1、BRITISH STANDARD BS 6656:2002 Assessment of inadvertent ignition of flammable atmospheres by radio-frequency radiation Guide ICS 13.280 BS 6656:2002 This British Standard, having been prepared under the direction of the Electrotechnical Sector Policy and Strategy Committee, was published under the a
2、uthority of the Standards Policy and S t r a t e g y C o m m i t t e e o n 30 October 2002 BSI 30 October 2002 First published February 1986 First revision November 1991 Second revision October 2002 The following BSI references relate to the work on this British Standard: Committee reference GEL/602
3、 ISBN 0 580 40595 8 Committees responsible for this British Standard The preparation of this British Standard was entrusted to Technical Committee GEL/602, upon which the following bodies were represented: BBC British Broadcasting Corporation BEAMA Ltd. GAMBICA Chamber of Shipping Engineering Equipm
4、ent and Materials Users Association ERA Technology Ltd HSE Health and Safety Executive Institute of Petroleum Institution of Gas Engineers Ministry of Defence UK Defence Standardization Radio Society of Great Britain SIRA Limited UK Offshore Operators Association Limited Co-opted member Amendments i
5、ssued since publication Date Amd. No. CommentsBS 6656:2002 BSI 30 October 2002 i Contents Page Committees responsible Inside front cover Foreword iii 1S c o p e 1 2 Normative references 1 3T e r m s a n d d e f i n i t i o n s 2 4 Symbols and abbreviations 3 5 General considerations 4 6 Transmitters
6、 and transmitter output parameters 5 7 Structures and spark-making mechanisms 7 8 Ignition of flammable atmospheres 11 9 Practical measurements and tests 13 10 Methods of assessment for determining potential RF ignition hazards on a plant containing hazardous areas 15 11 Plant safety measures 43 12
7、Special cases 45 Annex A (informative) Sources of information and addresses of some advisory bodies 51 Annex B (informative) Electromagnetic radiated fields and examples of radiating antenna and unintended receiving antenna characteristics 52 Annex C (informative) Subdivision of group II flammable g
8、ases and vapours 60 Annex D (normative) Measurement of electromagnetic fields 65 Annex E (normative) Methods of measurement on structures (on-site tests) 67 Annex F (informative) Derivation of vulnerable zone distances for Table 5, Table 6 and Table 10 72 Annex G (informative) Worked examples of ful
9、l assessment procedure 73 Annex H (informative) Ground-wave propagation (vertical polarization) Calculation of field strength 81 Bibliography 84 Figure 1 Typical loop-type structures 9 Figure 2 Assessment procedure flow chart 17 Figure 3 On-site test procedure flow chart see 10.5 and 10.1b) 18 Figur
10、e 4 Safe threshold field strength versus frequency for single unmodulated transmissions 26 Figure 5 Field strength versus distance from the transmitter (land path) 33 Figure 6 Field strength versus distance from the transmitter (sea path) 34 Figure 7 Modulus match powers for Q k= 5 39 Figure 8 Examp
11、le of reduction of structure efficiency 44 Figure B.1 Typical antenna installations (physical characteristics of transmitting antennas) 54 Figure B.2 Definition of angles 57 Figure E.1 Test arrangements for measurement of extractable power 69 Figure E.2 Test arrangement for incendivity tests 72 Figu
12、re G.1 Modulus match powers for structure resonated at 1,1 MHz 78 Table 1 Representative gases for gas groups 12 Table 2 Radio-frequency power thresholds 12 Table 3 Radio-frequency energy thresholds 13 Table 4 Representative gases for gas groups and the most easily ignited mixture (% volume with air
13、) 14BS 6656:2002 ii BSI 30 October 2002 Page Table 5 Radii of vulnerable zones 19 Table 6 Radii of vulnerable zones for small structures at frequencies up to and including 30 MHz 28 Table 7 Maximum radii of vulnerable zones 29 Table 8 Modulation factors 31 Table 9 Maximum radii of vulnerable zones f
14、or ships transmitters 47 Table 10 Radio frequency transmitters on offshore mobile and fixed installations (Gas Group IIA, threshold ignition value 6 W) 48 Table A.1 Advisory bodies 51 Table B.1 Antennas for vertical polarization, for frequencies up to and including 30 MHz 53 Table B.2 Antennas for h
15、orizontal polarization, for frequencies up to and including 30 MHz 53 Table B.3 Antennas for frequencies above 30 MHz 53 Table C.1 Subdivision of gases and vapours according to their maximum experimental safe gaps and minimum ignition currents 60 Table E.1 Current in calibration circuit 71 Table G.1
16、 Transmitter data (Example 1) 73 Table G.2 Plant data (Example 1) 74 Table G.3 Transmitter data (Example 2) 75 Table G.4 Plant data (Example 2) 75 Table G.5 Effective field strength for station transmitters (Example 2) 76 Table G.6 Maximum extractable power from station transmissions (Example 2) 74
