EMC and EMF in telecommunications

Transcription

1 EMC and EMF in telecommunications R. Pomponi ITU-T: International Telecommunication Union Telecommunication Sector ABSTRACT - THIS PAPER SUMMARIZES THE PRESENTATIONS OF ITU-T SG 5 IN THE PANEL SESSION EMC AND EMF IN TELECOMMUNICATIONS. AFTER A GENERAL OVERVIEW OF SG 5 FIELD OF ACTIVITY, THE SUBJECTS DISCUSSED ARE RESISTIBILITY REQUIREMENTS AND TESTING, LIGHTNING PROTECTION, EMF IN TELECOMMUNICATION, EMC RELATED TO BROADBAND ACCESS SYSTEMS, EMC AND EMF PREDICTION AND INTERFERENCE FROM POWER SYSTEMS. THE USE OF SG 5 PUBLICATIONS IS ALSO ILLUSTRATED. INTRODUCTION The paper summarizes the ITU-T Study Group 5 (SG 5) Technical Panel EMC and EMF in Telecommunications. The Technical Panel is organized into nine presentations. The first is a general overview of the field of activity and working methodology of SG 5, the following seven discuss specific technical subjects under consideration during the current Study Period ( ). The final presentation gives a guide to the use of SG 5 Publications aimed at achieving EMC. The speakers are the Rapporteurs of the Questions under study. These are P. Day (Telstra, Australia), C. Barbosa (CPQD, Brazil), J. Boksiner (Telcordia, USA), F. Brunello (Telecom Italia Lab, Italy), C. Monney (Swisscom, Switzerland), D. Carpenter (Btexact Technologies, UK), H-G Öhlin (Telia, Sweden) and G. Varju (MATAV, Hungary). GENERAL SURVEY OF THE FIELD OF ACTIVITY AND THE METHODOLOGY SG 5 Title, Mandate and Objective The title of SG 5 is Protection against electromagnetic environment effects. SG 5 is responsible for studies related to electromagnetic phenomena that can cause damage or disturbance to telecommunication installations or injury to telecommunication personnel or health effects to population. In this field, SG 5 is the most experienced and competent standardization body. The SG 5 objective is to define protective measures and installation techniques by means of Recommendations, Directives and Handbooks in order to limit the risk of damage to telecommunication installations and equipment, disturbances to telecommunications systems and injury to people. Working method The Management Team is composed of R. Pomponi (Telecom Italia Lab, Italy) SG 5 Chairman, G. Varju (MATAV, Hungary) SG 5 Vice-Chairman, A. Zeddam, (France Telecom, France) SG 5 Vice- Chairman and WP1 Chairman, M. Hattori (NTT, Japan) WP 2 Chairman and J. Katona Kiss, Counsellor, TSB. 60 exp - Volume 3 - n. 1 - March 2003

2 in search of inno vation Roberto Pomponi Roberto Pomponi, Electrical Engineer, works in TILAB (Telecom Italia Lab) in Turin, Italy, where he has carried out activities on electromagnetic compatibility, protection and safety of telecommunication installations. Actually he is involved in the Project Office activities. During the Study Period , he chairs the ITU-T Study Group 5 Protection against electromagnetic environment effects. The Management Team guides the SG 5 activities related to the Questions, that are the subjects to be studied generally - during the four years period ( ). SG 5 nominates a Rapporteur for each Question, who is the person charged to coordinate the activities to be carried out, and defines a Working Program. This information is published on the ITU-T web site: SG 5 Organization The Questions under study are based on competencies of Resistibility, Lightning, EMC (Emission & Immunity), Safety, Earthing & Bonding, and Protection to the business areas, telecommunication network and equipment and software development. Seven of the current fourteen Questions are new and seven are continuation of Questions studied during the previous Study Period ( ). The Questions are allocated among two Working Parties (WPs): WP 1 Preventing damages and safety, chaired by A. Zeddam (France Telecom) and WP 2 Emission, immunity and electromagnetic fields, chaired by M. Hattori (NTT). OVERVOLTAGE PROTECTION OF EQUIPMENT Protection of equipment at an installation requires a combination of adequate equipment resistibility, the installation of primary protection in lightning prone areas and adequate earthing and bonding. SG 5 made considerable changes to the overvoltage resistibility recommendations in the last study period to ensure that equipment can be protected by the addition of primary protection. These changes assume that the earthing and bonding of the in- stallation does not adversely affect the ability of the primary protector to protect the equipment. Earthing and bonding requirements at an installation are contained in Recommendation K.27 for exchanges, Recommendation K.31 for subscriber buildings and Recommendation K.35 for access network equipment. To prevent equipment damage, both the mains and telecommunication protectors need to be connected to a common earthing point by short bonding conductors (in the order of 1 metre maximum). Some contributions from SG 5 members have shown that in real life these conductors can be 10 or more metres in length and in the worst case the mains and telecommunications systems may be separately earthed. It would be prohibitively expense if all operators and customers had to modify their installations to a single earth configuration with a maximum bond conductor length of 1 metre. One way to achieve the required level of protection is to install additional protection at the equipment. Combination Protection Units (CPUs) have both the telecommunication and mains protectors in the same box and when installed correctly will provide the required level of protection. If installed incorrectly, they may allow damage to occur to the equipment being protected or to associated equipment. SG 5 is studying the requirements for installing CPUs. LIGHTNING PROTECTION FOR TELECOMMUNICATION SYSTEMS Since the early days of telecommunications, the protection of equipment and personnel against lightning has been a subject of prime concern. exp - Volume 3 - n. 1 - March

