CLC/TR 50427:2004
(Main)Assessment of inadvertent ignition of flammable atmospheres by radio-frequency radiation - Guide
Assessment of inadvertent ignition of flammable atmospheres by radio-frequency radiation - Guide
This standard provides guidance in assessing the possibility of inadvertent extraction of energy from an electromagnetic field propagated from communication, radar or other transmitter aerials and the coupling of this energy to an EED in a manner capable of causing initiation. It does not cover the similar hazard arising from electromagnetic fields generated by other means, for example electrical storms, electricity generating plant or power transmission lines, nor does it cover the potential hazards that may arise in oil and gas exploration and production operations
Leitfaden zur Verhinderung der unbeabsichtigten Zündung explosionsfähiger Atmosphären durch hochfrequente Strahlung
Evaluation des risques d'inflammation des atmosphères inflammables par des rayonnements de radiofréquence - Guide
Ugotavljanje nenamernega vžiga vnetljivih atmosfer z radiofrekvenčnim sevanjem - Vodilo
General Information
Standards Content (Sample)
SLOVENSKI STANDARD
01-april-2005
8JRWDYOMDQMHQHQDPHUQHJDYåLJDYQHWOMLYLKDWPRVIHU]UDGLRIUHNYHQþQLPVHYDQMHP
9RGLOR
Assessment of inadvertent ignition of flammable atmospheres by radio-frequency
radiation - Guide
Leitfaden zur Verhinderung der unbeabsichtigten Zündung explosionsfähiger
Atmosphären durch hochfrequente Strahlung
Evaluation des risques d'inflammation des atmosphères inflammables par des
rayonnements de radiofréquence - Guide
Ta slovenski standard je istoveten z: CLC/TR 50427:2004
ICS:
13.230 Varstvo pred eksplozijo Explosion protection
13.280 Varstvo pred sevanjem Radiation protection
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.
TECHNICAL REPORT CLC/TR 50427
RAPPORT TECHNIQUE
TECHNISCHER BERICHT December 2004
ICS 13.230; 33.060.20
English version
Assessment of inadvertent ignition of flammable atmospheres
by radio-frequency radiation –
Guide
Evaluation des risques d'inflammation Leitfaden zur Verhinderung
des atmosphères inflammables der unbeabsichtigten Zündung
par des rayonnements de explosionsfähiger Atmosphären
radiofréquence – durch hochfrequente Strahlung
Guide
This Technical Report was approved by CENELEC on 2004-08-28.
CENELEC members are the national electrotechnical committees of Austria, Belgium, Cyprus, Czech
Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy,
Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Slovakia, Slovenia,
Spain, Sweden, Switzerland and United Kingdom.
CENELEC
European Committee for Electrotechnical Standardization
Comité Européen de Normalisation Electrotechnique
Europäisches Komitee für Elektrotechnische Normung
Central Secretariat: rue de Stassart 35, B - 1050 Brussels
© 2004 CENELEC - All rights of exploitation in any form and by any means reserved worldwide for CENELEC members.
Ref. No. CLC/TR 50427:2004 E
Foreword
This Technical Report was prepared by the Technical Committee CENELEC TC 31, Electrical apparatus for
explosive atmospheres - General requirements.
The text of the draft was submitted to the formal vote and was approved by CENELEC as
CLC/TR 50427 on 2004-08-28.