17、Table G.7 Total extractable power at f r= 1,1 MHz (Example 2) 79 Table G.8 Total extractable power at f r= 1,2 MHz (Example 2) 79 Table G.9 Total extractable power at f r= 1,45 MHz (Example 2) 79 Table G.10 Transmitter data (Example 3) 80 Table G.11 Plant data (Example 3) 80 Table G.12 Peak field st
18、rength for radar transmitters (Example 3) 81 Table G.13 Energy in structure from pulse transmissions (Example 3) 81BS 6656:2002 BSI 30 October 2002 iii Foreword This British Standard has been prepared by Technical Committee GEL/602. It supersedes BS 6656:1991, which is withdrawn. The revision is bas
19、ed on BS 6656:1991, the main differences being modifications to Table 5 and Table 6, previously Table 4 and Table 5, in order to include new transmitters and to amend the assessment of transmitters at very high frequency (v.h.f.) and higher frequencies (h.f.) which in some cases could lead to an ove
20、r assessment of the hazard. NOTE The decimal comma is used throughout this standard in place of the decimal point. This publication does not purport to include all the necessary provisions of a contract. Users are responsible for its correct application. Compliance with a British Standard does not o
21、f itself confer immunity from legal obligations. Summary of pages This document comprises a front cover, inside front cover, pages i to iv, pages 1 to 85 and a back cover. The BSI copyright notice displayed in this document indicates when the document was last issued.iv blankBS 6656:2002 BSI 30 Octo
22、ber 2002 1 Introduction Electromagnetic waves produced by radio-frequency (RF) transmitters (e.g. radio, television and radar) will induce electric currents and voltages in any conducting structure on which they impinge. The magnitude of the induced current and voltages depends upon the shape and si
23、ze of the structure relative to the wavelength of the transmitted signal and on the strength of the electromagnetic field. When parts of the structure normally in contact are caused to break or separate momentarily (e.g. during maintenance or as a result of vibration) a spark may occur if the induce
24、d voltage and current is sufficiently large. If this happens in a location where a potentially flammable atmosphere may be present a hazardous situation can occur. However, the possibility of ignition will depend on many factors including whether the spark can deliver sufficient energy to ignite a p
25、articular flammable atmosphere. This British Standard provides a systematic approach to assist transmitter operators, plant managers and all others concerned with a logical method for the assessment and elimination of RF induced ignition hazards. The assessment procedures recommended in this British
26、 Standard are based on measurements of the powers and energy that can be extracted from typical structures, including cranes, and measurements of the minimum powers and energy that are required to ignite various flammable atmosphere gas groups. The assessment procedures for probability of ignition r
27、ecommended in this British Standard are based on the assumption that worst case conditions apply at all times. The critical features are the coincidence of the structure in resonance and the presence of the gas/air mixture in the optimum proportions for RF spark ignition. Deviation from these optimu
28、m conditions will result in significantly higher powers being required for ignition. NOTE 1 Several studies have been performed which indicate that the power could be twice as great for an assumed risk as detailed in reference 1, if due allowance is taken for probabilistic effects. In order to achie
29、ve a probability of ignition comparable with other risks, it would be necessary for effective extractable power calculated to be twice the values determined according to this British Standard. The probabilistic elements could be taken into consideration following further research work and practical
30、experience. NOTE 2 If allowances for probabilities are to be applied then expert advice should be sought. 1 Scope This British Standard provides guidance on assessing the potential ignition hazard from the inadvertent extraction of energy from electromagnetic fields, propagated from communication, r
31、adar or other transmitting antennas to plant where a potentially flammable atmosphere may be present. The frequency range covered by this British Standard is 9 kHz to 60 GHz. This British Standard does not apply to similar hazards arising from electromagnetic fields generated by other means, such as
32、 electric storms, electricity generating installations or other radiating electrical equipment, nor does it apply to any hazard arising within telecommunication or other electronic equipment. NOTE 1 The methods of assessment from 9 GHz to 60 GHz are based on extrapolation of data for frequencies bel