3 More recently, the widespread use of highly sensitive microelectronic components across the telecommunication plant has increased the need for adequate protection. In order to provide guidance for the protection of telecommunications systems against lightning discharges, the SG 5 produced a Handbook and a set of K Series Recommendations. The Handbook, The protection of telecommunications lines and equipment against lightning discharges, is made of 10 chapters written in a tutorial style covering the lightning phenomena, it s effects on telecommunications systems and the philosophy of protection. The set of K-Series Recommendations (K.11, K.25, K.39, K.40, K.46 and K.47) gives the requirements, methods and procedures to achieve a cost-effective protection against lightning discharge. Two new Recommendations related to lightning are under development, one dealing with the protection of mobile telephony Radio Base Stations (RBS) (K.rbs) and the other with the protection of lines made of coaxial cables (K.coax). In order to provide knowledge for the development of new and the upgrade of existing Recommendations, many technical subjects are under investigation during the current Study Period, as for example: Shielding effect of the metallic cable sheath upon the lightning induced surges; Protective effect provided by the installation of Surge Protection Devices (SPD) along a telecommunication line; Procedures intended to protect RBS against lightning discharge. This work is being conducted under the scope of SG 5 Question 5 Lightning Protection of Fixed, Mobile and Wireless Systems. EMF ENVIRONMENTAL CHARACTERIZATION, GUIDANCE FOR HUMAN EXPOSURE SG 5 began to address the issue of human exposure to Electromagnetic Fields (EMF) during the previous Study Period. SG 5 began this effort in response to concern over the human safety of EMF exposure and the proliferation of EMF emitters used for telecommunications. SG 5 decided that it should provide guidance for compliance with existing EMF exposure limits rather than developing new limits. Also, SG 5 decided to postpone consideration of mobile handsets. One of the goals of SG 5 was to develop a standardized approach to environmental characterization of EMF. To this end, SG 5 developed Recommendation K.52, Guidance on Complying with Limits for Human Exposure to Electromagnetic Fields. Recommendation K.52 provides techniques and procedures for assessing the severity of field exposure and for limiting the exposure to workers and the general public to these fields if the limits are exceeded. Recommendation K.52 classifies the accessible space around an emitter into the following three zones: Compliance Zone where the potential exposure to EMF is below the applicable limits; Occupational Zone where the potential exposure to EMF is below the limits for occupational exposure but exceeds the limits for general public exposure; Exceedance Zone where the potential exposure to EMF exceeds the limits for both occupational and general public exposure. Recommendation K.52 provides a simple analytical method using far-field expressions to calculate the field. With knowledge of the antenna pattern, radiated power and characteristics of the earth, it is possible to calculate the boundaries of the Compliance and Exceedance Zones. Figure 1 shows an example in three dimensions. Although it is possible to use simple techniques to estimate the exposure, calculation or measurement of EMF exposure with greater accuracy requires more sophisticated methods. SG 5 is addressing this need during the current study period. Problems to be addressed include: 62 exp - Volume 3 - n. 1 - March 2003

