1、INTERNATIONAL TELECOMMUNICATION UNION ITU=T TELECOMMUNICATION STANDARDIZATION SECTOR OF ITU K.39 (I 0196) SERIES K: PROTECTION AGAINST INTERFERENCE Risk assessment of damages to telecommunication sites due to lightning discharges ITU-T Recommendation K.39 (Previously CCITT Recommendation) STD-ITU-T
2、RECMN K-37-ENGL 177b W 48b259L Ilb327L4 b59 ITU-T K-SERIES RECOMMENDATIONS PROTECTION AGAINST INTERFERENCE For further details, please refer to ITU-T List of Recommendations. FOREWORD The IT-T (Telecommunication Standardization Sector) is a permanent organ of the International Telecommunication Unio
3、n (ITU). The ITU-T is responsible for studying technical, operating and tariff questions and issuing Recommen- dations on them with a view to standardizing telecommunications on a worldwide basis. The World Telecommunication Standardization Conference (WTSC), which meets every four years, establishe
4、s 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 (Helsinki, March 1-12, 1993). IT-T Recommendation K.39 was prepar
5、ed by IT-T Study Group 5 (1993-1996) and was approved by the WTSC (Geneva, 9-18 October 1996). NOTES 1. telecommunication administration and a recognized operating agency. 2. follows: In this Recommendation, the expression ?Administration? is used for conciseness to indicate both a The status of ann
6、exes and appendices attached to the Series K Recommendations should be interpreted as - - an annex to a Recommendation forms an integral part of the Recommendation; an appendix to a Recommendation does not form part of the Recommendation and only provides some complementary explanation or informatio
7、n specific to that Recommendation. O ITU 1997 Ail 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 microfilm, without permission in writing from the ITU. Recommendation K.39 (10196) i 1 2 3 4
8、5 6 7 8 9 10 11 STDmITU-T RECMN K.39-ENGL L99b W 48b2591 Ub329LS 421 CONTENTS Introduction Scope List of references Definitions General . Coupling mechanisms 6.1 Resistive coupling . 6.2 Magnetic coupling 6.3 Electric coupling . 6.4 Electromagnetic coupling . Frequency of damage, F . Risk of damage,
9、 R . Principles for the risk assessment . 7.1 7.2 Calculation of risk areas . Estimation of probability factors p . Consequential damage factor 6 Acceptable risk levels, Raccept Appendix I . Example . Risk assessment of damages to a telecommunication site with an adjacent antenna mast . Page 1 1 1 2
10、 2 2 2 2 3 3 3 3 4 5 6 9 9 10 11 Recommendation K.39 (10/96) - STD-ITU-T RECMN K-3S-ENGL 177b I 4b2571 Ob32917 3b m SUMMARY This Recommendation gives support to protection engineers in evaluating the risk for severe damages caused by lightning. It comprises a risk analysis that includes the effect f
11、rom direct and nearby lightning strikes as well as overvoltages originating from incoming services, mainly power and communication networks. The analysis also indicates the most efficient additional protective measures for sites with insufficient basic protection. Part of this Recommendation deals w
12、ith the risk of injuries to people being present at the site during thunderstorms. This Recommendation is based on a concept from IEC TC 81 and modified to be applicable to practical cases in telecommunication systems. The example in Appendix I applies to a telecommunication site with an adjacent an
13、tenna mast. . Recommendation K.39 (10196) 111 Recommendation K.39 RISK ASSESSMENT OF DAMAGES TO TELECOMMUNICATION SITES DUE TO LIGHTNING DISCHARGES (Geneva, 1996) 1 Introduction Protective measures against a randomly occurring overvoltage phenomenon like lightning discharges have to a great extent b
14、een empirically based. Modem equipment has shown to be more prone to damages compared with older ones. Serious consequences in complexed communication systems may also occur in built-up areas, earlier classified as unexposed. The widespread use of radio stations with high antenna masts for wireless
15、telecommunication has signifi- cantly increased the risk for damages due to direct lightning strikes to the site. The need for protection should be based on a risk assessment considering the cost and importance of the system, the electromagnetic environment at the particular site and the probability
16、 of damages. The protection levels and the type of protective methods should also be chosen regarding the costs of installation and maintenance of protective devices. An assessment of the probability of overvoltage occurrences and the sensitivity of the existing telecommunication installation shall
17、give a possibility to attain a well-balanced protection of the whole system. Protective measures to eliminate severe injuries to people during thunderstorms are especially important at exposed sites that are permanently or temporarily manned. 2 Scope This Recommendation comprises a method for the ri
