EN 60079-28:2007

SLOVENSKI STANDARD
01-november-2007
(NVSOR]LYQHDWPRVIHUHGHO=DãþLWDRSUHPHNLXSRUDEOMDRSWLþQRVHYDQMHLQ
VLVWHPRY]DSUHQRVRSWLþQHJDVHYDQMD,(&
Explosive atmospheres - Part 28: Protection of equipment and transmission systems
using optical radiation
Explosionsfähige Atmosphäre - Teil 28: Schutz von Einrichtungen und
Übertragungssystemen, die mit optischer Strahlung arbeiten
Atmospheres explosives - Partie 28: Protection du matériel et des systemes de
transmission utilisant le rayonnement optique
Ta slovenski standard je istoveten z: EN 60079-28:2007
ICS:
29.260.20 (OHNWULþQLDSDUDWL]D Electrical apparatus for
HNVSOR]LYQDR]UDþMD explosive atmospheres
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

EUROPEAN STANDARD
EN 60079-28
NORME EUROPÉENNE
March 2007
EUROPÄISCHE NORM
ICS 29.260.20
English version
Explosive atmospheres -
Part 28: Protection of equipment and transmission systems
using optical radiation
(IEC 60079-28:2006)
Atmosphères explosives -  Explosionsfähige Atmosphäre -
Partie 28: Protection du matériel Teil 28: Schutz von Einrichtungen
et des systèmes de transmission und Übertragungssystemen,
utilisant le rayonnement optique die mit optischer Strahlung arbeiten
(CEI 60079-28:2006) (IEC 60079-28:2006)

This European Standard was approved by CENELEC on 2006-10-01. CENELEC members are bound to comply
with the CEN/CENELEC Internal Regulations which stipulate the conditions for giving this European Standard
the status of a national standard without any alteration.

Up-to-date lists and bibliographical references concerning such national standards may be obtained on
application to the Central Secretariat or to any CENELEC member.

This European Standard exists in three official versions (English, French, German). A version in any other
language made by translation under the responsibility of a CENELEC member into its own language and notified
to the Central Secretariat has the same status as the official versions.

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: rue de Stassart 35, B - 1050 Brussels

© 2007 CENELEC - All rights of exploitation in any form and by any means reserved worldwide for CENELEC members.
Ref. No. EN 60079-28:2007 E
Foreword
The text of document 31/631/FDIS, future edition 1 of IEC 60079-28, prepared by IEC TC 31, Equipment
for explosive atmospheres, was submitted to the IEC-CENELEC parallel vote and was approved by
CENELEC as EN 60079-28 on 2006-10-01.
The following dates were fixed:
– latest date by which the EN has to be implemented
at national level by publication of an identical
national standard or by endorsement (dop) 2007-10-01
– latest date by which the national standards conflicting
with the EN have to be withdrawn (dow) 2009-10-01
This European Standard has been prepared under a mandate given to CENELEC by the European
Commission and the European Free Trade Association and covers essential requirements of
EC Directive 94/9/EC. See Annex ZZ.
Annexes ZA and ZZ have been added by CENELEC.
__________
Endorsement notice
The text of the International Standard IEC 60079-28:2006 was approved by CENELEC as a European
Standard without any modification.
In the official version, for Bibliography, the following notes have to be added for the standards indicated:
IEC 60079-1 NOTE Harmonized as EN 60079-1:2004 (not modified).
IEC 60079-2 NOTE Harmonized as EN 60079-2:2004 (not modified).
IEC 60079-7 NOTE Harmonized as EN 60079-7:2007 (not modified).
IEC 60079-14 NOTE Harmonized as EN 60079-14:2003 (not modified).
IEC 60079-15 NOTE Harmonized as EN 60079-15:2005 (not modified).
IEC 60079-26 NOTE Harmonized as EN 60079-26:2004 (not modified).
IEC 61241-0 NOTE Harmonized as EN 61241-0:2006 (modified).
IEC 61241-4 NOTE Harmonized as EN 61241-4:2006 (not modified).
IEC 61241-10 NOTE Harmonized as EN 61241-10:2004 (not modified).
IEC 61241-11 NOTE Harmonized as EN 61241-11:2006 (not modified).
IEC 61241-18 NOTE Harmonized as EN 61241-18:2004 (not modified).
__________
- 3 - EN 60079-28:2007
Annex ZA
(normative)
Normative references to international publications
with their corresponding European publications

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.

