oSIST prEN IEC 60079-29-3:2026

SLOVENSKI STANDARD
01-marec-2026
Eksplozivne atmosfere - 29-3. del: Plinski detektorji - Navodilo o funkcijski
varnosti fiksnih sistemov za odkrivanje plina
Explosive atmospheres - Part 29-3: Gas detectors - Guidance on functional safety of
fixed gas detection systems
Explosionsfähige Atmosphäre - Teil 29-3: Gasmessgeräte - Leitfaden zur funktionalen
Sicherheit von ortsfesten Gaswarnsystemen
Atmosphères explosives - Partie 29-3: Détecteurs de gaz - Recommandations relatives à
la sécurité fonctionnelle des systèmes fixes de détection de gaz
Ta slovenski standard je istoveten z: prEN IEC 60079-29-3:2025
ICS:
13.230 Varstvo pred eksplozijo Explosion protection
13.320 Alarmni in opozorilni sistemi Alarm and warning systems
29.260.20 Električni aparati za Electrical apparatus for
eksplozivna ozračja explosive atmospheres
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

31/1941/CDV
COMMITTEE DRAFT FOR VOTE (CDV)
PROJECT NUMBER:
IEC 60079-29-3 ED2
DATE OF CIRCULATION: CLOSING DATE FOR VOTING:
2025-12-26 2026-03-20
SUPERSEDES DOCUMENTS:
31/1883/CD, 31/1923A/CC
IEC TC 31 : EQUIPMENT FOR EXPLOSIVE ATMOSPHERES
SECRETARIAT: SECRETARY:
United Kingdom Mr Tom Stack
OF INTEREST TO THE FOLLOWING COMMITTEES: HORIZONTAL FUNCTION(S):

SC 65A
ASPECTS CONCERNED:
SUBMITTED FOR CENELEC PARALLEL VOTING NOT SUBMITTED FOR CENELEC PARALLEL VOTING
Attention IEC-CENELEC parallel voting
The attention of IEC National Committees, members of CENELEC,
is drawn to the fact that this Committee Draft for Vote (CDV) is
submitted for parallel voting.
The CENELEC members are invited to vote through the CENELEC
online voting system.
This document is still under study and subject to change. It should not be used for reference purposes.
Recipients of this document are invited to submit, with their comments, notification of any relevant patent rights of which they are
aware and to provide supporting documentation.
Recipients of this document are invited to submit, with their comments, notification of any relevant “In Some Countries” clauses to be
included should this proposal proceed. Recipients are reminded that the CDV stage is the final stage for submitting ISC clauses. (SEE
AC/22/2007 OR NEW GUIDANCE DOC).

TITLE:
Explosive atmospheres – Part 29-3: Gas detectors – Guidance on functional safety of fixed gas detection
systems
PROPOSED STABILITY DATE: 2030
NOTE FROM TC/SC OFFICERS:
electronic file, to make a copy and to print out the content for the sole purpose of preparing National Committee positions.
You may not copy or "mirror" the file or printed version of the document, or any part of it, for any other purpose without
permission in writing from IEC.

Link to Committee Draft for Vote (CDV) online document:
Click here
How to access
This link leads you to the Online Standards Development (OSD) platform for National Mirror Committee’s
(NMC) comments. The project draft may be found further down this document.

Resource materials
We recommend NCs to review the available materials to better understand the member commenting on the
OSD platform. This includes the:
• OSD NC roles overview
• How to add and submit comments to the IEC

Contact
Should you require any assistance, please contact the IEC IT Helpdesk.