18、sk assessment of damages to equipment at telecommunication sites and personnel safety due to overvoltages and overcurrents caused by lightning discharges. 3 List of references The following IT-T Recommendations and other references contain provisions which, through reference in this text, constitute
19、 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 therefore encouraged to investigate the possibility of applying the most recent edition of the Rec
20、ommendations and other references listed below. A list of the currently valid IT-T Recommendations is regularly published. IEC 1662:1995, Assessment of the rW.k of damage due to lightning. IEC 1024-1 : 1990, Protection of structures against lightning, Part 1: General principles. IEC 1024- 1 - 1 : 19
21、93, Protection of structures against lightning, Part 1 : General principles - Section 1 : Guide A - Selection ofprotection levels for lightning protection systems. IEC 13 12- 1: 1995, Protection against lightning electromagnetic impulse, Part 1: Generalprinciples. ITU-T Recommendation K20 (1996), Re
22、sistibility of telecommunication switching equipment ro overvollages and overcurrents. ITU-T Recommendation K.2 1 (1 996), Resistibility of subscriber s terminal to overvoltages and overcurrents. IT-T Recommendation K.22 (1995), Overvoltage resistibility of equipment connected to an ISDN T/S bus. IT
23、U-T Recommendation K.27 (1 996), Bonding configurations and earthing inside a telecommunication building. ITU-T Recommendation K.40 ( 1996), Protection against LEW in telecommunications centres. Recommendation K.39 (1096) 1 4 Definitions This Recommendation defines the following terms. 4.1 currents.
24、 It contains at least one non-linear component. surge protective device (SPD): A device that is intended to limit transient overvoltages and divert surge 4.2 exchange at the interface to the external telecommunication lines. subscriber line circuit (SLIC): A printed circuit board containing subscrib
25、er line functions and placed in an 5 General Telecommunication centres and other information systems contain large amounts of electronic equipment sensitive to lightning discharges. Unprotected sites may be damaged by cloud to ground flashes at a distance of several kilometres. Interferences may eve
26、n be caused by intracloud discharges. The risk of damages depends on a number of factors, among which the following are the more important: - - type of incoming services, especially power supply and telecommunication cables; size and shape of the building containing the equipment and the shielding e
27、ffect of the building material; - layout of the electronic system including the cable routing and the utilization of internal shields; - protective measures, integrated in the building construction, inherent of the equipment or at the interface between external and internal cables. Physical damages
28、to telecommunication sites are dominated by hardware damages of the electronic equipment due to overvoltages and overcurrents on incoming lines. Direct lightning strikes to a building can cause extensive damages to the building itself in form of fire, explosion or other physical destruction. In such
29、 cases the installed equipment usually suffers severe losses. Also software parts of the system may be destroyed or partly affected, e.g. erased memories and false or inhibited alarm signals. However, the most severe consequential effect of a thunderstorm is often the loss of communication service t
30、hat will lead to high costs for both the subscriber and the telecommunication operator. 6 Coupling mechanisms Lightning transients will affect a telecommunication centre or similar systems by the following coupling mechanisms. 6.1 Resistive coupling A lightning discharge to an object creates a poten
31、tial rise that may reach several hundreds of kV at the point of impact with respect to the remote earth. In a telecommunication centre, the potential rise will generate currents that are distributed amongst metallic services like telecommunication and power supply cables bonded to the reference poin
32、t of the system. Currents in cable shields cause overvoltages between the shield and the cable core proportional to the transfer impedance. 6.2 Magnetic coupling Lightning currents in a conductor or in the lightning discharge channel create magnetic fields, which for a distance up to some hundred me
33、tres may be assumed to have the same time variation as the current. However, the magnetic field is often disturbed and attenuated by building materials or surrounding objects and therefore a more precise analysis may be necessary in some cases. 2 Recommendation K.39 (10/96) STDmITU-T RECMN K.39-ENGL
34、 L79b I qBb259L Ob32720 952 The magnetic field variations induce voltages and currents in the internal wiring and in the outdoor cable plant. The distribution of lightning current parameters is given in IEC Document 1024-1-1, Guide A - Selection of protection levels for lightning protection systems.