NOTE  When an international publication has been modified by common modifications, indicated by (mod), the relevant EN/HD
applies.
Publication Year Title EN/HD Year
IEC 60079 Series Electrical apparatus for explosive gas EN 60079 Series
atmospheres
1) 2)
IEC 60079-0 (mod) - Electrical apparatus for explosive gas EN 60079-0 2006
atmospheres -
Part 0: General requirements
1) 2)
IEC 60079-10 - Electrical apparatus for explosive gas EN 60079-10 2003
atmospheres -
Part 10: Classification of hazardous areas

1) 2)
IEC 60079-11 - Explosive atmospheres - EN 60079-11 2007
Part 11: Equipment protection by intrinsic
safety "i"
1) 2)
IEC 60825-2 - Safety of laser products - EN 60825-2 2004
Part 2: Safety of optical fibre communication
systems (OFCS)
IEC 61508 Series Functional safety of EN 61508 Series
electrical/electronic/programmable electronic
safety-related systems
IEC 61511 Series Functional safety - Safety instrumented EN 61511 Series
systems for the process industry sector

1)
Undated reference.
2)
Valid edition at date of issue.

Annex ZZ
(informative)
Coverage of Essential Requirements of EC Directives
This European Standard has been prepared under a mandate given to CENELEC by the European
Commission and the European Free Trade Association and within its scope the standard covers only the
following essential requirements out of those given in Annex II of the EC Directive 94/9/EC:
– ER 1.0.1 to ER 1.0.4, ER 1.0.5 (partly)
– ER 1.2.1, ER 1.2.6, ER 1.2.8 to ER 1.2.9
– ER 1.3.1
– ER 1.5.1
– ER 2.1.1 (partly)
– ER 2.2.1 (partly)
– ER 2.3.1 (partly)
Compliance with this standard provides one means of conformity with the specified essential
requirements of the Directive concerned.
WARNING: Other requirements and other EC Directives may be applicable to the products falling within
the scope of this standard.
NORME CEI
INTERNATIONALE
IEC
60079-28
INTERNATIONAL
Première édition
STANDARD
First edition
2006-08
Atmosphères explosives –
Partie 28:
Protection du matériel et des systèmes de
transmission utilisant le rayonnement optique

Explosive atmospheres –
Part 28:
Protection of equipment and transmission
systems using optical radiation

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60079-28  IEC:2006 – 3 –
CONTENTS
FOREWORD.7
INTRODUCTION.11
1 Scope.13
2 Normative references .15
3 Terms and definitions .15
4 General requirements .21
4.1 Optical equipment .21
4.2 Risk levels.21
5 Types of protection.23
5.1 General .23
5.2 Requirements for inherently safe optical radiation “op is” .23
5.3 Requirements for protected optical radiation “op pr” .27
5.4 Optical radiation interlock with optical fibre breakage “op sh” .29
5.5 Suitability of types of protection.29
6 Type verifications and tests .31
6.1 Test set-up for ignition tests .31
6.2 Reference test.33
6.3 Test mixtures .35
6.4 Tests for pulse trains and pulses between 1 ms and 1 s duration .35
7 Marking .35
7.1 General .35
7.2 Marking information.37
7.3 Examples of marking .37

Annex A (normative) Reference test data.39
Annex B (informative) Ignition mechanisms .41
Annex C (normative) Ignition hazard assessment.53
Annex D (informative)  Typical optical fibre cable design .57
Annex E (informative) Introduction of an alternative risk assessment method
encompassing “equipment protection levels” for Ex equipment .59

Bibliography.69

Figure 1 – Figure B.1 with limit lines for intermediate areas for non-combustible
targets, T1 – T4 atmospheres, apparatus group IIA, IIB or IIC.25
Figure B.1 – Minimum radiant igniting power with inert absorber target (α
=83 %, α =93 %) and continuous wave-radiation of 1 064 nm.47
064 nm 805 nm
Figure B.2 – Minimum radiant igniting power with inert absorber target (α
=83 %, α =93 %) and continuous wave-radiation (PTB: 1 064 nm, HSL:
064 nm 805 nm
805 nm, [24]: 803 nm) for some n-alkanes .49
Figure C.1 – Ignition hazard assessment .53
Figure D.1 – Example multi-fibre optical cable design for heavy duty applications.57
Figure D.2 – Typical single optical fibre cable design.57