IEC CDV 60079-29-3 ED2 © IEC 2025

CONTENTS
FOREWORD . 4
INTRODUCTION . 7
1 Scope . 13
2 Normative references . 14
3 Terms and definitions . 14
4 Safety specifications . 16
4.1 General . 16
4.2 Demand rate . 16
5 Gas detection unique features . 16
5.1 Objective . 16
5.2 Features . 17
5.2.1 General . 17
5.2.2 Sensor location . 17
5.2.3 Sensor filter elements (passive) . 17
5.2.4 Sensor filter elements (active) . 17
5.2.5 Sensor principles . 17
5.2.6 Poisoning and adverse chemical reaction . 18
5.2.7 ppm.hr or %vol.hr lifetime . 18
5.2.8 Negative gas readings . 18
5.2.9 Hazard and risk analysis . 18
5.2.10 Preventative effectiveness or mitigation effectiveness . 18
5.2.11 Cross sensitivities. 19
5.2.12 Special states . 19
5.2.13 Metrological performance standards . 19
5.2.14 Fault signal handling. 19
5.2.15 Overrange indication . 19
5.2.16 Surrogate calibration . 19
5.2.17 Maximum/minimum alarm set points . 19
6 Functional safety management . 20
6.1 Objective . 20
6.2 Recommendations . 20
6.3 Competence . 21
7 General recommendations . 21
7.1 Objective . 21
7.2 Recommendations . 22
7.2.1 General . 22
7.2.2 Safety and non-safety functions . 22
7.2.3 Safety functions of different integrity targets . 22
7.2.4 Behaviour under dangerous failure conditions . 22
7.2.5 Behaviour under safe failure conditions . 22
7.2.6 Behaviour under special state conditions . 23
7.2.7 Power supply . 23
7.2.8 Gas detector . 24
7.2.9 Gas detection control unit (logic solver) . 24
7.2.10 Final element . 24
7.2.11 Visual indication . 24
IEC CDV 60079-29-3 ED2 © IEC 2025

7.2.12 Switching outputs . 25
7.2.13 Protocol outputs . 26
7.2.14 Protocol inputs . 26
7.2.15 System architecture, PFD and PFH values . 26
8 Gas detection unique requirements . 27
8.1 Objectives . 27
8.2 Recommendations . 27
8.2.1 Introduction to gas sampling . 27
8.2.2 Gas sampling . 27
8.2.3 Gas multiplexer . 28
8.2.4 Gas multiplexer control system . 29
8.2.5 Conditioning of measured gas . 29
8.2.6 Gas sampling by diffusion mode . 30
8.2.7 Automatic calibration and adjustment . 31
8.2.8 Automatic calibration and adjustment control system . 31
9 Alternative control units (logic solvers) . 32
9.1 Objectives . 32
9.2 Recommendations . 32
9.2.1 Performance (metrological) . 32
9.2.2 Programming of logic . 32
10 Factory acceptance testing . 32
10.1 Objectives . 32
10.2 Recommendations . 33
10.2.1 Planning . 33
10.2.2 Execution . 33
11 Installation and commissioning . 34
11.1 Objectives . 34
11.2 Recommendations . 34
11.2.1 Planning . 34
11.2.2 Execution . 34
12 System validation . 35
12.1 Objectives . 35
12.2 Recommendations . 35
12.2.1 Planning . 35
12.2.2 Execution . 35
13 Operation and maintenance . 36
13.1 Objectives . 36
13.2 Recommendations . 36
13.2.1 Planning . 36
13.2.2 Execution . 37
14 System modification . 37
14.1 Objectives . 37
14.2 Recommendations . 37
14.2.1 Planning . 37
14.2.2 Execution . 38
15 System decommissioning . 38
15.1 Objectives . 38
15.2 Recommendations . 38
IEC CDV 60079-29-3 ED2 © IEC 2025

