CLC TR 50484-2009 Recommendations for shielded enclosures《屏蔽罩的建议》.pdf

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1、raising standards worldwideNO COPYING WITHOUT BSI PERMISSION EXCEPT AS PERMITTED BY COPYRIGHT LAWBSI Standards PublicationRecommendations for shielded enclosuresPD CLC/TR 50484:2009National forewordThis Published Document is the UK implementation of CLC/TR 50484:2009.The UK participation in its prep

2、aration was entrusted by Technical CommitteeGEL/210, EMC - Policy committee, to Subcommittee GEL/210/12, EMC basic,generic and low frequency phenomena Standardization.A list of organizations represented on this committee can be obtained onrequest to its secretary.This publication does not purport to

3、 include all the necessary provisions of acontract. Users are responsible for its correct application. BSI 2010ISBN 978 0 580 64295 1ICS 17.220.01; 31.240Compliance with a British Standard cannot confer immunity fromlegal obligations.This Published Document was published under the authority of theSt

4、andards Policy and Strategy Committee on 31 July 2010.Amendments issued since publicationAmd. No. Date Text affectedBRITISH STANDARDPD CLC/TR 50484:2009TECHNICAL REPORT CLC/TR 50484 RAPPORT TECHNIQUE TECHNISCHER BERICHT April 2009 CENELEC European Committee for Electrotechnical Standardization Comit

5、 Europen de Normalisation Electrotechnique Europisches Komitee fr Elektrotechnische Normung Central Secretariat: avenue Marnix 17, B - 1000 Brussels 2009 CENELEC - All rights of exploitation in any form and by any means reserved worldwide for CENELEC members. Ref. No. CLC/TR 50484:2009 E ICS 17.220.

6、01; 31.240 Supersedes R210-005:1999English version Recommendations for shielded enclosures This Technical Report was approved by CENELEC on 2009-03-20. CENELEC members are the national electrotechnical committees of Austria, Belgium, Bulgaria, Cyprus, the Czech Republic, Denmark, Estonia, Finland, F

7、rance, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, the Netherlands, Norway, Poland, Portugal, Romania, Slovakia, Slovenia, Spain, Sweden, Switzerland and the United Kingdom. PD CLC/TR 50484:2009CLC/TR 50484:2009 2 Foreword This Technical Report was prepar

8、ed by the Technical Committee CENELEC TC 210, Electromagnetic compatibility (EMC). The text of the draft was submitted to vote in accordance with the Internal Regulations, Part 2, Subclause 11.4.3.2 (simple majority) and was approved by CENELEC as CLC/TR 50484 on 2009-03-20. This document supersedes

9、 R210-005:1999. _ PD CLC/TR 50484:2009 3 CLC/TR 50484:2009 Contents 1 Scope 4 2 Normative references 4 3 Definitions 4 4 General .4 5 Shielding 5 5.1 Shielding attenuation.6 5.2 Evaluation of shielding effectiveness . 10 5.3 Shielding components and selection of materials 11 5.4 Shielding attenuatio

10、n values (see Figure 8 measured according to EN 50147-1) 14 Bibliography . 16 Figures Figure 1 Illustrated set-up for shielding .4 Figure 2 Wave impedance versus distance of the field source .5 Figure 3 Schematic diagram of the partial reflections (subscript R) and transmissions (subscript T) at the

11、 two surfaces of a shield .6 Figure 4 S results calculated for a low-impedance magnetic field source .9 Figure 5 Calculated S results for a low-impedance magnetic field source 9 Figure 6 Shielding attenuation measurement. 11 Figure 7 Examples of door contacts . 13 Figure 8 Shows typical performance

12、values. 14 Table Table 1 Summary SE aspects . 10 PD CLC/TR 50484:2009CLC/TR 50484:2009 4 1 Scope This Technical Report applies to shielded enclosures used for EMC testing which are to be validated according to the EN 50147 series of standards and the corresponding international standards. The object

13、 of this report is to give guidance to the selection of the shielding materials and components. The frequency range for this document is 10 kHz to 40 GHz. 2 Normative references The following referenced documents are indispensable for the application of this document. For dated references, only the

14、edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies. EN 50147-1:1996, Anechoic chambers Part 1: Shield attenuation measurement EN 50147-2, Anechoic chambers Part 2: Alternative test site suitability with respect to site atte