60079-28  IEC:2006 – 5 –
Table 1 – Relationship between EPL and the probability of an ignition source.21
Table 2 – Safe optical power and irradiance for hazardous locations categorized by
apparatus group and temperature class .23
Table 3 – Optical interlock availability or ignition risk reduction factor by EPL.29
Table 4 – Application of types of protection for optic systems based on EPLs .31
Table A.1 – Reference values for ignition tests with a mixture of propane in air at
40 °C mixture temperature .39
Table B.1 – AIT (auto ignition temperature), MESG (maximum experimental safe gap)
and measured ignition powers of the chosen combustibles for inert absorbers as the
target material (α =83 %, α =93) .45
1 064 nm 805 nm
i,min
Table B.2 – Comparison of measured minimum igniting optical pulse energy (Q )
e,p
at 90 µm beam diameter with auto ignition temperatures (AIT) and minimum ignition
energies (MIE) from literature [25] at concentrations in percent by volume (ϕ) .51
Table E.1 – Traditional relationship of EPLs to zones (no additional risk assessment) .63
Table E.2 – Description of risk of ignition protection provided .65

60079-28  IEC:2006 – 7 –
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
EXPLOSIVE ATMOSPHERES –
Part 28: Protection of equipment and transmission systems
using optical radiation
FOREWORD
1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising
all national electrotechnical committees (IEC National Committees). The object of IEC is to promote
international co-operation on all questions concerning standardization in the electrical and electronic fields. To
this end and in addition to other activities, IEC publishes International Standards, Technical Specifications,
Technical Reports, Publicly Available Specifications (PAS) and Guides (hereafter referred to as
“IEC Publication(s)”). Their preparation is entrusted to technical committees; any IEC National Committee
interested in the subject dealt with may participate in this preparatory work. International, governmental and
non-governmental organizations liaising with the IEC also participate in this preparation. IEC collaborates
closely with the International Organization for Standardization (ISO) in accordance with conditions determined
by agreement between the two organizations.
2) The formal decisions or agreements of IEC on technical matters express, as nearly as possible, an international
consensus of opinion on the relevant subjects since each technical committee has representation from all
interested IEC National Committees.
3) IEC Publications have the form of recommendations for international use and are accepted by IEC National
Committees in that sense. While all reasonable efforts are made to ensure that the technical content of
IEC Publications is accurate, IEC cannot be held responsible for the way in which they are used or for any
misinterpretation by any end user.
4) In order to promote international uniformity, IEC National Committees undertake to apply IEC Publications
transparently to the maximum extent possible in their national and regional publications. Any divergence
between any IEC Publication and the corresponding national or regional publication shall be clearly indicated in
the latter.
5) IEC provides no marking procedure to indicate its approval and cannot be rendered responsible for any
equipment declared to be in conformity with an IEC Publication.
6) All users should ensure that they have the latest edition of this publication.
7) No liability shall attach to IEC or its directors, employees, servants or agents including individual experts and
members of its technical committees and IEC National Committees for any personal injury, property damage or
other damage of any nature whatsoever, whether direct or indirect, or for costs (including legal fees) and
expenses arising out of the publication, use of, or reliance upon, this IEC Publication or any other
IEC Publications.
8) Attention is drawn to the Normative references cited in this publication. Use of the referenced publications is
indispensable for the correct application of this publication.
9) Attention is drawn to the possibility that some of the elements of this IEC Publication may be the subject of
patent rights. IEC shall not be held responsible for identifying any or all such patent rights.
International Standard IEC 60079-28 has been prepared by IEC technical committee 31:
Equipment for explosive atmospheres.
The text of this standard is based on the following documents:
FDIS Report on voting
31/631/FDIS 31/650/RVD
Full information on the voting for the approval of this standard can be found in the report on
voting indicated in the above table.
This publication has been drafted in accordance with the ISO/IEC Directives, Part 2.