15.2.1 Planning . 38
15.2.2 Execution . 38
16 Documentation . 39
16.1 Objectives . 39
16.2 Recommendations . 39
Annex A (informative) Typical Applications . 40
A.1 Typical diffusion applications . 40
A.1.1 Application 1 . 40
A.1.2 Application 2 . 40
A.1.3 Application 3 . 41
A.1.4 Application 4 . 41
A.2 Typical sampling applications . 42
A.2.1 Point to Point sampling . 42
A.2.2 Multi-stream sampling . 43
Annex B (informative) Transformation of requirements . 44
B.1 General . 44
B.2 SIL 1 capability . 44
B.2.1 Characteristic . 44
B.2.2 Transformation . 44
B.3 SIL 2 capability . 44
B.3.1 Characteristic . 44
B.3.2 Transformation . 45
B.4 SIL 3 capability . 45
B.4.1 Characteristic . 45
B.4.2 Transformation . 45
Bibliography . 46

Figure 1 – Determination of Functional Safety Requirements of Gas Detection Safety
System . 8
Figure 2 – Related Safety Instrumented System Standards . 13
Figure A.1 – Gas detection safety loops . 40
Figure A.2 – Typical gas detector aspiration configurations . 42

Table 1 – Typical Roles and Their Most Relevant Clauses . 10
Table 2 – Demand for Functional Safety Management (see IEC 61508-1) . 21

IEC CDV 60079-29-3 ED2 © IEC 2025

INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
Explosive atmospheres -
Part 29-3: Gas detectors - Guidance on functional safety of fixed gas
detection systems
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 itself does not provide any attestation of conformity. Independent certification bodies
provide conformity assessment services and, in some areas, access to IEC marks of conformity.
IEC is not responsible for any services carried out by independent certification bodies.
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) IEC draws attention to the possibility that the implementation of this document may involve
the use of (a) patent(s). IEC takes no position concerning the evidence, validity or applicability
of any claimed patent rights in respect thereof. As of the date of publication of this document,
IEC had received notice of any patent, which may be required to implement this document.
IEC CDV 60079-29-3 ED2 © IEC 2025

However, implementers are cautioned that this may not represent the latest information, which
may be obtained from the patent database available at https://patents.iec.ch. IEC shall not be
held responsible for identifying any or all such patent rights.
IEC 60079-29-3 has been prepared by IEC technical committee 31: Equipment for explosive
atmospheres. It is an International Standard.
This second edition cancels and replaces the first edition published in 2014 This edition
constitutes a technical revision.
This edition includes the following significant technical changes with respect to the previous
edition:
a) .;
The text of this International Standard is based on the following documents:
Draft Report on voting
XX/XX/FDIS XX/XX/RVD
Full information on the voting for its approval can be found in the report on voting indicated in
the above table.
The language used for the development of this International Standard is English [change
language if necessary].
This document was drafted in accordance with ISO/IEC Directives, Part 2, and developed in
accordance with ISO/IEC Directives, Part 1 and ISO/IEC Directives, IEC Supplement, available
at www.iec.ch/members_experts/refdocs. The main document types developed by IEC are
described in greater detail at www.iec.ch/publications.
This part of IEC 60079-29 is to be used in conjunction with the following standards:
IEC 60079-29-0, Explosive atmospheres - Part 29-0: Gas detection equipment - General
requirements and test methods
IEC 60079-29-1, Explosive atmospheres - Part 29-1: Gas detectors - Performance requirements
of detectors for flammable gases
IEC 60079-29-2, Explosive atmospheres - Part 29-2: Gas detectors - Selection, installation, use
and maintenance of detectors for flammable gases and oxygen
IEC 60079-29-4, Explosive atmospheres - Part 29-4: Gas detectors - Performance requirements
of open path detectors for flammable gases
IEC 62990-1, Workplace atmospheres - Part 1: Gas detectors - Performance requirements of
detectors for toxic gases
IEC 62990-2, Workplace atmospheres - Part 2: Gas detectors - Selection, installation, use and
maintenance of detectors for toxic gases and vapours
A list of all parts of the IEC 60079 series, under the general title: Explosive atmospheres, can
be found on the IEC website.
The committee has decided that the contents of this document will remain unchanged until the
stability date indicated on the IEC website under webstore.iec.ch in the data related to the
specific document. At this date, the document will be
IEC CDV 60079-29-3 ED2 © IEC 2025