33、ow 9 GHz. NOTE 2 The ignition of dust is not covered in this British Standard. This British Standard also provides advice on how to mitigate the hazard in cases where the assessment indicates that a hazard may exist. This British Standard does not cover the hazards associated with the use of electro
34、-explosive devices (EED) (see BS 6657), or the biological hazards of exposure to RF fields. 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 editi
35、on of the referenced document (including any amendments) applies. BS EN 50014, Electrical apparatus for potentially explosive atmospheres General requirements. BS EN 50020, Electrical apparatus for potentially explosive atmospheres Intrinsic safety “i” BS EN 60079-10, Electrical apparatus for explos
36、ive gas atmospheres Part 10: Classification of hazardous areas. IEC 60079-10BS 6656:2002 2 BSI 30 October 2002 3 Terms and definitions For the purposes of this British Standard the following terms and definitions apply. 3.1 circuit factor, Q k performance parameter for a structure acting as a receiv
37、ing antenna (see 2) NOTE Assuming the structure to be tuned to the transmission frequency f t , Q kis the ratio of f tto f, where f is the difference between those frequencies, one above and one below f t , at which the structure resonates when it is re-tuned so that the open circuit voltage at f th
38、as fallen by 3 dB. Q kis closely related to the Q factor of a tuned circuit. 3.2 extractable power, P max power dissipated in a resistive load connected across a discontinuity in a structure acting as a receiving antenna NOTE The extractable power reaches its maximum when the structure is tuned to t
39、he frequency of the transmitter (under these conditions the impedance of the structure presents a resistive value only, with no reactive components), and the load resistance is a value equal to that of the structure. 3.3 modulus match power, P mm maximum value of extractable power that can be achiev
40、ed with a resistive load at a frequency to which the structure is not tuned (see 2) 3.4 structure efficiency ratio of the extractable power that the structure can deliver to a matched load and the maximum extractable power delivered by a lossless short dipole in free space immersed in the same field
41、 3.5 thermal initiation time time during which energy deposited by the spark accumulates in a small volume of gas around it without significant thermal dissipation NOTE For times shorter than the thermal initiation time the total energy deposited by the spark will determine whether or not ignition o
42、ccurs. For increasingly longer times, the power or rate at which energy is deposited becomes the determining factor for ignition. 3.6 vulnerable zone region surrounding a transmitter in which a potential hazard could arise within a hazardous area of a plant 3.7 far field region, distant from the tra
43、nsmitter, in which the field strength is inversely proportional to distance in the absence of ground reflection NOTE The inner limit of the far field is generally regarded as the distance d from the transmitter defined as follows. For frequencies up to and including 30 MHz, d=8 H 2 / where H is the
44、height of the top of the antenna above ground and is the wavelength. At frequencies above 30 MHz, d = 2W 2 / where W is the width of the antenna. 3.8 near field region close to the transmitter which lies within the far field region NOTE In the near field region the dependence of the field strength o
45、n distance is complex and mutual coupling effects can also affect the value of extractable power. 3.9 flammable atmosphere gas/air or vapour/air mixture capable of being ignited which can occur in a hazardous area NOTE See BS EN 60079-10 for further information.BS 6656:2002 BSI 30 October 2002 3 3.1
46、0 equivalent isotropically radiated power (EIRP) product of the power supplied to the antenna and the antenna gain in a given direction relative to an isotropic antenna (absolute or isotropic gain) 3.11 effective field strength value of electric field strength due to a single transmitter which is de
47、rived from the transmitter characteristics, modulation factors (see 6.5) and distance, and is used for the calculation of extractable power 3.12 antenna gain gain produced by an antenna concentrating radiation in a particular direction NOTE 1 The gain of an antenna is always related to a specified r
48、eference antenna. NOTE 2 The gain, G, of an antenna in a particular direction is given by the equation: G = (1) where R is the power in Watts, W, that should be radiated from the reference antenna; A is the power in Watts, W, that should be radiated from the given antenna to give the same field stre
49、ngth at a fixed distance in that direction. NOTE 3 The gain, which is often expressed in logarithmic form, is stated in decibels. 3.13 hazard potential source of danger to life, limb or health, or of discomfort to a person or persons, or of damage to property 3.14 safe distance distance outside which it is considered that there is no pot