60079-28  IEC:2006 – 9 –
A list of all parts of the IEC 60079 series, under the general title Explosives atmospheres, can
be found on the IEC website.
The committee has decided that the contents of this publication will remain unchanged until
the maintenance result date indicated on the IEC web site under "http://webstore.iec.ch" in
the data related to the specific publication. At this date, the publication will be
• reconfirmed;
• withdrawn;
• replaced by a revised edition, or
• amended.
60079-28  IEC:2006 – 11 –
INTRODUCTION
Optical equipment in the form of lamps, lasers, LEDs, optical fibers, etc. is increasingly used
for communications, surveying, sensing and measurement. In material processing, optical
radiation of high irradiance is used. Often the installation is inside or close to potentially
explosive atmospheres, and radiation from such equipment may pass through these
atmospheres. Depending on the characteristics of the radiation it might then be able to ignite
a surrounding explosive atmosphere. The presence or absence of an additional absorber
significantly influences the ignition.
There are four possible ignition mechanisms.
a) Optical radiation is absorbed by surfaces or particles, causing them to heat up, and, under
certain circumstances, this may allow them to attain a temperature which will ignite a
surrounding explosive atmosphere.
b) Thermal ignition of a gas volume, where the optical wavelength matches an absorption
band of the gas.
c) Photochemical ignition due to photo dissociation of oxygen molecules by radiation in the
ultraviolet wavelength range.
d) Direct laser induced breakdown of the gas at the focus of a strong beam, producing
plasma and a shock wave both eventually acting as the ignition source. These processes
can be supported by a solid material close to the breakdown point.
The most likely case of ignition occurring in practice with lowest radiation power of ignition
capability is case a). Under some conditions for pulsed radiation, case d) also will become
relevant.
Optical equipment is used in most cases in conjunction with electrical equipment, for which
clear and detailed requirements and standards for use in potentially explosive atmospheres
exist. One purpose of this standard is to inform industry about potential ignition hazards
associated with the use of optical systems in hazardous locations as defined in IEC 60079-10
and the adequate protection methods.
This standard details the integrated system used to control the ignition hazard from equipment
using optical radiation in hazardous locations.

60079-28  IEC:2006 – 13 –
EXPLOSIVE ATMOSPHERES –
Part 28: Protection of equipment and transmission systems
using optical radiation
1 Scope
This part of IEC 60079 explains the potential ignition hazard from equipment using optical
radiation intended for use in explosive gas atmospheres. It also covers equipment, which
itself is located outside but its emitted optical radiation enters such atmospheres. It describes
precautions and requirements to be taken when using optical radiation transmitting equipment
in explosive gas atmospheres. It also outlines a test method, which can be used to verify a
beam is not ignition capable under selected test conditions, if the optical limit values cannot
be guaranteed by assessment or beam strength measurement.
This standard contains requirements for optical radiation in the wavelength range from
380 nm to 10 µm. It covers the following ignition mechanisms:
• optical radiation is absorbed by surfaces or particles, causing them to heat up and, under
certain circumstances, this may allow them to attain a temperature which will ignite a
surrounding explosive atmosphere;
• direct laser induced breakdown of the gas at the focus of a strong beam, producing
plasma and a shock wave both eventually acting as the ignition source. These processes
can be supported by a solid material close to the breakdown point.
NOTE 1 See items a) and d) of the introduction.
This standard does not cover ignition by ultraviolet radiation and by absorption of the radiation
in the explosive mixture itself. Explosive absorbers or absorbers that contain their own
oxidizer as well as catalytic absorbers are also outside the scope of this standard.
This standard specifies requirements for equipment intended for use under atmospheric
conditions.
This standard supplements and modifies the general requirements of IEC 60079-0. Where a
requirement of this standard conflicts with a requirement of IEC 60079-0, the requirement of
this standard willll take precedence.
NOTE 2 Although one should be aware of ignition mechanism b) and c) explained in the introduction, they are not
addressed in this standard due to the very special situation with ultraviolet radiation and with the absorption
properties of most gases (see Annex B).
NOTE 3 Safety requirements to reduce human exposure hazards from fibre optic communication systems are
found in IEC 60825-2:2000.
NOTE 4 Types of protection "op is", "op pr", and "op sh" can provide equipment protection levels (EPL) Ga, Gb,
or Gc. For further information, see Annex E.