– reconfirmed,
– withdrawn, or
– revised.
IEC CDV 60079-29-3 ED2 © IEC 2025

INTRODUCTION
Fixed gas detection systems have been used for many years to perform safety instrumented
functions. Like any instrumented system, a fixed gas detection system commonly comprises of
a single or multiple gas detector input(s), a control unit and a single or multiple final element(s)
or output(s). Additional peripheral equipment may be incorporated into a fixed gas detection
system, for example a gas sampling system or a gas conditioning system. If a fixed gas
detection system, including any relevant peripheral equipment is to be effectively used for
safety instrumented functions, it is preferable that the total system achieves certain minimum
standards and performance levels.
It is important to understand that the number of sensing points and their appropriate location,
their redundancy, the management of regular maintenance, specifically response checking or
calibration, and other gas detection specific features (such as design of gas sampling systems)
are all likely to have a far greater effect on the integrity of the overall Safety Instrumented
System (SIS) than the required Safety Integrity Level (SIL) or SIL capability of any of the
individual functional units. This, however, does not exclude the need to consider each Safety
Instrumented Function (SIF) having a stand-alone functional integrity.
This document gives guidance to the considerations for fixed gas detection systems which are
based on the use of electrical, electronic or programmable electronic systems (E/E/PES) where
there is either a risk reduction target stated or if the gas detection system is used as an
additional safe guarding system. The intent of this document is to provide guidance for gas
detection systems to comply with the IEC 61508 (all parts) or IEC 61511 Series standards.
This document does not apply to portable gas detectors or fixed gas detection systems when
there is no stated risk reduction target. However, this document could be used as best practice
for such devices or systems.
The expression 'gas detection system' within this document is generic and applies to a range
of devices from standalone fixed gas detectors, which might have their own internal alarm trip
levels and switching outputs, up to complex fixed gas detection systems (Annex A - Typical
Applications).
This document takes into consideration the possible complexity of the supply chain which a gas
detection equipment manufacturer, seller or system integrator might encounter which includes,
but is not limited to:
– the use of standalone gas detectors which are integrated into an overall safety system by a
gas detection equipment manufacturer, seller or system integrator (or equivalent)
– the design and use of fixed gas detection sub-systems, including any associated and/or
peripheral gas detection equipment which are integrated into an overall safety system by a
gas detection equipment manufacturer, seller or system integrator (or equivalent)
– the design and use of a complete fixed gas detection system, including associated and/or
peripheral gas detection equipment which is the overall safety system
NOTE 1 IEC 61508 Parts 1, 2and 3are applicable to the stand-alone gas detector system which may include gas
sensors, detector or control unit and final element such as beacon lights, sounders or actuators. Guidance on the
design of peripheral equipment is included within this document.
Before this document can be applied, it is important to understand and categorise the
application of the fixed gas detection system. The main applications are:
– as a prevention or mitigation system - the total system or an individual instrumented control
loop has a safety function and safety integrity clearly defined.
– as an additional safe guarding system - this covers fixed gas detection systems which
operate in parallel (secondary) to an instrumented safety system, where the demand on the
fixed gas detection system is only when the primary instrumented safety system fails or
another layer of protection fails.
IEC CDV 60079-29-3 ED2 © IEC 2025

The use of an additional safeguarding gas detection system should not contribute to the
Hardware Fault Tolerance (HFT) declaration for the instrumented safety system.
A fixed gas detection system, as shown in Figure 1, may operate several times per year subject
to the application, therefore this international standard accepts that the demand rate associated
with 'on demand' (low demand) should be specified in the safety requirements (for example, a
reference could be "> 1/yr but <10/yr").