4 in search of inno vation Figure 1 Example of Exceedance Zone (Frequency = 900 MHz, Limit for general public = 3 W/m2, Power to antenna = 100 W) Measurements Instrumentation requirements, Procedures for dealing with time-varying sources (multi-channel systems, systems with automatic power control, adaptive antennas). 2. Numerical prediction Near field models, the use of ray tracing and other techniques. The following section presents one approach for numerical prediction. Other topics of interest are detailed installation guidelines for operators and standardization of data and data interchange for the numerical prediction. EMF NUMERICAL PREDICTION The analysis of the EMF radiated by a set of antennas in an urban context is today a fundamental task for telecommunications companies. New concerns, in fact, took place with the growth of wireless systems. Electromagnetic pollution and, consequently, the respect of limits defined by governmental authorities is an example; coexistence of several antennas from different providers in the same place is another one. Therefore it s very important to have a software tool able to accurately analyse the EMF radiated by different antennas in the same urban context. The software tool, called GUARDIAN (Graphical User Application for the RaDiation Intensity in Antenna Neighbourhoods) features a high-resolution colour graphical interface and effectively shows any peak in the EMF exceeding the exposure limits established under national or international authority dispositions. Depending on the set threshold value, GUARDIAN can also display compliance volumes in relation to the individual installations. Through the management of vector databases (DXF and SHP formats), GUARDIAN represents EMF values and compliance volumes, and integrates views of buildings close to radio sites using databases containing vector representations of buildings and terrain orography in the area surrounding the installations. Additionally, through a dedicated Building Editor, GUARDIAN enables users to create vector maps of areas not included in the database; the representation can be acquired from a graphic file in conventional formats (tif, jpg, etc.). Once the site s technical data has been imported, the program s interface allows users to change the parameters (location, antenna tilt, overall antenna connector power, power per channel, antenna type) on the basis of system extension assumptions. GUARDIAN s computing potential provides different types of computations, shown in three-dimension graphic form as resulting from the interaction with the surrounding buildings and envi- exp - Volume 3 - n. 1 - March

5 ronment. The program s computation and viewing options are: 1. EMF computation using the UTD (Uniform Theory of Diffraction) method; 2. Assessment of the Compliance Zone of one or more antenna systems with respect to the set threshold, with the option to use the near-field or far-field method; 3. Point computation of three-dimensional iso-surfaces in relation to a specific values of the electric field; 4. Computation of the EMF near all accessible surfaces (fronts of buildings and streets) and inside buildings, by means of ray-tracing and UTD. Thanks to its advanced processing and graphical representation functionalities, the program ultimately aims to offer a valuable support tool in performing radio site compliance analyses. EMC RELATED TO BROADBAND ACCESS SYSTEMS The structure of the telephone network has evolved since its birth at the end of the 19 th century. Each time that a new technology was available, this last was implemented in the telephone network, but the older one remained in use. That s why the network is now composed of overhead lines, twisted or not, paper or synthetic insulated cables. The major concern was the crosstalk, e.g. the disagreement to hear the conversion of another person. Now, the new technologies like HD- SL, ADSL or VDSL make a better use of the bandwidth at disposal, enabling many conversations or higher bit rates on a single copper pair. But the transmission frequencies used are higher than previously and have the potential to disturb medium or short wave broadcasting. SG 5 has began work on this topic at the beginning of the present study period. The outcome will be the definition and validation of a measurement method and to assert whether actual radiation limits for information technology equipment are sufficient or not in order to protect users of the radio spectrum. EMC PREDICTION THROUGH MATHEMATICAL MODELLING Modern digital telecommunications Systems are typically composed of large numbers of separately procured Apparatus, each individually certified to meet a radiated emissions limit (generally CISPR22 Class A). The existence of common emissions frequencies among the Apparatus means that the System emissions level, the result of the cumulation of the individual Apparatus emissions, could exceed the limits to which the Apparatus are individually certified. The System emissions level will be free to propagate beyond the exchange. Given that most exchanges are located in populated areas, the probability that a telecommunications System could cause interference to an adjacent spectrum user is intuitively high enough to demand a method through which this risk may be managed. SG 5 is studying a method proposed by BTexact Technologies, the new high-technology company spun out from BT Labs, Suffolk, England. The approach derives the statistical distributions that describe the System emissions level from the constituent Apparatus emission level(s), information that is generally readily available (being obtained during certification measurement). Figures 1 and 2 display the distributions generated for an example System containing 10 identical Apparatus items each with an emissions level of 25 dbmv/m (when measured at 10 metres) at a common frequency (in this case 100 MHz). Figure 2 displays the Probability Distribution (PD). This shows the relative likelihood with which the System emissions level will be found to adopt any value between the physical limits of the worst case (i.e. highest emissions level) and the best case (i.e. lowest emissions level). For this example, the worst case System emissions level is ( log 10 {10}) 45 dbmv/m, i.e. 5 db above the CISPR22 Class A limit of 40 dbmv/m. However, figure 2 indicates that the System emissions level that is most likely to occur is 33 dbmv/m: some 12 db below the worst case level and some 7 db below the CISPR22 Class A limit. 64 exp - Volume 3 - n. 1 - March 2003

