is that the flux linkage in the area between
34、 the two conductors shall be null. This condition comes from the application of Maxwells equation: Rec. ITU-T K.101 (12/2014) 3 AtHlE AL dddd (1) Figure 1 Representation of a transient current being split into two parallel conductors where the integration of the electric field E follows the path L a
35、long the dashed line in Figure 1 and the magnetic field H is integrated in the area A = ab delineated by the dashed rectangle. As the electric field within a perfect conductor is null, the left-hand side of equation (1) is null. Therefore, the integral of the magnetic field time derivative within th
36、e loop area must be null. Consider that the magnetic field has an harmonic time dependency given by: tjHtH exp)( 0 (2) where is the angular frequency and j = (1)1/2 is the imaginary unit. This is not a limitation, as any impulsive signal can be broken down into a number of components of different fr
37、equencies. For each frequency, the integral of the magnetic field in the area between the conductors must be null. This means that the magnetic flux generated by the current I1 in the dashed loop must be equal to the magnetic flux generated by the current I2, as the corresponding magnetic fields hav
38、e opposite directions. This condition is expressed by: 1221d2d2 21 rbrrbrxxIaxxIa (3) Note that the integration limits are b r2 and b r1 because, due to the skin effect, the magnetic field within perfect conductors is null. A physical interpretation of equation (3) is that the smaller conductor 2 ha
39、s a larger area to integrate the magnetic field, so that a lower current I2 is necessary to generate the same field that is generated by the current I1. Solving equation (3) gives: 212121 lnln r rbIr rbI (4) The total current is given by: 21 III (5) Defining the shielding factor () as the fraction o
40、f the total current I that is left in conductor 1 after the placement of conductor 2, gives: 4 Rec. ITU-T K.101 (12/2014) II1 (6) Inserting equation (5) into equation (4) and rearranging gives: 21 212 1 lnln rr rbrbr rb(7) In the situations where b r1 and b r2, equation (7) can be simplified as: 212
41、2lnln rr brb (8) 6.2 Guard-wire A common protection of buried cables against lightning flashes is the installation of a guard-wire above the cable, as shown in Figure 2. Besides protecting the cable from being directly struck by lightning (see ITU-T K.47), a guard-wire regularly bonded to the cable
42、metallic sheath also diverts to earth part of the lightning current of the cable screen. The model described in clause 6.1 can be applied to this situation, as the two conductors are in parallel. Making reference to Figure 2, Equation (8) can be rewritten as: gcg rrdrd 2lnln (9) where rg is the guar
43、d-wire radius, rc is the telecommunication cable radius, and d is the distance between these conductors. For example, consider a guard-wire with radius rg = 3 mm placed in parallel with a buried cable with rc = 10 mm metallic sheath and bonded to it at regular intervals. The distance between the two
44、 conductors is d = 300 mm. Equation (9) provides the shielding factor = 0.58. If the cable of Figure 2 is connected to a structure (e.g., a telecommunication tower) that is struck by lightning, the cable metallic sheath will carry 58 per cent of the total current, while the guard-wire will carry the
45、 remaining 42 per cent. The reduction of the lightning current in the cable may have a significant effect on the cable protection, as described in ITU-T K.47. Figure 2 Guard-wire above a telecommunication cable 6.3 Telecommunication towers The model developed in clause 6.2 may be used to assess the
46、current distribution on towers struck by lightning. In this case, the relevant shielding factor is related to the fraction of the total lightning current that flows through the cable feeders. This clause shows some examples that are good approximations to common tower structures, where the shielding
47、 factor provided by the tower is designated T to be in line with ITU-T K.56 and ITU-T K.97. Rec. ITU-T K.101 (12/2014) 5 6.3.1 Tubular tower If telecom cables are placed inside a tubular tower, the total lightning current flows through the tower and no current flows through the cable, so that T = 0.
48、 Of course, this is an approximation that neglects the tower resistance. Otherwise, a more detailed calculation is needed, which takes into account the tower resistance. In this case, the cables are affected by the electric field developed along the internal surface of the tower. This electric field
49、 is given by the product of the internal surface impedance of the tower and the current in the tower. If telecom cables are placed outside the tower, the distribution of current is determined by equations (7) or (8), which are rewritten here as equations (10) and (11), with the tower radius as rt, the conductor radius as rc, and the distance between the t