Figure 1 – Determination of Functional Safety Requirements of Gas Detection Safety
System
Figure 1 shows a generalized determination of functional safety requirements of gas detector
safety systems.
To assist with the possible complexity and unique requirements associated with fixed gas
detection systems, a fixed gas detection system may be broken down into functional units. Each
functional unit can vary in complexity; a functional unit may be a simple gas detector or a
combination of components which form peripheral equipment. Each safety element can be
independently assessed during the initial design phase of the functional unit, thus allowing its
safety data to contribute to the functional unit's overall assessment.
NOTE 2 Basic elements of a sub-system/system (for example, a gas detector, logic controller/solver, etc.) can be
designed as a product consistent with the approach of IEC 61508 Parts 1, 2 and 3.
Each functional unit is then assembled to deliver a complete fixed gas detection system. It is
not necessary to re-assess individual functional units when they are used in a different
configuration - it is only necessary to evaluate the combination of functional units.
This document is based on the safety lifecycle model detailed in IEC 61508 (all parts) but adds
additional and supportive information particular to gas detection equipment to assist with this
safety lifecycle.
IEC CDV 60079-29-3 ED2 © IEC 2025

This document specifies requirements under 'Functional Safety Management' with which all
parties involved in the supply of fixed gas detection systems should comply.
NOTE 3 Functional Safety Management applies to all stages of the safety lifecycle irrespective of the product,
subsystem, system supply or service being supplied.
For this document, the SIL capability excludes consideration of gas detection coverage or the
transport of gas or vapour to the measuring point: IEC 60079-29-2 addresses these two
subjects.
Table 1 suggests the relevance of clauses in this document by role in the safety lifecycle.
IEC CDV 60079-29-3 ED2 © IEC 2025

Table 1 – Typical Roles and Their Most Relevant Clauses
Gas Alterna Factory Installatio Syste Operatio System System Document
Applies Definiti Conform Gas Functio General
to ons ance to detect nal requirem detectio tive accepta n and m n and modifica de- ation
this ion safety ents n unique control nce commissi validat mainten tion commissi
documen uniqu manage requirem units testing oning ion ance oning
t e ment ents (logic (SAT)
featur solvers
es )
Clause Claus Clause Clause Clause Clause Clause
Clause Clause 4 Clause 6 Clause 7 Clause 8 Clause 11 Clause 15 Clause 16
3 e 5 9 10 12 13 14
Consultan +++ +++
t
Contractor +++ +++
Vendor +++ +++
System +++ +++
Integrator
Manufactu +++ +++
rer
NOTE 1 Each category above will have personnel in several of the categories below.
General + + + + + + + + + ++ ++ + +

managem
ent
Design +++ + +++ +++ +++ +++ + + + ++ +++ ++ ++

engineerin
g /
managem
ent
System +++ + +++ +++ +++ +++ +++ ++ ++ + +++ ++ ++

engineer /
managem
ent
IEC CDV 60079-29-3 ED2 © IEC 2025

Applies Definiti Conform Gas Functio General Gas Alterna Factory Installatio Syste Operatio System System Document
to ons ance to detect nal requirem detectio tive accepta n and m n and modifica de- ation
this ion safety ents n unique control nce commissi validat mainten tion commissi
documen uniqu manage requirem units testing oning ion ance oning
t e ment ents (logic (SAT)
featur solvers
es )
Clause Clause Clause 4 Claus Clause 6 Clause 7 Clause 8 Clause Clause Clause 11 Clause Clause Clause Clause 15 Clause 16
3 e 5 9 10 12 13 14
Installatio ++ + ++ + ++ + + +++ ++ + ++ ++ ++

n
engineerin
g /
managem
ent
Commissi ++ + ++ ++ ++ + + +++ ++ + ++ ++ ++

oning
engineer/
managem
ent
Service ++ + ++ ++ ++ + + ++ ++ +++ +++ +++ ++

engineer /
managem
ent
Quality ++ + +++ +++ +++ + +++ ++ +++ + ++ + +++

engineer /
managem
ent
Training +++ + +++ +++ +++ ++ + + + +++ + + ++

officers
Operation + + ++ + ++ + ++ + +++ +++ +++ +++ +++

&
maintenan
ce
NOTE 2 See Annex B for guidance on the life cycle of gas detection.
IEC CDV 60079-29-3 ED2 © IEC 2025