6 in search of inno vation Figure 2 Example of Probability Distribution Apparatus 25 dbuv/m (100 MHz Vertical) Probability [1*-0.3] E Field [dbuv/m] Figure 3 displays the Cumulative Probability Distribution (CPD). This shows the likelihood with which the System emissions level will be found equal to or below a specified level. When the specified level in question is a compliance limit, the CPD provides the Compliance Probability. Hence figure 3 indicates that the System emissions level has a 96% Compliance Probability with the CISPR22 Class A limit. The statistical distributions combine to indicate that while the worst case System emissions level is like- Figure 3 Example of Cumulative Probability Distribution Apparatus 25 dbuv/m (100 MHz Vertical) Cumulative Probability E Field [dbuv/m] exp - Volume 3 - n. 1 - March

7 ly to be high (i.e. above CISPR22 Class A) emission at these levels is relatively unlikely to occur. Instead, much lower System emissions levels can be expected. In practice (depending upon the number of Apparatus sharing a common emissions frequency, their individual emissions level and their respective distances from a hypothetical measurement position) it is possible for the System emissions level to meet the same emissions requirements as their constituent Apparatus. Hence this approach avoids the need for expensive, time-consuming and problematic re-engineering of the System to reduce the emissions levels. INTERFERENCE FROM POWER SYSTEMS SHARING RESPONSIBILITY There are two situations when power systems, including electrified railways, may interfere with telecommunication systems: During normal operation; Under fault condition. During normal operation, both the fundamental frequency (16 2 / 3, 50 or 60 Hz) and the related harmonics may disturb telecommunications. The fundamental frequency may interfere with signalling and remote power supply of for example PCM systems and sometimes even cause telephones to ring. The harmonics may cause noise in speech transmission, but could also interfere with for example Carrier Frequency and Pair Gain systems. Power systems under fault condition may cause dangerous overvoltages that could damage equipment and injure people. This is due to the large currents going to earth in directly earthed systems. This is also the case with double earth faults in non-directly earthed systems. Telecomm systems have to be resistant to these phenomena to some extent. However, when speaking about danger to people, one has to consider which voltage or rather which current a human being can withstand. Work has been done in co-operation with CIGRÉ to set the limits in terms of admissible voltages versus time duration. This has lead to Recommendation K.33. For many years the same limit has been used for both the safety of people and for the resistibility of equipment. This has been the starting point for the new limits found in Recommendation K.53, which also defines the boundary of responsibility between power and telecom companies. The limits set in K.53 indicate up to which level telecoms plants have to be immune and therefore designed for at the owner s expense. When the limits are exceeded the responsible power company has to pay for measures that have to be taken to reduce the levels. GUIDE TO USE SG 5 PUBLICATIONS AIMED AT ACHIEVING EMC The publications produced by SG 5 aim to define measures to achieve EMC for telecommunication equipment and installations. The types of SG 5 publications and their relationship with other ITU-T documents are shown on figure 4. The publication set supports not only the equipment designers and operators but is also very helpful for users in developing countries. Directives Concerning the Protection of Telecommunication Lines against Harmful Effects from Electric Power and Electrified Railway Lines has been the first SG 5 publication related to the compatibility with electricity lines [1]. The historical milestones of its development are as follow: The 1 st Interference booklet about interference prepared by SG 1 in 1927 (75 years ago); The 1 st (Rome) version of the Directives, in 1937 (65 years ago); The 2 nd version of the Directives, approved in New Delhi in 1963, published in1963 (40 years ago); The complete renewal of the 1963 edition of the Directives, issued in Geneva, The renewed Directives are composed of series of nine volumes that cover the study of the induction problems in its complexity accordingly to the scheme of figure 5. Because the Directives have been developed and endorsed with close collaboration between 66 exp - Volume 3 - n. 1 - March 2003