___________
– 3 – CLC/TR 50427:2004
Contents
Introduction .6
1 Scope.7
2 Normative references.7
3 Terms and definitions.7
4 Symbols and abbreviations .9
4.1 Modulation codes.9
4.2 Polarization codes.10
5 General considerations .10
5.1 Radio-frequency hazard.10
5.2 Philosophy of systematic method of approach .11
5.3 Responsibility for making the hazard assessment.11
6 Transmitters and transmitter output parameters .12
6.1 Types of transmitter .12
6.2 Frequency range.12
6.3 Transmitter output power .12
6.4 Antenna gain.12
6.5 Modulation factors.12
6.5.1 General.12
6.5.2 Frequency modulation (FM).13
6.5.3 Amplitude modulation (AM).13
6.5.4 Single sideband (SSB) operation.13
6.5.5 Pulsed radar.13
7 Structures and spark-making mechanisms .14
7.1 Structures.14
7.2 Loop-type structures .14
7.3 Vertical structures .16
7.4 Spark-making mechanisms.17
8 Ignition of flammable atmospheres.17
8.1 Flammable atmospheres .17
8.2 Ignition by radio-frequency discharges .17
8.3 Criteria for ignition.18
8.3.1 Effectively continuous transmissions .18
8.3.2 Radar transmissions .18
9 Practical measurements and tests.19
9.1 Measurement of electromagnetic fields .19
9.2 Measurement of extractable power.19
9.3 Test transmissions .20
9.4 Incendivity tests.20
10 Methods of assessment for determining potential RF ignition hazards on a plant
containing hazardous areas.21
10.1 General.21
10.2 Basis of the theoretical assessments .21
10.2.1 General.21
10.2.2 Initial assessment.22
10.2.3 Full assessment .22
10.3 Initial assessments.32
10.3.1 Initial assessment of the risk from a particular transmitter site.32
10.3.2 Initial assessment for a particular plant.33
10.4 Full assessment procedure.34
10.4.1 Procedure.34
10.4.2 Information to be obtained .35
10.4.3 Calculation of effective field strengths .35
10.4.4 Calculation of extractable power or energy.41
10.4.5 Comparison of the total extractable power or energy from the structure with the
threshold values detailed in Clause 8 .43
10.5 Practical on-site tests.46
10.5.1 Procedure.46
10.5.2 Plant and transmitter both in existence (Case 1 of Figure 3).46
10.5.3 Existing plant and proposed transmitter (Case 2 of Figure 3) .47
10.5.4 Existing transmitter and proposed plant (Case 3 of Figure 3) .48
11 Plant safety measures .49
11.1 General.49
11.2 Bonding.49
11.3 Insulation.50
11.4 Reducing the structure efficiency.50
11.5 De-tuning of structures.50
12 Special cases.51
12.1 Cranes.51
12.2 Mobile and portable transmitters.51
12.3 Ships.51
12.3.1 General.51
12.3.2 Ships in harbour areas.52
12.3.3 Ships at sea .52
12.4 Offshore oil and gas installations.53
12.4.1 General.53
12.4.2 Structures on offshore installations.53
12.4.3 Assessment procedures.53
12.4.4 Radio frequency transmitters and vulnerable zones.54
12.4.5 Safety measures and recommendation .55
– 5 – CLC/TR 50427:2004
Annex A (informative) Sources of information and addresses of some advisory bodies.59
Annex B (informative) Electromagnetic radiated fields and examples of radiating antenna and
unintended receiving antenna characteristics .61
Annex C (informative) Subdivision of group II flammable gases and vapours.69
Annex D (normative) Measurement of electromagnetic fields .74
Annex E (normative) Methods of measurement on structures (on-site tests).78
Annex F (informative) Derivation of vulnerable zone distances for Table 5, Table 6 and Table 10 .84
Annex G (informative) Worked examples of full assessment procedure .85
Annex H (informative) Ground-wave propagation (vertical polarization) -
Calculation of field strength .94
Bibliography.96
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 size 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 induced 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 particular flammable atmosphere.
This European Technical Report 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 European Technical Report 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 recommended in this European Technical Report
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 optimum 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 achieve 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
European Technical Report. The probabilistic elements could be taken into consideration following further research work and
practical experience.
NOTE 2 If allowances for probabilities are to be applied then expert advice should be sought.
– 7 – CLC/TR 50427:2004
1 Scope
This European Technical Report provides guidance on assessing the potential ignition hazard from the
inadvertent extraction of energy from electromagnetic fields, propagated from communication, radar or
other transmitting antennas to plant where a potentially flammable atmosphere may be present. The
frequency range covered by this European Technical Report is 9 kHz to 60 GHz. This European
Technical Report does not apply to similar hazards arising from electromagnetic fields generated by other
means, such as 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 below 9 GHz.
NOTE 2 The ignition of dust is not covered in this European Technical Report. This European Technical Report also provides
advice on how to mitigate the hazard in cases where the assessment indicates that a hazard may exist. This European Technical
Report does not cover the hazards associated with the use of electro-explosive devices (EED) (see CLC/TR 50426), 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 edition of the referenced
document (including any amendments) applies.
Publication Year Title
EN 60079-0 Electrical apparatus for explosive gas atmospheres —
Part 0: General requirements (IEC 60079-0)
EN 50020 Electrical apparatus for potentially explosive atmospheres —
Intrinsic safety “i”
EN 60079-10 Electrical apparatus for explosive gas atmospheres —
Part 10: Classification of hazardous areas (IEC 60079-10)
3 Terms and definitions
For the purposes of this European Technical Report the following terms and definitions apply.