15、nuation EN 55011, Industrial, scientific and medical (ISM) radio-frequency equipment Electromagnetic disturbance characteristics Limits and methods of measurement (CISPR 11, mod.) EN 55022, Information technology equipment Radio disturbance characteristics Limits and methods of measurement (CISPR 22

16、, mod.) IEC 60050(161), International Electrotechnical Vocabulary (IEV) Chapter 161: Electromagnetic compatibility 3 Definitions Void. 4 General Depending on the particular circumstances, it may be necessary to shield a room from the electromagnetic environment. Conversely it may be necessary to pro

17、tect the environment from electromagnetic energy generated within the room. Figure 1 illustrates this. Figure 1 Illustrated set-up for shielding PD CLC/TR 50484:2009 5 CLC/TR 50484:2009 5 Shielding The shielding effectiveness ( SE ) of a shielded enclosure can be measured, e.g. as described in EN 50

18、147-1, or calculated, e.g. as in 5.1. In general, the SE of a shielded enclosure can only be calculated for simple cases. To do this a number of assumptions are made. The most important of these assumptions is that the envelope formed by the enclosure is homogeneous and consists of material whose pr

19、operties such as thickness ( t ), conductivity ( ) and permeability ( ) are well defined. Another assumption is that the shielded enclosure has a simple geometric structure. Normally, steel, copper or aluminium sheets are used to meet the SE requirements. The SE not only depends on the shield materi

20、al parameters but also on the wave impedance of the field to be shielded. Consequently, the SE depends on the distance ( r ) between source and shield, relative to the wavelength 0of the field, normally expressed in the quantity r = 00/2/2 cfrr = , where f is the frequency and smc /10380= the propag

21、ation velocity of the field. Then, three regions are distinguished: Figure 2 Wave impedance versus distance of the field source In the far-field (plane wave, free space) the wave impedance is a constant 0= 377 . In the near-field, the wave impedance depends on r and, consequently, on the type of sou

22、rce. The two most important types of source are: 1) the magnetic dipole having a wave impedance ZwH 0, and therefore normally called a low-impedance source. In the near-field ZwHis proportional to r; 2) the electric dipole having a wave impedance ZwE 0, and therefore normally called a high-impedance

23、 source. In the near-field ZwEis in inversely proportional to r. In the near-field region (normally the lower frequency range, say up to 10 MHz) the minimum SE of an enclosure is determined by the SE for the magnetic field component of a low-impedance source. A high SE value is then achieved by usin

24、g a shield of an adequate thickness with a high value of the relative permeability. In the higher frequency range (normally f larger than 10 MHz) and in the case that r I a shield with a good conductivity is important. In this range constructional details of the enclosure, such as joints/seems, door

25、s, inserts and resonance effects will limit the final SE of the enclosure, in particular when the largest dimensions of slits and openings in the enclosure are smaller than 0. The cable feed-throughs are another source of limitation of the SE . PD CLC/TR 50484:2009CLC/TR 50484:2009 6 5.1 Shielding a

26、ttenuation In many SE calculations, SE is considered to be equal to the attenuation S of the amplitude of the electric or magnetic component of the EM field as caused by an infinitely large planar shield. In general, this is not correct. For example, in S calculations resonance effects in the field

27、distribution inside a shielded enclosure which will affect the SE are not taken into account. However, S calculations allow a good estimate of SE when considering shielded enclosure requirements. In these calculations the direction of propagation of the EM wave to be shielded is generally taken perp

28、endicular to the shield. The major basic theories and concepts of shielding were established by Schelkunoff 1 and Kaden 2. More condensed and detailed practical information can be found in EMC textbooks 3. The incoming field wave is represented by 1H . Figure 3 Schematic diagram of the partial refle

29、ctions (subscript R) and transmissions (subscript T) at the two surfaces of a shield According to the Schelkunoff theory, the total attenuation TS provided by a shield results from three mechanisms, their relation being given by (see Figure 3): =RRTITITMRRATHHHHHHHHHHSSSS33322212(1)where H represent

30、s the amplitude of the field component to be shielded. When expressed in dB MRRASSSS +=(dB) (2)These terms are elucidated in 5.1.1 to 5.1.3, and numerical examples are given in 5.1.4. PD CLC/TR 50484:2009 7 CLC/TR 50484:2009 5.1.1 The absorption loss term 21/ HHSTA= i.e. the contribution to RS as a