35、 6.3 Electric coupling Adjacent to the charge in the lower part of the lightning channel, a very high electric field arises. It can have an effect on rod antenna equipment. Inside a building the interference from the field may normally be neglected. 6.4 Electromagnetic coupling At large distances, e
36、lectromagnetic fields from lightning discharges impress substantial overvoltages on an extended telecommunication network. This conducted interference appears at the equipment interface. Directly radiated fields will hardly damage any telecommunication equipment inside a building or cabinet. 7 Princ
37、iples for the risk assessment A risk assessment for damages in telecommunication centres should be performed in two steps: firstly, a basic assessment of the need of lightning protection for the object; secondly, the selection of protection methods and the energy withstanding of protection component
38、s, SPDs, on telecommunication and power supply services. 7.1 Frequency of damage, F Risk areas should be defined for coupling mechanisms described above. Lightning discharges confined to these areas are assumed to damage the telecommunication building itself, TS, or at least the installed electronic
39、 equipment inside, see Figure 1. TO507080-96/dOl Equivalent risk area for direct strikes to the building. Risk area for discharges to ground nearby the building affecting the telecommunication centre by resistive and inductive coupling. Risk area for incoming services (power supply, telecommunicatio
40、n cables, etc.). Risk area for direct strikes to adjacent objects, e.g. antenna masts, with metallic connection to the telecommunication centre. Figure 1K.39 - Risk areas for lightning discharges It should be noticed that different risk areas may cover each other, which should be considered when the
41、 total risk area is calculated. Higher structures will, to a certain extent, shield lower nearby buildings or areas from direct lightning strikes. Direct strikes will normally cause more severe damages than indirect strikes. Recommendation K.39 (10/96) 3 STD.ITU-T RECMN K.39-ENGL L99b II 48b2571 Ob3
42、2921 879 Also the risk area A, for incoming cables, is assumed to have a higher pnonty than A, for strikes to ground nearby the site to be protected. The sum of the areas gives the total risk area for the site and corresponds to the average number of damages every year, F, at a normalized lightning
43、intensity N, of 1 flash to ground per km2 and year. It is assumed here that the telecommunication equipment has a resistibility against lightning transients according to Recommendations K.20, K.21 and K.22 and the power supply equipment to other international standards. The requirements shall compri
44、se parameter values of the transients concerning maximum voltage, current, charge, steepness, specific energy, etc. The risk areas are calculated on condition that the communication site is not provided with any particular protective measures or such protection that may be included as natural protec
45、tive parts of the construction of the building or in the outside cable plant. In reality there are always some characteristics of the site that reduce the penetrating fields and currents. The likelihood that damage should occur is therefore significantly lower than in the theoretical case. Influence
46、s from inherent and additional protective measures are considered in clause 9 -“Estimation of probability factors p”. Regarding the local lightning flash density, the number of yearly prospective number of damages, F, is expressed by the formula: where the different values of p depend on the existin
47、g or planned protective measures, which all decrease the probability of damages. The four terms represent damages caused by direct strikes to the site, (d), nearby strikes to ground, (n), lightning discharges to or in the vicinity of incoming cables, (s), and direct discharges to adjacent objects, e
48、.g. antenna towers, with metallic connection to the telecommunication site (a). In most cases the third term will dominate, but for large buildings or buildings with high antenna towers the influence from other terms may be substantial. It is advantageous to keep the terms separated during the calcu
49、lations in order to be able to identi the main reason of failures and to implement the most efficient protective methods. Information about the lightning flash density can often be obtained from detailed keraunic maps. Otherwise the following approximate formula may be used: N, = 0.04 T225 per km2 and year, where Td is the average number of thunderstorm days registered per year. 7.2 Risk of damage, R The risk R that the telecommunication site will suffer senous damages can be estimated to: which in most cases is simplified to: R = F.6 = CFi - tji fort=lyearandF 20 kV) Stand