60079-28  IEC:2006 – 15 –
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.
IEC 60079 (all parts), Electrical apparatus for explosive gas atmospheres
IEC 60079-0, Electrical apparatus for explosive gas atmospheres – Part 0: General
requirements
IEC 60079-10, Electrical apparatus for explosive gas atmospheres – Part 10: Classification of
hazardous areas
IEC 60079-11, Explosive atmospheres – Part 11: Equipment protection by intrinsic safety "i"
IEC 60825-2, Safety of laser products – Part 2: Safety of optical fibre communication systems
IEC 61508 (all parts), Functional safety of electrical/electronic/programmable electronic
safety-related systems
IEC 61511 (all parts), Functional safety – Safety instrumented systems for the process
industry sector
3 Terms and definitions
For the purposes of this document, the terms and definitions given in IEC IEC 60079-0 and
the following apply.
NOTE Additional definitions applicable to explosive atmospheres can be found in IEC 60050-426 [1] .
3.1
absorption
in a propagation medium, the conversion of electromagnetic wave energy into another form of
energy, for instance heat
[IEV 731-03-14]
3.2
beam diameter (or beam width)
the distance between two diametrically opposed points where the irradiance is a specified
fraction of the beam’s peak irradiance
[IEV 731-01-35]
NOTE Most commonly applied to beams that are circular or nearly circular in cross section.
3.3
beam strength
a general term used in this standard referring to an optical beam’s power, irradiance, energy,
or radiant exposure
—————————
Figures in square brackets refer to the bibliography.

60079-28  IEC:2006 – 17 –
3.4
core
the central region of an optical fibre through which most of the optical power is transmitted
[IEV 731-02-04]
3.5
cladding
that dielectric material of an optical fibre surrounding the core
[IEV 731-02-05]
3.6
fibre bundle
an assembly of unbuffered optical fibres
[IEV 731-04-09]
3.7
fibre optic terminal device
an assembly including one or more opto-electronic devices which converts an electrical signal
into an optical signal, and/or vice versa, which is designed to be connected to at least one
optical fibre
[IEV 731-06-44]
NOTE A fibre optic terminal device always has one or more integral fibre optic connector(s) or optical fibre
pigtails(s).
3.8
inherently safe optical radiation
visible or infrared radiation that is incapable of producing sufficient energy under normal or
specified fault conditions to ignite a specific hazardous atmospheric mixture
NOTE This definition is analogous to the term “intrinsically safe ” applied to electrical circuits.
3.9
irradiance
the radiant power incident on an element of a surface divided by the area of that element
[IEV 731-01-25]
3.10
light (or visible radiation)
any optical radiation capable of causing a visual sensation directly on a human being
[IEV 731-01-04]
NOTE 1 Nominally covering the wavelength in vacuum range of 380 nm to 800 nm.
NOTE 2 In the laser and optical communication fields, custom and practice in the English language have
extended usage of the term light to include the much broader portion of the electromagnetic spectrum that can be
handled by the basic optical techniques used for the visible spectrum.

60079-28  IEC:2006 – 19 –
3.11
minimum ignition energy
MIE
lowest electrical energy stored in a capacitor which upon discharge is sufficient to effect
ignition of the most ignitable explosive atmosphere under specified test conditions
3.12
optical fibre
filament shaped optical waveguide made of dielectric materials
[IEV 731-02-01]
3.13
optical fibre cable
an assembly comprising one or more optical fibres or fibre bundles inside a common covering
designed to protect them against mechanical stresses and other environmental influences
while retaining the transmission qualities of the fibres
[IEV 731-04-01]
3.14
optical fibre communication system
OFCS
engineered, end-to-end assembly for the generation, transference and reception of optical
radiation arising from lasers, LEDs or optical amplifiers, in which the transference is by means
of optical fibre for communication and/or control purposes
3.15
free space optical communication system
FSOCS
an installed, portable, or temporarily mounted, through-the-air system typically used, intended
or promoted for voice, data or multimedia communications and/or control purposes via the use
of modulated optical radiation produced by a laser or IR-LED. "Free space" means indoor and
outdoor optical wireless applications with both non-directed and directed transmission.
Emitting and detecting assemblies may or may not be separated.
NOTE The above definitions are from IEC TC 76. This standard is not only dealing with “communication systems”,
so a more general definition could be useful.
3.16
optical (or radiant) power
the time rate of flow of radiant energy with time
[IEV 731-01-22]
3.17
optical radiation
electromagnetic radiation at wavelengths in vacuum between the region of transition to X-rays
and the region of transition to radio waves, that is approximately between 1 nm and 1 000 µm
[IEV 731-01-03]
NOTE In the context of this standard, the term “optical” refers to wavelengths ranging from 380 nm to 10 µm.

