SIST-TP CLC/TR 50484:2009
Recommendations for shielded enclosures
Recommendations for shielded enclosures
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 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.
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Standards Content (Sample)
SLOVENSKI STANDARD
SIST-TP CLC/TR 50484:2009
01-september-2009
1DGRPHãþD
SIST CLC/R 210-005:2000
3ULSRURþLOD]D]DVORQMHQHSURVWRUH
Recommendations for shielded enclosures
Ta slovenski standard je istoveten z: CLC/TR 50484:2009
ICS:
17.220.01 Elektrika. Magnetizem. Electricity. Magnetism.
Splošni vidiki General aspects
31.240 Mehanske konstrukcije za Mechanical structures for
elektronsko opremo electronic equipment
SIST-TP CLC/TR 50484:2009 en
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.
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SIST-TP CLC/TR 50484:2009
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SIST-TP CLC/TR 50484:2009
TECHNICAL REPORT
CLC/TR 50484
RAPPORT TECHNIQUE
April 2009
TECHNISCHER BERICHT
ICS 17.220.01; 31.240 Supersedes R210-005:1999
English 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, France, 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.
CENELEC
European Committee for Electrotechnical Standardization
Comité Européen de Normalisation Electrotechnique
Europäisches Komitee für 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
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SIST-TP CLC/TR 50484:2009
CLC/TR 50484:2009 – 2 –
Foreword
This Technical Report was prepared 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 R210-005:1999.
________________
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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 attenuation 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 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 values. 14
Table
Table 1 – Summary SE aspects . 10
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SIST-TP CLC/TR 50484:2009
CLC/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 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 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
attenuation
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, 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 protect the environment from
electromagnetic energy generated within the room. Figure 1 illustrates this.
Figure 1 – Illustrated set-up for shielding
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5 Shielding
The shielding effectiveness ( SE ) of a shielded enclosure can be measured, e.g. as described in
EN 50147-1, 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 properties such as thickness (t), conductivity (σ ) and permeability ( µ ) 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 material 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 λ of the field, normally expressed in the quantity βr = 2πr /λ = 2πfr /c ,
0
0 0
8
where f is the frequency and c = 3⋅10 m / s the propagation velocity of the field. Then, three
0
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 η = 377 Ω. In the near-
0
field, the wave impedance depends on βr and, consequently, on the type of source. The two most
important types of source are:
1) the magnetic dipole having a wave impedance Z ≪ η , and therefore normally called a
wH 0
‘low-impedance source’. In the near-field Z is proportional to βr;
wH
2) the electric dipole having a wave impedance Z ≪ η, and therefore normally called a
wE 0
Z is in inversely proportional to βr.
‘high-impedance source’. In the near-field
wE
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 using 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 βr ≫ I a shield with
a good conductivity is important. In this range constructional details of the enclosure, such as
joints/seems, doors, 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 λ . The cable
0
feed-throughs are another source of limitation of the SE .
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SIST-TP CLC/TR 50484:2009
CLC/TR 50484:2009 – 6 –
5.1 Shielding attenuation
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 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 perpendicular 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 H .
1
Figure 3 – Schematic diagram of the partial reflections (subscript R)
and transmissions (subscript T) at the two surfaces of a shield
According to the Schelkunoff theory, the total attenuation S provided by a shield results from three
T
mechanisms, their relation being given by (see Figure 3):
H H H H H
IT 1 2 2R 3
S = S ⋅ S ⋅ S = ⋅ ⋅ ⋅ ⋅ ⋅⋅⋅⋅⋅
(1)
T A R MR
H H H H H
2 IT 2T 3 3R
where
H represents the amplitude of the field component to be shielded.
When expressed in dB
S = S + S + S (dB)
(2)
τ A R MR
These terms are elucidated in 5.1.1 to 5.1.3, and numerical examples are given in 5.1.4.
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5.1.1 The absorption loss term S = H / H i.e. the contribution to S as a result of the energy
A 1T 2 R
absorption when the field passes once through the shield. S can be calculated from
A
t δ
S = e
(3)
A
where
δ is the skin depth of the shielding material, given by
2
(4)
δ =
ωσµ
and
ω = 2πf .
7
NOTE 1 The conductivity can be written as σ = σ · σ · c , where σ = 5,8 · 10 S/m is the conductivity of copper and σ
r u cu r
-7
the conductivity of the shield material relative to copper. Similarly, µ can be written as µ = µ µ , where µ = 4π · 10 S/m and
r o 0
µ the relative permittivity of the shield. Expressing the frequency in MHz, δ can be written as
r
66
δ =
(µm) (5)
f (MHz)σ µ
r r
NOTE 2 S does not depend on the distance between source and shield, it only depends on the shield material parameters t,
A
υ, µ and the frequency f. From Equation (3) it follows that S ≈ 8 · t/δ (dB).
A
S = (H / H )(H / H )
5.1.2 The reflection loss term , i.e. the contributions to S as a result
R 1 1T 2 2T
T
of the reflection of the field when entering and leaving the shield. This contribution is proportional to
the wave impedance of the field and, hence, in the near field S depends on the type of source via
R
the factor βr as indicated in Clause 5.
a) Near field ()βr << I :
In the case of an electric dipole source, S can be estimated from
R
η
σδ
o
S = S = ⋅
(6)
R RE
βr
4 2
Note that S → ∞ when βr → 0 , i.e. when f → 0 and/or r → 0 .
RE
In the case of a magnetic dipole source, S can be estimated from
R
σδ
S = S = η βr
(see the Note to S ) (7)
R RH 0
MR
4 2
Note that S → 0 when βr → 0 , i.e. when f →
...
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