31、result of the energy absorption when the field passes once through the shield. AS can be calculated from tAeS =(3)where is the skin depth of the shielding material, given by 2= (4)and f 2= . NOTE 1 The conductivity can be written as = r cu, where cu= 5,8 107S/m is the conductivity of copper and rthe

32、 conductivity of the shield material relative to copper. Similarly, can be written as = ro, where 0= 4 10-7S/m and r the relative permittivity of the shield. Expressing the frequency in MHz, can be written as rrMHzf )(66=(m) (5)NOTE 2 SAdoes not depend on the distance between source and shield, it o

33、nly depends on the shield material parameters t, , and the frequency f. From Equation (3) it follows that SA 8 t/ (dB). 5.1.2 The reflection loss term )/)(/(2211 TTRHHHHS =, i.e. the contributions to TS as a result of the reflection of the field when entering and leaving the shield. This contributio

34、n is proportional to the wave impedance of the field and, hence, in the near field RS depends on the type of source via the factor r as indicated in Clause 5. a) Near field ()Ir : In the far-field the wave impedance is a constant independent of the type of source, and RS can be estimated from 024=RS

35、(8)5.1.3 The multiple reflection factor )./)(/(3332 RRMRHHHHS = i.e. the reduction factor of the reflection loss )(RHRERSorSorS due to multiple reflections of the waves inside the shield. This term is only of importance when AS is small. MRS can be estimated from /21tMReS=(9)NOTE 3 The product of th

36、e reflection loss term and the multiple reflection factor reducing the effective reflection loss is always 1. This consideration is of importance in the case Equation (6) applies. Therefore, in the aforementioned estimates the following additional condition shall be used: 1 . At frequencies f 1 MHz

37、ST 150 dB, being completely determined by AS , which in praxis means that at those frequencies the SE is determined by imperfections of the enclosure, see 5.3.1. The values of TS in Figure 4 just comply with curve 2, the standard performance curve in Figure 8 in 5.4. PD CLC/TR 50484:2009 9 CLC/TR 50

38、484:2009 The curve labelled conv allows conversion of a frequency value into a r value, taking r = 0,3 m. Figure 4 S results calculated for a low-impedance magnetic field source Assuming a much large value of r , say r = 5 m, r = 1 at f = 10 MHz. An example of results of )( fS , assuming additionall

39、y that t = 1 mm, r= 0,6 (e.g. aluminium), and r= 1, is given in Figure 5. RES (high impedance electric field source) is also indicated. In the far field RRERHSSS = . The curve “Conv” assumes r = 5,0 m. Figure 5 Calculated S results for a low-impedance magnetic field source PD CLC/TR 50484:2009CLC/TR

40、 50484:2009 10 In this example RHS clearly contributes to TS . The curve TS complies with Curve 1, the high performance curve, in 5.4, Figure 8. This example illustrates a consideration often met in SE discussions, in which three frequency ranges are considered, see Table 1 containing a summary of v

41、arious SE aspects. Table 1 Summary SE aspects Frequency range I f 100 MHz 0 30 m 30 m 3 m 1 GHz is caused by imperfections and not by property of the shield material. In certain cases, i.e. high ambient field strengths caused by transmitters or the generation of very high field strengths within the

42、shielded room, higher values are required. Figure 8 Shows typical performance values PD CLC/TR 50484:2009 15 CLC/TR 50484:2009 CALCULATION EXAMPLE (only showing the tendency): Outside: Allowed field strength limit 30 dB V/m at ambient field strength between 30 MHz and 230 MHz (EN 55011 and EN 55022)

43、 Inside: Produced field strength 140 dB V/m (10 V/m) Immunity test for industrial environment Required shielding attenuation = 110 dB (10 dB to 20 dB shielding attenuation may be achieved by concrete walls of a building, also anechoic material on enclosure walls may reduce the electromagnetic field

44、going through) PD CLC/TR 50484:2009CLC/TR 50484:2009 16 Bibliography 1 S.A. Schelkunoff, Elektromagnetic Waves. Princeton, N.J.: Van Nostrand, 1943 2 Kaden, H.: Die elektromagnetische Schirmung in der Fernmelde- und Hochfrequenztechnik. Berlin, Gttingen, Heidelberg: Springer-Verlag 1950 3 EMC textbo

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