3.1
circuit factor, Q
k
performance parameter for a structure acting as a receiving antenna (see [2])
NOTE Assuming the structure to be tuned to the transmission frequency f , Q is the ratio of f to Δf, where Δf is the difference
t k t
between those frequencies, one above and one below f , at which the structure resonates when it is re-tuned so that the open circuit
t
voltage at f has fallen by 3 dB. Q is closely related to the Q factor of a tuned circuit.
t k
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 the 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 achieved 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
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 occurs. 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 transmitter, 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 = 8H /λ where H is the height of the top of the antenna above ground and λ is the wavelength. At
frequencies above 30 MHz, d = 2W /λ 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 on 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 EN 60079-10 for further information.
3.10
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 derived 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 reference antenna.
– 9 – CLC/TR 50427:2004
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 strength at a fixed distance in that direction.
R
G = (1)
A
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 strength 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 potential hazard
4 Symbols and abbreviations
4.1 Modulation codes
AM Amplitude-modulated speech or music transmission. Carrier power quoted.
MCW Amplitude-modulated tone transmission. Carrier power quoted.
TV Amplitude-modulated video transmission. Peak power quoted.
R ( ) Pulse-modulated radar transmission. Peak power quoted. The number in brackets indicates
the pulse duration in s where known.
FM Frequency modulation.
FSK Frequency shift keying.
GFSK Gaussian frequency shift key modulation.
SSB Single sideband transmission. Peak envelope power quoted.
CW Continuous wave.
MSK Minimum shift keying.
GMSK Gaussian minimum shift keying.
CDMA Code division multiple access.
PCM Pulse code modulation.
PSK Phase shift keying.
PM Phase modulation.
DQPSK Differential quadrature phase shift keying.
4.2 Polarization codes
V Vertical polarization.
H Horizontal polarization.
V/H Either vertical or horizontal polarization, or both simultaneously.
5 General considerations
5.1 Radio-frequency hazard
For a radio-frequency hazard assessment, detailed consideration should be taken of the conditions that
have to be satisfied simultaneously for a hazard to exist. These are as follows:
NOTE 1 The simultaneous occurrence of these four conditions is unlikely.
NOTE 2 See [3] for further information.
a) electromagnetic radiation of sufficient intensity;
NOTE 3 An electromagnetic field of sufficient intensity may be generated by a fixed/mobile or portable transmitter, the
magnitude of the field depending upon the transmitted power, the antenna gain and the proximity to the site under
consideration.
NOTE 4 Intense electromagnetic fields are also generated by the intentional radio frequency sources in industrial, scientific
and medical (ISM) equipment. Field strengths in the order of 10 V/m may be present in the near vicinity of the equipment.
Typical characteristics of industrial equipment are:
2,5 GHz to 10 kW
915 MHz to 100 kW industrial microwaves
27 MHz to (10 to 50) kW
welding or drying techniques
13,56 MHz to (10 to 50) kW
b) presence of a structure capable of behaving as a receiving antenna. Only structures in a
hazardous area (as defined in EN 60079-10) should be considered (see Clause 7 and Clause 8);
c) existence of a mechanism whereby the received energy or power can be delivered as a spark;
d) presence of a flammable atmosphere (see Clause 8).
All conducting structures behave as receiving antennas, but the magnitude of induced current and voltage
depends upon the method of construction and the configuration. Experience gained in practical
measurements on structures has shown that for frequencies up to and including 30 MHz, the loop
configuration is the most efficient receiving system (see [4]). At higher frequencies all structures are large
compared with a wavelength and their behaviour is conveniently treated by the use of long dipole theory.
The behaviour of these structures is described in Clause 7.
The generation of a spark is dependent upon the appearance of a small discontinuity in a receiving
structure, for example when parts of a structure normally in contact are caused to separate, either during
maintenance or at any time by flexing, mechanical vibration or similar actions.
The spark energy required to ignite a flammable atmosphere depends upon the nature and composition
of the flammable atmosphere. In making the assessment, it is assumed that the composition is at its
optimum for ignition to take place. This in itself provides a margin of safety under most circumstances,
since the energy required for ignition of a particular atmosphere generally increases rapidly as its
composition moves away from the optimum.