60079-28  IEC:2006 – 21 –
3.18
protected optical fibre cable
optical fibre cable protected from releasing optical radiation into the atmosphere during
normal operating conditions and foreseeable malfunctions by additional armouring, conduit,
cable tray or raceway
3.19
radiant energy
energy that is emitted, transmitted or received via electromagnetic waves
[IEV 731-01-21]
3.20
radiant exposure
the radiant energy incident on an element of a surface divided by the area of that element
)
[IEV 393-14-84, modified, and IEV 845-01-42, modified]
4 General requirements
4.1 Optical equipment
All electrical parts and circuits inside and outside optical equipment shall comply with the
appropriate standards for electrical apparatus.
4.2 Risk levels
Three different equipment protection levels Ga, Gb, Gc are defined (see Annex E). Table 1
shows the relationship between the EPL and the probability of an ignition source:
Table 1 – Relationship between EPL and the probability of an ignition source
EPL Protection required
Ga Ignition not likely with one fault and two independent faults or in the case of rare
malfunctions
Gb Ignition not likely with one fault or in the case of expected malfunctions
Gc Ignition not likely in normal operation

An ignition hazard assessment, as given in Annex C, has to be carried out to identify the
ignition mechanisms and ignition sources caused by the specific working principle of the
equipment using optical radiation.
The types of protection selected from section 5 to protect the specific equipment depend on
this ignition hazard assessment considering the table of ignition probabilities given above for
the different EPLs.
NOTE In IEC TC 31, the introduction of "equipment protection levels (EPL) Ga, Gb, Gc" was decided.
—————————
IEC 60050-393:2003, International Electrotechnical Vocabulary (IEV) – Part 393: Nuclear instrumentation –
Physical phenomena and basic concepts
IEC 60050-845:1987, International Electrotechnical Vocabulary (IEV) – Chapter 845: Lighting