Key:
+Useful
++ Advisable
+++ Most appropriate
The following items were listed in the bibliography but not cited in the text. Please find a suitable
place to cite them to justify their inclusion in the bibliography:
IEC 61511-1, Functional safety - Safety instrumented systems for the process industry sector -
Part 1: Framework, definitions, system, hardware and application programming
requirementsIEC 61511-1 [1]
IEC 61511-2, Functional safety - Safety instrumented systems for the process industry sector -
Part 2: Guidelines for the application of IEC 61511-1:2016IEC 61511-2 [2]
IEC 61511-3, Functional safety - Safety instrumented systems for the process industry sector -
Part 3: Guidance for the determination of the required safety integrity levelsIEC 61511-3 [3]
IEC CDV 60079-29-3 ED2 © IEC 2025

1 Scope
This document sets out safety integrity related considerations for fixed gas detection systems
and associated equipment in terms of the framework and philosophy of IEC 61508 (all parts).
The document introduces the requirements described by Figure 2 and includes equipment for
the detection of flammable and toxic gases and vapours, and oxygen.
Other local, national and international standards specify the performance requirements of gas
detection equipment. These standards are commonly known as Metrological Performance
Standards and are concerned with the accuracy of the measured value, the overall system
performance, but not with the device or system integrity with respect to the safety function. This
document applies to the integrity of the safety instrumented function.
NOTE 1 In some jurisdictions, it can be a requirement to certify the performance of equipment for the measurement
of flammable gases, vapours, toxic gases and/or oxygen used in life safety applications.

Figure 2 – Related Safety Instrumented System Standards
This document does not consider the Safety Integrity Level SIL 4. SIL 4 is considered to be
practically unattainable for gas detection systems
NOTE 2 It is rare for any risk study to determine a Safety Integrity higher than SIL 2 for a fixed gas detection
system.
This document is applicable to fixed gas detection systems which can include the following
functional units:
– Gas sensor/transmitter
– Gas detection control unit (logic solver)
– Gas sampling (single and multiplexed streams)
– Gas conditioning
– Automatic gas calibration and adjustment
– Output module (if not part of the control unit)
IEC CDV 60079-29-3 ED2 © IEC 2025

2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements 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 60079-29-0, Explosive atmospheres - Part 29-0: Gas detectors - General requirements and
test methods
IEC 60079-29-1, Explosive atmospheres - Part 29-1: Gas detectors - Performance requirements
of detectors for flammable gases
IEC 60079-29-2, Explosive atmospheres - Part 29-2: Gas detectors - Selection, installation, use
and maintenance of detectors for flammable gases and oxygen
IEC 60079-29-4, Explosive atmospheres - Part 29-4: Gas detectors - Performance requirements
of open path detectors for flammable gases
IEC 61508 (all parts), Functional safety of electrical/electronic/programmable electronic safety-
related systems
IEC 62990-1, Workplace atmospheres - Part 1: Gas detectors - Performance requirements of
detectors for toxic gases
IEC 62990-2, Workplace atmospheres - Part 2: Gas detectors - Selection, installation, use and
maintenance of detectors for toxic gases and vapours
3 Terms and definitions
For the purposes of this document, the terms and definitions given in IEC 60079-29-0, IEC
61508-4 and the following apply.
ISO and IEC maintain terminology databases for use in standardization at the following
addresses:
– IEC Electropedia: available at https://www.electropedia.org/
– ISO Online browsing platform: available at https://www.iso.org/obp
NOTE Additional definitions applicable to explosive atmospheres can be found in IEC 60050-426.
3.1
additional safe guarding system
fixed gas detection system which operate in parallel (secondary) to an instrumented safety
system, where the demand on the fixed gas detection system is only when the primary
instrumented safety system fails or another layer of protection fails
3.2
associated gas detection equipment
equipment additional to the gas detection equipment covered by IEC 60079-29-0, IEC 60079-
29-1, IEC 62990-1 or IEC 60079-29-4 which is part of the overall fixed gas detection system
and is essential with respect to the safety function
Note 1 to entry: Examples of associated gas detection equipment are gas sampling or gas multiplexing.
IEC CDV 60079-29-3 ED2 © IEC 2025

