1、INTERNATIONAL TELECOMMUNICATION UNION ITU-T TE LEC0 M MU N I CAT I ON STAN DARD IZATIO N SECTOR OF ITU K.25 (02/2 o 00) SERIES K: PROTECTION AGAINST INTERFERENCE Protection of optical fibre cables ITU-T Recommendation K.25 (Formerly CCITT Recommendation) ITU-T RECOMMENDATION K.25 PROTECTION OF OPTIC
2、AL FIBRE CABLES Summary This Recommendation provides a procedure to protect the telecommunication lines using fibre optics against direct lightning discharges to the line itself or to the structures that the line enters. The protection procedure is related to the exposure of the line to direct light
3、ning discharges and includes the selection of cable characteristics/installation, use of shield wires, bonding/earthing of the cable shield, installation of surge protective devices (SPD) and route redundancy. Source ITU-T Recommendation K.25 was revised by ITU-T Study Group 5 (1997-2000) and was ap
4、proved under the WTSC Resolution No. 1 procedure on 25 February 2000. Recommendation K.25 (02/2000) 1 FOREWORD ITU (International Telecommunication Union) is the United Nations Specialized Agency in the field of telecommunications. The ITU Telecommunication Standardization Sector (ITU-T) is a perman
5、ent organ of the ITU. The ITU-T is responsible for studying technical, operating and tariff questions and issuing Recommendations on them with a view to standardizing telecommunications on a worldwide basis. The World Telecommunication Standardization Conference (WTSC), which meets every four years,
6、 establishes the topics for study by the ITU-T Study Groups which, in their turn, produce Recommendations on these topics. The approval of Recommendations by the Members of the ITU-T is covered by the procedure laid down in WTSC Resolution No. 1. In some areas of information technology which fall wi
7、thin ITU-Ts purview, the necessary standards are prepared on a collaborative basis with IS0 and IEC. NOTE In this Recommendation, the expression “Administration“ is used for conciseness to indicate both a telecommunication administration and a recognized operating agency. INTELLECTUAL PROPERTY RIGHT
8、S The ITU draws attention to the possibility that the practice or implementation of this Recommendation may involve the use of a claimed Intellectual Property Right. The ITU takes no position concerning the evidence, validity or applicability of claimed Intellectual Property Rights, whether asserted
9、 by ITU members or others outside of the Recommendation development process. As of the date of approval of this Recommendation, the ITU had not received notice of intellectual property, protected by patents, which may be required to implement this Recommendation. However, implementors are cautioned
10、that this may not represent the latest information and are therefore strongly urged to consult the TSB patent database. o ITU 2000 All rights reserved. No part of this publication may be reproduced or utilized in any form or by any means, electronic or mechanical, including photocopying and microfil
11、m, without permission in writing fi-om the ITU. 11 Recommendation K.25 (02/2000) CONTENTS 1 2 3 4 5 5.1 5.2 5.3 6 7 7.1 7.2 7.3 7.4 7.5 Scope and object . References . Definitions Reference configwation Construction characteristics of the cable General Failure current for buried cable or aerial cabl
12、e with earth connections of the metal sheath Failure current for aerial cable without earth connections of the metal sheath Need for protection . Protective measures General Dielectric or metal-fiee cables Choice of cable characteristics for both buried and aerial installations Use of shield wire fo
13、r buried cables . Route redundancy . Annex A . Frequency of primary failures . A . 1 Buried cable A.2 Aerial cables A.3 Annex B - Sheath breakdown current . B . 1 Buried cable B.2 Aerial cable . Aerial cable without earth connections of the metal sheath Aerial cable with earth connections of the met
14、al sheath . Annex C - Tests for surge current resisitivity . C.l General C.2 Breakdown voltage test . C.3 C.4 C.5 Cables entering a structure exposed to direct lightning strokes B.2.1 B.2.2 Test for surge current resistibility of the interconnecting elements Sand box test for buried cables . Test fo
15、r aerial cables . Recommendation K.25 (02/2000) Page 1 1 1 2 3 3 3 4 5 5 5 6 6 6 7 7 7 8 8 9 9 9 9 10 10 10 11 11 11 12 . 111 Page Appendix I - Shielding factor I. 1 Shielding factor for one shield wire 13 13 1.2 Shielding factor for multiple shield wires . 14 1.3 Minimum length of buried shield wir
16、es 15 Appendix II - Tolerable frequency of primary failures (Fa) 16 Appendix III - Environmental factor () . 16 17 Appendix IV - Bibliography iv Recommendation K.25 (02/2000) Recommendation K.25 PROTECTION OF OPTICAL FIBRE CABLES (Melbourne, 1988, revised in 1996 and 2000) 1 Scope and object The sco