60079-28  IEC:2006 – 23 –
5 Types of protection
5.1 General
Three types of protection can be applied to prevent ignitions by optical radiation in potentially
explosive atmospheres. These types of protection encompass the entire optical system.
These types of protection are
a) inherently safe optical radiation, type of protection “op is”;
b) protected optical radiation, type of protection “op pr”;
c) optical system with interlock, type of protection “op sh”.
5.2 Requirements for inherently safe optical radiation “op is”
5.2.1 General
Inherently safe optical radiation means visible or infrared radiation that is incapable of
supplying sufficient energy under normal or specified fault conditions to ignite a specific
explosive atmosphere. The concept is a beam strength limitation approach to safety. Ignition
by an optically irradiated target absorber requires the least amount of energy, power, or
irradiance of the identified ignition mechanisms in the visible and infrared spectrum. The
inherently safe concept applies to unconfined radiation and does not require maintaining an
absorber-free environment.
NOTE Research to date [17-22] has concluded the following values of visible and infrared beam strength are safe
for explosive gas atmospheres. The safe values incorporate a modest safety factor on observed ignition values
obtained under severe test conditions. Ignition of a carbon disulfide-air mixture has been reported recently using
24 mW optical power.
5.2.2 Continuous wave radiation
Optical powers or optical irradiance shall not exceed the values listed in Table 2, categorized
by apparatus group and temperature class. The irradiance values are safe up to a maximum
2 2
irradiated surface area of 400 mm . For irradiated surface areas above 400 mm , the
temperature limits of the relevant temperature class apply. Table 2 contains information on
combustible and on non-combustible absorbers. As an alternative to Table 2, for intermediate
target surface areas where combustible solid targets can be excluded safe power values can
be drawn from Figure 1.
Table 2 – Safe optical power and irradiance for hazardous locations
categorized by apparatus group and temperature class
IIC
Apparatus group I IIA IIA IIB
Temperature class T3 T4 T4 T4 T6
Temperature class (°C) <150 < 200 < 135 < 135 < 135 < 85
Power (mW) 150 150 35 35 35 15
Irradiance (mW/mm )
a a
(surface area not 20 20 5 5 5 5
exceeding 400 mm )
a 2 2
For irradiated areas greater than 30 mm where combustible materials may intercept the beam, the 5 mW/mm
irradiance limit applies.
60079-28  IEC:2006 – 25 –
hydrogen ethyne carbon disulfide
2 000
[1] methane n-pentane
Adler CS
iso-propylic alcohol propane
diethyl ether ethene
1 000
dimethyl ether
THF
Ignition
5 mW/mm
35 mW
Non-ignition
–4 –3 –2 –1 0 1 2
10 10 10 10 10 10 10
Irradiated area  mm
IEC  1489/06
Figure 1 – Figure B.1 with limit lines for intermediate areas
for non-combustible targets, T1 – T4 atmospheres, apparatus group IIA, IIB or IIC
5.2.3 Pulsed radiation
For optical pulse duration of less than 1 ms, the optical pulse energy shall not exceed the
minimum spark ignition energy (MIE) of the respective explosive gas atmosphere.
For optical pulse duration between 1 ms and 1 s, an optical pulse energy equal to 10 times
the MIE of the explosive gas atmosphere shall not be exceeded.
For optical pulse duration greater than 1 s, the peak power shall not exceed the safety levels
for continuous wave radiation (see 5.2.2, Table 2). Such pulses are considered as continuous
wave radiation.
For optical pulse trains, the single pulse criterion applies for each pulse. With repetition rates
above 100 Hz, the average power shall not exceed the safety levels for continuous wave
radiation. With repetition rates below 100 Hz, a higher average power may be applicable if
demonstrated by tests according to Clause 6.
5.2.4 Ignition tests
Ignition tests to demonstrate inherent safety may be performed in special cases such as
– beams of intermediate dimensions or duration that may exceed the minimum optical
ignition criteria but are still incapable of causing ignition;
Minimum igniting power  mW
60079-28  IEC:2006 – 27 –
– beams with complex time waveforms so that pulse energies and/or average power are not
easily resolved;
– specific atmospheres, targets, or other specific applications that are demonstrably less
severe than test conditions studied to date.
The test shall be done with 10 samples of the light source as specified in Clause 6. The test is
passed if there is no ignition during the 10 tests.
5.2.5 Optical devices incorporating the inherently safe concept
Optical devices incorporating the inherently safe concept shall provide over-power/energy
fault protection to prevent excessive beam strengths in potentially explosive atmospheres.
The risk/hazard analysis shall determine when these additional devices are required. The
failure modes of the optical source, the supply barrier, and the presence of an explosive
atmosphere shall be considered during normal operation and during fault conditions to
determine the requirement for additional protection.
Optical sources such as laser diodes or light-emitting diodes will fail if over-heated under
over-power fault conditions. The thermal failure characteristic of certain optical sources may
provide the necessary over-power fault protection (test of 10 samples).
Electrical circuits such as current and/or voltage limiters placed between the optical source
and the electrical power source can provide over-power fault protection similar to intrinsically
safe circuits.
Over-power fault protection shall be provided to the degree necessary for the intended EPL
(see for example IEC 60079-11). For Ga equipment, for example, current and/or voltage
limiters shall provide over-power fault protection after two countable faults are applied to the
current and/or voltage limiter. For Gb equipment, the two-fault requirement can be reduced to
one failure. For Gc equipmen,t the rated values shall be taken without assuming any fault.
The thermal failure characteristic of certain low power optical sources such as light-emitting
diodes is acceptable to provide adequate over-power protection for any EPL.
5.3 Requirements for protected optical radiation “op pr”
5.3.1 General
This concept requires radiation confined inside optical fibre or other transmission medium
based on the assumption that there is no escape of radiation from the confinement. In this
case, the performance of the confinement defines the safety level of the system.
The risk analysis provides the safety requirements based on postulated conditions (fault
conditions or normal operation).
Optical fibre may be used for situations where there are no postulated conditions so that an
external force may cause a break of the protective barrier. Additional protective means (e.g.
robust cabling, conduit or raceway) shall be used when external forces may cause a break
during normal or abnormal operations. The risk analysis will dictate the protective measures
required to prevent a break and escape of radiation.
Where enclosures are used, they may allow an ignition source inside without igniting the
atmosphere outside, provided they meet the requirements of the standard types of protection
concerned (IEC 60079 series).
60079-28  IEC:2006 – 29 –
5.3.2 Radiation inside fibre, etc. (no mechanical damage to be expected)
The optical fibre protects the release of optical radiation into the atmosphere during normal
operating conditions. For foreseeable malfunctions, this can be provided by additional
armouring, conduit, cable tray, or raceway.
5.3.3 Radiation inside enclosures
Incendive radiation inside enclosures is acceptable if the enclosure complies with recognised
types of protection for electrical apparatus where an ignition source may be present inside
(flameproof "d" enclosure, pressurised "p" enclosure, restricted breathing enclosure.)
according to IEC 60079 series. It shall, however, be considered, that any radiation escaping
from the enclosure has to be protected according to this standard.
5.4 Optical radiation interlock with optical fibre breakage “op sh”
This type of protection is applicable when the radiation is not inherently safe with interlock
cut-off if the protection by the confinement fails and the radiation becomes unconfined on time
scales suitably shorter than the ignition delay time.
The interlock cut-off shall be required to perform according to the requirements defined by the
risk analysis. The methods given in appropriate standards (e.g. IEC 61508, IEC 61511) may
be used to analyse equipment performance to have an availability or risk reduction factor,
depending on the equipment protection level, as shown in Table 3.
Table 3 – Optical interlock availability or ignition risk reduction factor by EPL
EPL Safety availability Ignition risk reduction factor
Ga 0,999 to 0,9999 1 000 to 10 000
Gb 0,99 to 0,999 100 to 1 000
Gc 0,9 to 0,99 10 to 100
NOTE The values listed in Table 3 were derived from recommendations of the SAFEC report (Wilday 2000).
Where it can be demonstrated by the ignition hazard assessment (see Annex C), that the
conditions for ignition are not attained readily after breakage of the fibre, shut down times
used for eye protection purposes may be used (see IEC 60825-2:2000). This will typically be
the case for Gc equipment, but may also apply for Gb equipment.
5.5 Suitability of types of protection
Where the ignition hazard assessment given in Annex C shows that ignitions due to optical
radiation are to be expected, the following principles of using the types of protection can be
applied.
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Table 4 – Application of types of protection for optic systems based on EPLs
Type(s) of protection Ga Gb Gc
Inherently safe optical radiation “op is” (see 5.2)
Safe with two faults Yes Yes Yes
Safe with one fault No Yes Yes
safe in normal operation No No Yes
Protected fibre optic media with ignition capable beam “op
pr” (see 5.3)
With additional mechanical protection No Yes Yes
Without additional mechanical protection No No Yes
Protected fibre optic media with ignition capable beam
interlocked with fibre breakage “op sh” (see 5.4)