3.3
dangerous failure
failure which has the potential to put the safety-related system in a hazardous or fail-to- function
state
Note 1 to entry: Whether or not the potential is realised can depend on the channel architecture of the system; in
systems with redundant/multiple channels to improve safety, a dangerous hardware failure is less likely to lead to
the overall dangerous or fail-to-function state.
Note 2 to entry: An example of a dangerous failure is the loss of a sensing head.
3.4
element
part of a subsystem comprising a group of components
3.5
fail safe mode
operating mode capable of preserving safety in the case of failure
Note 1 to entry: In the case of loss of power supply the output will be de-energised and the signal is active.
3.6
fault signal
audible, visible or other type of output, different from the alarm signal, permitting, directly or
indirectly, a warning or indication that the equipment is not working satisfactorily
3.7
functional unit
entity of hardware or software, or both, capable of accomplishing a specified purpose which
may consist of several elements
3.8
hardware module
entity of hardware capable of accomplishing a specified purpose, for example a transmitter or
control unit
3.9
measuring point
point in space from which the ambient atmosphere is sampled by the gas detection system
3.10
peripheral equipment
equipment which is part of the overall fixed gas detection system but which can be non-essential
with respect to the safety function
Note 1 to entry: Data storage is an example independent from the safety function.
3.11
proof test
periodic test performed to detect hidden failures in a safety-related system
3.12
safe failure
failure which does not have the potential to put the safety-related system in a hazardous or fail-
to-function state
Note 1 to entry: An example of a safe failure is a fault in an EMC filtering circuit which has no influence in normal
operation but may cause a spurious alarm when EMC disturbances are present.
IEC CDV 60079-29-3 ED2 © IEC 2025

3.13
safe state
state of the equipment under control (EUC) when safety is achieved
Note 1 to entry: In going from a potentially hazardous condition to the final safe state, the EUC may have to go
through a number of intermediate safe states. For some situations, a safe state exists only so long as the EUC is
continuously controlled. Such continuous control can be for a short or an indefinite period of time.
3.14
sample line
means by which the gas being sampled is conveyed to the sensor
Note 1 to entry: Accessories such as filters or water traps are often included in the sample line.
3.15
sample point
end of a sample line from where the sampled gas is drawn
Note 1 to entry: A sample point (hardware) normally comprises a physical housing containing a particle filter or
equivalent.
3.16
SIL capability
characterisation of a functional unit against the requirements of IEC 61508-2 and IEC 61508-3
as being suitable for use in SIL rated systems
3.17
special state
state of the equipment other than that in which monitoring of gas concentration takes place
Note 1 to entry: for example warm-up, calibration mode or fault condition
4 Safety specifications
4.1 General
Consideration is to be given to all subsequent relevant clauses of this document.
4.2 Demand rate
A fixed gas detection system may operate several or many times subject to the application. This
document acknowledges that the demand rate associated with 'on demand' may not be specified
in the safety requirements. This requirement needs to be defined to allow other functional safety
requirements to be defined.
NOTE Such a reference could be in the form "> 1/yr but < 10/yr".
Proof test intervals for low demand mode are determined with the assumption that the demand
rate is maximum 1 per year. If the specified demand rate is higher tha
...