8 in search of inno vation Figure 4 Relationship between SG 5 and other ITU-T documents ITU - R ITU - T ITU - D SG 5 Recommendations Other publications Handbooks Directives the ITU-T, the International Union of Railways (UIC) and the International Council on Large Electric Systems (CIGRE), the Directives have high level of legislation. The Handbooks provide more detailed information on three key areas of EMC in telecommunication [2], [3] and [4]. Regarding the Earthing Handbook [3], the preparation of a new version is in the final stage, which follows the concept of bonging and earthing rather than the earthing only. The K-series Recommendations [5] constitute the essential, standard-like publications. Accordingly to their type and subject the following classification is applied: Figure 5 Procedure for study of induction problems by the use of the Directives START Identify power or electrified railways and Telecommunication facilities involved Volume I Determine electric and geometrical parameters Required to estimate the effects Volume IV, V and IX Calculate measure induced voltages and currents Volume II, III and IX Stipulate Protective measures Volumes VII and VIII Are permitted levels exceeded? Vol. VI Yes No END exp - Volume 3 - n. 1 - March

9 " Basic # General # Calculation/estimation # Testing/measuring " Generic " Protection Or # General mitigation # Protection practice oriented # Protection components " Safety to induction effects # Specification of limits # Protection practice " Safety to EMF # Guide on compliance effects with human exposure # Measurements/ numerical prediction # Installation guidelines The EMC requirements for telecommunication equipment and installations are specified for the following environmental locations: Equipment and installations located in telecommunications centres, at radio installations or at outdoor locations; Telecommunication lines at outdoor locations; Equipment located in customer premises. The development of new Recommendations and the revision of existing ones are based on the experience of SG 5 delegates, the available technical information within the literature and the research activities carried out by the organizations participating in SG 5. Therefore, the SG-5 publications are permanently updated to reflect the stateof-the-art in EMC protection requirements. Finally, it is worth mentioning that the use of the SG 5 publications are supported by the Guide [6] and the collection of Terminology that is available as a supplement to the Recommendations. CONCLUSION This paper has presented some key subjects under study by ITU-T SG 5 during the current Study Period ( ). The objective of these studies is the publication of new and revised Recommendations and Handbooks, and the maintenance of Directives. To this end, a Management Team guides the SG 5 work, which is organized in several Questions. A Rapporteur co-ordinates the activity carried out under each Question. The results of these studies are discussed during regular Rapporteur s Group, Working Party and SG 5 meetings. When a new document (e.g. a new or revised Recommendation) is ready for publication, a fast approval procedure is followed in order to fulfil the time to market requirements. 68 exp - Volume 3 - n. 1 - March 2003

10 in search of inno vation GLOSSARY CIGRE International Council on Large Electric Systems CISPR International Special Committee on Radio Interference CPU Combination Protection Unit EMC ElectroMagnetic Compatibility EMF ElectroMagnetic Field ITU-D International Telecommunication Union Development Sector ITU-R International Telecommunication Union Radio Sector RBS Radio Based Station SG Study Group SPD Surge Protection Devices UIC International Union of Railways UTD Uniform Theory of Diffraction REFERENCES [1] Directives Concerning the Protection of Telecommunication Lines against Harmful Effects from Electric Power and Electrified Railway Lines has been the first publication related to the compatibility with electricity lines; ITU-T Geneva, 1989; [2] Lightning Handbook, The Protection of telecommunication lines and equipment against lightning discharges (1974) (contains Chapters 1 to 5) The Protection of telecommunication lines and equipment against lightning discharges; ITU-T Geneva, Chapters 1 to , Chapters 6 to , Chapters 9 and ; [3] Earthing Handbook, Earthing of Telecommunication Installation; Geneva 1974; [4] Measuring Handbook Handbook on interference measuring techniques, Geneva, 02/2000; [5] Series K Recommendations, Protection against interference, Geneva 1988 (K.1) to 2000 (K 54); BySeries.asp?serie=K [6] Guide to the use of ITU-T Publications produced by Study Group 5 aimed at achieving Electromagnetic Compatibility and Safety, Geneva under edition. CONTACTS Roberto Pomponi Telecom Italia Lab Tel.: Fax: roberto.pomponi@telecomitalia.it exp - Volume 3 - n. 1 - March