With additional mechanical protection Yes Yes Yes
Without additional mechanical protection No Yes Yes
None (unconfined, ignition capable beam) No No No

6 Type verifications and tests
6.1 Test set-up for ignition tests
6.1.1 Test vessel
Diameter >150 mm, height above ignition source >200 mm.
6.1.2 Energy and power measurements
Total uncertainty of energy and power measurement shall be less than 5 % relative, including
variations of light source.
6.1.3 Ignition criterion
A temperature increase of at least 100 K, measured by a 0,5 mm diameter thermocouple
bead, 100 mm above the hot spot , or the appearance of a flame.
6.1.4 Mixture temperature
40 °C or the maximum temperature of the specific application.
6.1.5 Mixture pressure
Ambient pressure according to IEC 60079-0.
6.1.6 Safety factor
A safety factor of 1,5 for cw radiation and 3 for pulsed radiation shall be applied to all results
(as non-ignition results) obtained by the tests according to 6.3 or 6.4 before using these data
as inherently safe data.
Where no ignition can be obtained during test (e.g. because the power or energy cannot be
increased further more in the test), this factor shall also be applied to the highest non
incendive beam strength data obtained.

60079-28  IEC:2006 – 33 –
Another possibility to obtain safe beam strength data (including a safety factor) is to use a
test gas that is more sensitive to ignition. For cw-equipment to be used in IIA/T3
atmospheres, this test gas can be ethene up to a size of the beam area of about 2 mm .
NOTE As the ignition by a small hot surface is a process containing considerable statistical deviations, a safety
factor is justified. Due to the same reason, great care is to be applied when judging experiments as non-incendive
because small variations in test parameters may influence the results remarkably.
6.2 Reference test
6.2.1 Reference gas
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