– 11 – CLC/TR 50427:2004
5.2 Philosophy of systematic method of approach
This European Technical Report is based on a series of graded assessments, each requiring a
progressively more detailed analysis.
The initial assessment procedure is designed to eliminate from further consideration those locations
where it is highly unlikely that a hazard exists. It is based on “realistic worst case” estimates of the radius
of the zone around different classes of transmitter within which a hazard might result from the presence of
a structure in a hazardous area.
If the initial assessments indicate that a hazard might exist, the full assessment procedure given in 10.4
should be followed. This provides a method of computing the maximum power available in any spark
produced, based on more detailed information about the actual transmitter and plant and their relative
location. This calculated power should then be compared with the minimum power required to ignite the
particular flammable atmosphere concerned (see Table 2, Table 3 and Figure 4).
When this procedure is followed, it will quickly become apparent whether the available information is
adequate for an assessment to be made with a high degree of confidence or whether additional
information is required from practical measurements on site. If doubt exists, then expert opinion should be
sought (see Annex A).
The assessment procedures in this European Technical Report determine whether ignition is possible
under worst case conditions. No account is taken of any effects that could influence the probability of
ignition. An inherent safety factor exists for many circumstances.
The assessment procedures recommended in Clause 10 apply generally to most circumstances. For
cranes, mobile transmitters and oil rigs the special considerations described in Clause 12 should be taken
into account.
5.3 Responsibility for making the hazard assessment
The radio-frequency (RF) environment is becoming increasingly severe, with the proliferation of
transmitting sources, increased transmitter powers and the exploitation of new techniques.
NOTE 1 Legislation, (for example in the UK see [5]), requires that employers safeguard both their employees and others who may
be placed at risk by their activities. Hence, both operators of RF transmitters and users of plant in which a flammable atmosphere
may be present have a responsibility to ensure safe operation.
NOTE 2 Particular locations such as mines and quarries may exist where additional responsibilities are placed on the owners and
managers.
Operators of a proposed plant in which a flammable atmosphere may be present should send details to
the transmitter operators and request information about relevant transmitters in the locality of the site. If a
potential hazard is indicated, the plant operator should then use the assessment procedures given in this
European Technical Report, in consultation with the transmitter operators concerned.
Similarly, the operator of a proposed new (or altered) transmitter should contact all operators of plant with
potentially flammable atmospheres within the vulnerable zone for their transmitter, and use the procedure
given in this European Technical Report to assess the potential hazard at each location.
Where both plant and transmitter already exist but an assessment is required, the plant operator should
be held responsible for ensuring that an assessment is made. If for some reason relevant information
cannot be made available to the body responsible for the assessment, the responsibility for having an
assessment carried out should be assumed by the body unable to release the necessary information.
NOTE 3 As an aid to those who need to make a hazard assessment but do not have the necessary technical resources, a list of
sources of information and specialist organizations capable of providing consultation or test facilities is given in Annex A.
6 Transmitters and transmitter output parameters
6.1 Types of transmitter
This clause provides information on various types of transmitter and transmitting systems. This
information is necessarily rather brief for certain types of radar and other military equipment but basic
details are given and further information may be sought from the specialist organizations listed in Annex
A. Typical types of antenna are shown in Figure B.1.
6.2 Frequency range
The main frequency range covered in this guide is 9 kHz to 60 GHz. The types of transmitter considered
include the following:
a) radio and television broadcast transmitters in specific bands in the range 0,15 MHz to 1 000 MHz;
b) fixed and mobile transmitters for communication purposes, private, commercial and amateur, in
specific bands above 0,4 MHz and for military use above 0,15 MHz;
c) radar, in specific bands at 220 MHz, 600 MHz and above 1 GHz;
d) navigational equipment, non-directional beacons, etc., from 9 kHz upwards.
6.3 Transmitter output power
Transmitter output power from several watts up to megawatts may be encountered depending on the
frequency range and the application. The method of specifying the power varies, which is significant for
hazard assessment when highly directional antennas are in use and when considering different
modulation systems. In general, values are specified in the technical documentation for either the carrier
or the peak power output from the transmitter together with the antenna gain, although the product of the
two is often quoted in the technical documentation to give the equivalent isotropically radiated power
(EIRP).