17、pe of this Recommendation is the protection against lightning of telecommunication lines in fibre optics installations. The object of this Recommendation is to limit the number of possible primary failures occurring in the optical fibre cable in a specified installation within values which are lower
18、 than or equal to the limit value, defined as the tolerable fiequency of primary failures. Consequently, this Recommendation points out a method for both calculating the possible number of primary failures and choosing the feasible protective measures. 2 References The following ITU-T Recommendation
19、s and other references contain provisions which, through reference in this text, constitute provisions of this Recommendation. At the time of publication, the editions indicated were valid. All Recommendations and other references are subject to revision: all users of this Recommendation are therefo
20、re encouraged to investigate the possibility of applying the most recent edition of the Recommendations and other references listed below. A list of the currently valid ITU-T Recommendations is regularly published. i CCITT Recommendation K.29 (1992), Coordinated protection schemes for telecommunicat
21、ion cables below ground. 2 ITU-T Recommendation K. 1 1 (1993), Principles of protection against overvoltages and overcurren ts. 3 ITU-T Recommendation K.39 (1996), Risk assessment of damages to telecommunication sites due to lightning discharges. 3 Definitions This Recommendation defines the followi
22、ng terms: 3.1 as consequence the interruption of service. 3.2 in an optical fibre installation due to direct lightning flashes. 3.3 installation due to direct lightning flashes. 3.4 failures in an optical fibre installation not requiring additional protective measures. 3.5 due to direct lightning fl
23、ashes not requiring additional protective measures. 3.6 buried cable, in which it is assumed that all lightning current is conducted to the cable. primary failure: Damage caused by a lightning strike in a telecommunication line that has frequency of primary failures (Fp): Average annual number of ex
24、pected primary failures risk of primary failures (Rd): Probable average annual loss of function in the optical fibre tolerable frequency of primary failures (Fa): Average annual fiequency of primary tolerable risk of primary failures (Ra): Maximum level of the risk of primary failures (Rd) direct li
25、ghtning flash: Lightning flash to an aerial cable or to the ground surface near to a Recommendation K.25 (02/2000) 1 3.7 direct lightning flash frequency (Nd): Expected average annual number of direct lightning flashes to an optical fibre installation. 3.8 striking distance (D): Distance from a buri
26、ed cable that, when multiplied by 2, by the line length (L) and the ground flash density (Ng), gives the direct lightning flash frequency (Nd) of the cable. 3.9 failure current (Ia): Minimum peak value of the lightning current giving rise to a direct arc on the cable and causing primary failures. 3.
27、10 sheath breakdown current (Is): Minimum current flowing in the metallic sheath which causes breakdown voltages between metallic elements inside the cable core and the metallic sheath, thus leading to primary failures. 3.11 connection current (Ic): Minimum current flowing in the interconnecting ele
28、ments that causes a primary failure due to thermal or mechanical effects. 3.12 interconnecting elements: Metallic elements connecting metallic sheath(s) or the metallic strength member(s) of optical fibre cable at joints and cable ends. 3.13 test current (I,): Minimum current injected by arc in the
29、cable metallic sheath that causes a primary failure due to thermal or mechanical effects. 3.14 breakdown voltage (Ub): Impulse breakdown voltage between metallic components in the core and the metallic sheath of the optical cable. 3.15 damage correction factor ( type B: cable with dielectric core an
30、d metal sheath or sheaths - there are no metal elements in the core of the cable which has a metal sheath (for example the moisture barrier) or a metallic supporting wire; type C: cable with metal elements in the core and with a metal sheath or sheaths - there are metal elements, such as conductors
31、or strength members, in the core of the cable which has one or more metal sheaths; type D: cable with metal elements in the core and without a metal sheath. - - - For cable types B, C and D, the value of failure current (Ia) shall be evaluated, except for cables with more than one metal sheath. 5.2
32、Failure current for buried cable or aerial cable with earth connections of the metal sheath The failure current (Ia) is the lower value among the following values (see Figwe 2): - twice the connection current (I,) evaluated with the test for surge current resistibility of the interconnecting element
33、s (C.3); the test current (It) evaluated with the test for surge current resistibility shown in C.4 for buried cables or C.5 for aerial cables (see Bibliography 2); for type C cables, twice the sheath breakdown current (I,) flowing in the cable metallic sheath (with or without an insulating covering
34、). This current is calculated using the procedure of Annex B. - - Recommendation K.25 (02/2000) 3 Then: or: or: metal sheath strength metal Il interconnecting elements A I, test current - I, sheath breakdown current IC connection current ub breakdown voltage T0509700-00 Figure 2/K.25 - Lightning cur
35、rents in an optical fibre cable Ia = It if It 21, and I, 21, and I, J and J 21, and 21, ). It can be realized that the distance x between the shield wires and the cable has little effect on the shielding factor. For the other tables this value is kept constant as x = 0.25 m. Shielding factor for mul
36、tiple shield wires s=5mm 0.62 0.60 0.59 0.58 Table I.3/K.25 - Shielding factor for two wires disposed in a circle around the cable s=8mm s=12mm 0.59 0.55 0.57 0.54 0.56 0.54 0.55 0.53 x(m) 0.15 0.25 0.50 1 .o0 The same calculations have been performed for three wires disposed in a circle around the
37、cable, where the angle g is with respect to the vertical see Figure 1.1 (c). The values are shown in Table 1.4, where x = 0.25 m. g = 30 g = 45 g = 60 g = 90 0.38 0.36 0.34 0.33 0.38 0.35 0.34 0.33 0.37 0.35 0.34 0.33 0.37 0.35 0.34 0.33 Table I.4/K.25 - Shielding factor for three wires disposed in
38、a circle around the cable g = 30 0.33 g = 60 g = 90 g = 120 0.26 0.23 0.22 Table I.5/K.25 - Shielding factor for n wires symmetrically disposed in a circle around the cable n=4 0.16 I I I I n=6 n=8 0.09 0.06 14 Recommendation K.25 (02/2000) 1.3 In order to protect a telecommunication line in an expo
39、sed environment, a buried shield wire shall follow the cable in all extensions of the exposed section and it shall also be continued for a length Y beyond this section. The same procedure applies for the protection of lines entering exposed structures, where the shield wire shall follow the cable ro
40、ute fiom the exposed structure for a length Y. Naturally, the shield wires shall be bonded to the cable sheath and the structure earthing system. The length Y can be evaluated by equation (1-2): Minimum length of buried shield wires where: p = soil resistivity in l2.m. - - -7 - -. cable x L :Io:-: c
41、 .-e-. 0 . / / I &o$ I T0508870-99 Figure I.UK.25 - Configurations of shield wires Recommendation K.25 (02/2000) 15 APPENDIX II Tolerable frequency of primary failures (Fa) The damage caused by lightning to optical fibre installations may produce an unacceptable loss of services to the public. In th
42、is case, the decision whether or not to provide protective measures should be taken by a comparison of the actual value of frequency of primary failures (Fp) to the optical fibre installation with the limit value of the tolerable frequency of primary failures (Fa), fixed by each Network Operator. Th
43、e value Fa can be estimated with the following equation: Ra Fa =- 6 where: Ra tolerable risk of primary failures 6 relative amount of the expected losses per damage The following values of Ra and 6 are suggested: 6= Ra = 1 OP4 Therefore: APPENDIX III Environmental factor (&) The evaluation of l& sha
44、ll be performed based on the typical construction parameters of the region considered: - - - - - Urban area with tall buildings (above 6 floors): K, = 0.01 Urban area with medium buildings (between 3 and 6 floors): K, = O. 1 Suburban area with houses (one or two floors): K, = 0.5 Rural area without
45、constructions (flat ground): K, = 1 Rural area without constructions (top of hill): K, = 2 16 Recommendation K.25 (02/2000) APPENDIX IV Bibliography 11 The protection of telecommunication lines and equipment against lightning discharges. Chapter 9: Fibre optic cable lightning damage assessment, ITU,
46、 1994. 2 The protection of telecommunication lines and equipment against lightning discharges - ITU, 1974 and 1978. 3 UNGAR (S.G.): Effects of lightning punctures on the core-shield voltage of buried cable, The Bell System Technical Journal, Vol. 59, No. 3, March 1980. 4 SUNDE (E.): Earth conduction effects in transmission system, Dover Publications, Inc., New York. 5 BENDAYAN (J.): Cbles rsistant aux dommages causs par la foudre, Cables & Transmission, October 1972. Recommendation K.25 (02/2000) 17