6.4 Antenna gain
The reference antenna is often an isotropic antenna that radiates uniformly in all directions. Although this
is a purely hypothetical concept it is nevertheless very useful for reference purposes. When the gain of an
antenna relative to an isotropic reference antenna is stated in decibels it is denoted by dBi. In practice,
the gain of an antenna is often expressed relative to a half-wave dipole which itself has a gain of 1,64 (or
2 dB) relative to an isotropic antenna. In special circumstances other reference antennas may be used,
for example a short monopole. It is therefore important that the reference antenna is correctly specified.
The maximum gains for typical antennas are listed in Table B.1, Table B.2 and Table B.3.
6.5 Modulation factors
6.5.1 General
Most transmissions are modulated in order to convey information or to enable them to carry out specific
tasks (for example, radar). The characteristics of the principal modulating systems are described in 6.5.2
to 6.5.5. A modulation factor, m, is necessary for calculating the effective field strength E in 10.4.3.
Modulation factors are listed in Table 8 for different types of modulation.
NOTE Radio-frequency transmission may be unmodulated in which case the radiated power is constant and the power quoted in
the technical documentation should be used. Such transmissions are sometimes referred to as continuous wave (CW).
– 13 – CLC/TR 50427:2004
6.5.2 Frequency modulation (FM)
For frequency modulation the frequency of the transmission is varied to carry the information but the
output power remains constant as with CW transmissions. Frequency shift keying (FSK) and gaussian
frequency shift keying (GFSK), minimum shift keying (MSK) and gaussian minimum shift keying (GMSK)
are forms of frequency modulation. Phase shift keying (PSK) and phase modulation (PM) are treated
similarly in that no allowance for modulation is necessary since the output power is not affected by the
modulation.
6.5.3 Amplitude modulation (AM)
6.5.3.1 General
The amplitude of the transmission is varied to carry the information. This information may consist of
speech or music or a coded transmission or it may be a television picture waveform.
6.5.3.2 Speech and music
When speech and music are transmitted, the power quoted in the technical documentation is that of the
unmodulated or carrier transmission but the instantaneous field strength at the modulation peaks may be
up to two times greater when the speech or music is at its loudest volume. Since the duration of the
modulation peaks may exceed the thermal initiation time of the gas, it is necessary to make some
allowance for the power increase (modulation) when making an assessment. As a result of experience
gained with amplitude modulated broadcast transmitters, a modulation factor, m, of 1,4 should be used
for calculations of effective field strengths for RF ignition effects (see [6] and [7]).
6.5.3.3 Coded transmissions
A version of amplitude modulation used for Morse and other coded transmissions is known as modulated
continuous wave (MCW). For modulated continuous wave, whereby the carrier is fully modulated by a
continuous tone, a modulation factor, m, of 2 should be used.
6.5.3.4 Television transmissions (TV)
For television transmissions the peak power is usually quoted in the technical documentation but in this
case no allowance for modulation is necessary, because the mean power is approximately equal to the
peak power.
6.5.4 Single sideband (SSB) operation
For single sideband (SSB) operation the peak envelope power is usually quoted in the technical
documentation and a value of 0,7 for the modulation factor, m, should be used.
6.5.5 Pulsed radar
Pulsed radar consists of pulses transmitted at frequent intervals. The transmissions are characterized by
a peak power P , a mean power P , a pulse duration t (in s) and a pulse repetition rate of n pulses per
o m
second. These are related by the equation:
P
m
P = (2)
o
nt
where
nt is the duty cycle.
The peak power, P , should be used for assessment purposes and no additional allowance for
o
modulation should be considered (see [8] and [9]).
7 Structures and spark-making mechanisms
7.1 Structures
Voltage and current may be induced in a metal object in an electromagnetic field; when two such
conducting objects are in intermittent contact an RF discharge may result. The source of RF power for
ignition by discharge is, in effect, an antenna situated in the incident electromagnetic field. The antenna
configuration may be formed by any metallic object or assembly of objects, such as pipelines, storage
tanks bridged by walkways, tanker loading facilities and vent stacks. However, not all the metallic objects
on a site are capable of delivering significant energy into a discharge and some classification of structures
can be achieved in order to identify possible hazard situations.
For analogy with simple antenna types, structures may be divided into loop types or vertical (monopole)
types for consideration at frequencies up to and including 30 MHz and may be easily identified in a survey
of the plant. At higher frequencies, sections or small parts of the structure, whose physical dimensions
are comparable with a wavelength of the transmitter frequencies, may behave as efficient antennas.
There is no requirement to identify these since due allowance for their characteristics is made in the
assessment procedure (see Clause 10) where it is assumed that their behaviour is not significantly
different from that of a long dipole (see [10]).
7.2 Loop-type structures
At frequencies up to and including 30 MHz, structures which form loops (see Figure 1) are more efficient
as receiving antennas than other types of structure (see [2], [4], [11] and [12]). When the internal loop
perimeter is about one half-wavelength, the structure is self-resonant and its efficiency is at a maximum.
Loop structures having a smaller loop perimeter than the optimum can, however, be brought into
resonance by the presence of stray capacitance across the discontinuity. The efficiency decreases rapidly
when the ratio of the loop perimeter to the wavelength is below 0,5 and remains approximately constant
for values of this ratio greater than 0,5.
At any potential discontinuity in the structure, it is possible to calculate the maximum extractable power,
P (see 3.2) provided that information is available regarding the following:
max
a) the structure perimeter;
b) the frequency of the transmission;
c) the incident field strength at the structure location.
NOTE Cranes form particularly efficient loop-type structures and are thus treated as a special case (see 12.1).
– 15 – CLC/TR 50427:2004
p
p
Key Key
1 Possible discontinuities 1 Possible discontinuity
p Internal perimeter of the loop p Internal perimeter of the loop
a) Loop formed by columns and pipes b) Loop formed by columns and pipes
p
p
h 3
Key Key
1 Possible discontinuity 1 Loading arm
p Internal perimeter of the loop 2 Discontinuity
h Height of the structure 3 Antistatic bonds
p Internal perimeter of the loop
c) Horizontal loop d) Tanker loading facility
Figure 1 — Typical loop-type structures
p
p
Key Key
1 Walkway 1 Discontinuity
2 Possible discontinuity p Internal perimeter of the loop
p Internal perimeter of the loop
e) Storage tanks f) Fixed crane
p
Key
1 Discontinuity
p internal perimeter of the loop
g) Mobile crane
Figure 1 — Typical loop-type structures (concluded)
7.3 Vertical structures
Many vertical structures may be observed in a plant or area where flammable material is used. Probably
the most easily identifiable vertical structure is the vent or flare stack, although any free-standing structure
such as a fractionating column comes into the same category.
However, even for structures that are situated on a concrete base, the base affords a low impedance path
to earth at radio frequencies, and the power that can be extracted at this point is extremely low (see [4]).
Free-standing vertical structures are therefore not classified among the more efficient types of structures
acting as receiving antennas and generally may be omitted from consideration except where part of a
vertical structure forms one side of a loop. The structure should then be assessed on the basis of the
internal perimeter of the loop.
– 17 – CLC/TR 50427:2004
7.4 Spark-making mechanisms
Although a high RF potential may exist between two parts of a structure, the nature of the gap and the
spark-making mechanism is highly relevant to the RF ignition of gases or vapours present around the
discontinuity. Radio-frequency discharges occur most easily if two parts initially in contact are drawn
apart, i.e. if a break-spark occurs. The breakdown of gaps that are not in intermittent contact requires
considerably higher voltages than those necessary to produce ignitions from break-sparks, and therefore
discharges across a fixed gap are not considered to be a significant problem (see [13]).
The types of construction or operation that are likely to result in an intermittent contact across a
discontinuity are to be encountered in, for example, road tanker loading facilities, where rubbing contact
can occur between the loading arm and the tanker; very often the loop circuit is completed by anti-static
bonds from the tanker or the base of the loading system. Similarly, when flanges are parted or valves are
removed during maintenance procedures, an intermittent contact may be obtained thereby providing a
discontinuity in the loop. Such a condition is worthy of consideration, particularly if the loop formed by the
pipework is of the optimum dimension for resonance. Discharges are also more easily obtained if one of
the conducting members forms a sharp point, as for example at the ends of the strands of a wire hawser.
8 Ignition of flammable atmospheres
8.1 Flammable atmospheres
Spark ignition in a flammable atmosphere occurs when the power or energy in the spark exceeds a
certain threshold value, which depends both on the nature and the concentration of the flammable gas or
vapour. Table C.1 shows flammable gases and vapours grouped according to their ease of ignition. The
threshold power or energy of an RF spark for the different groups has been determined using a
representative gas for each group as given in Table 1.
Table 1 — Representative gases for gas groups
Gas group Representative gas
a
I Methane
IIA Propane
IIB Ethylene
IIC
...
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