prEN ISO/IEC 80079-20-1

• Abstract:

  This document is published as a dual log standard and provides guidance on classification of gases and vapours. It describes a test method intended for the measurement of the maximum experimental safe gaps (MESG) for gas-air mixtures or vapour-air mixtures under normal conditions of temperature and pressure (20 °C, 101,3 kPa) so as to permit the selection of an appropriate group of equipment. This document also describes a test method intended for use in the determination of the auto-ignition temperature (AIT) of a vapour-air mixture or gas-air mixture at atmospheric pressure, so as to permit the selection of an appropriate temperature class of equipment. Values of chemical properties of materials are provided to assist in the selection of equipment to be used in hazardous areas. Further data may be added as the results of validated tests become available. The materials and the characteristics included in a table (see Annex B) have been selected with particular reference to the use of equipment in hazardous areas. The data in this document have been taken from a number of references which are given in the bibliography. These methods for determining the MESG or the AIT may also be used for gas-air-inert mixtures or vapour-air-inert mixtures. However, data on air-inert mixtures are not tabulated. Keywords: classification of gases and vapours, measurement of the maximum experimental safe gaps (MESG)

• Status: Not Published
• Publication Date: 08-Feb-2028
• ICS: 29.260.20 - Electrical apparatus for explosive atmospheres
• Technical Committee: CEN/TC 305 - Potentially explosive atmospheres - Explosion prevention and protection
• Drafting Committee: CEN/TC 305/WG 1 - Test methods for determining the flammability characteristics of substances (provisional title)
• Current Stage: 4020 - Submission to enquiry - Enquiry
• Start Date: 20-Aug-2026
• Completion Date: 20-Aug-2026
• Directive: 2014/34/EU - Directive 2014/34/EU Of The European Parliament And Of The Council of 26 February 2014 on the harmonisation of the laws of the Member States relating to equipment and protective systems intended for use in potentially explosive atmospheres (recast)
• Mandate: M/596 - [C(2023)4798] Commission Implementing Decision of 20.7.2023 on a standardisation request as regards equipment and protective systems intended for use in potentially explosive atmospheres in support of Directive 2014/34/EU (ATEX) of the European Parliament and of the Council
• Ref Project: oSIST prEN ISO/IEC 80079-20-1:2026 - Explosive atmospheres - Part 20-1: Material characteristics for gas and vapour classification - Test methods and data (ISO/IEC DIS 80079-20-1:2026)

• Revises: EN ISO/IEC 80079-20-1:2019 - Explosive atmospheres - Part 20-1: Material characteristics for gas and vapour classification - Test methods and data (ISO/IEC 80079-20-1:2017, including Cor 1:2018)
• Effective Date: 18-Dec-2024

prEN ISO/IEC 80079-20-1 is a dual-logo standard from CEN and ISO/IEC for explosive atmospheres. It specifies how to classify gases and vapours using maximum experimental safe gap (MESG), minimum igniting current (MIC), auto-ignition temperature (AIT), and explosion characteristics such as pmax, Kmax, and dp/dt, so that the right equipment group and temperature class can be selected for hazardous areas. It is written for engineers, test laboratories, certification bodies, and anyone specifying or buying equipment for flammable gas and vapour atmospheres.

What does prEN ISO/IEC 80079-20-1 specify?

prEN ISO/IEC 80079-20-1 gives guidance for classifying gases and vapours and for selecting equipment for explosive atmospheres. It covers MESG and AIT test methods, tabulated material data for equipment selection, and a method for measuring explosion characteristics of flammable gases and vapours in air in an enclosed space.

The document is organized into:

• Clause 1 scope
• Clause 2 normative references
• Clause 3 terms and definitions
• Clauses 4 to 5 classification rules and data for equipment selection
• Clauses 6 to 8 test methods for MESG, AIT, and explosion characteristics
• Annexes A to F with ovens, verification data, tabulated values, and supporting methods

Annex	What it covers
Annex A	Ovens used for auto-ignition temperature tests
Annex B	Tabulated values for materials and characteristics
Annex C	Determination of cool flames
Annex D	Volume dependence of auto-ignition temperature
Annex E	Verification of maximum explosion pressure values and maximum rates of explosion pressure rise
Annex F	Noise cancelling and data smoothing techniques filtering

The scope also notes that the methods can be used for gas-air-inert and vapour-air-inert mixtures, but data for air-inert mixtures are not tabulated.

What are the key requirements of prEN ISO/IEC 80079-20-1?

Classification rules for equipment group and temperature class

prEN ISO/IEC 80079-20-1 classifies gases and vapours mainly by MESG or MIC ratio, and all flammable materials by AIT into temperature classes. In practice, this is the core of selecting flameproof or other Ex equipment for a specific gas or vapour.

Clause 4.2 assigns equipment groups from MESG:

• Group IIA for larger safe gaps
• Group IIB for intermediate values
• Group IIC for the smallest gaps

When MESG falls in the borderline zone, both MESG and MIC ratio are needed, and MIC ratio then decides the group. This matters because borderline substances need both tests, not just one number.

Clause 4.3 classifies by MIC ratio against laboratory methane. It also requires both MESG and MIC ratio in certain overlap ranges, so users do not rely on a single test result when the classification is close to a boundary.

Clause 4.4 allows provisional classification by chemical similarity within a homologous series. Clause 4.5 says gas mixtures generally need a special MESG or MIC ratio determination, with estimation methods given for some mixtures. That means mixture data should be handled carefully, especially when a process stream is not a pure substance.

Data used for equipment selection

Clause 5 collects data for flammable gases and vapours that help with equipment selection in hazardous areas. It covers equipment group, flammable limits, flash point, temperature class, MIC, and AIT, and it points users to the preferred data sources in the annexes.

The temperature class table links AIT to T1 through T6. In practice, users compare the measured or tabulated AIT with the equipment’s permitted temperature class so the equipment surface temperature stays below the ignition risk of the gas or vapour.

Clause 5.1.6 defines MIC testing in a 24 V DC circuit with a 95 mH air-cored coil. That matters for labs because the test circuit is part of the result, not just the gas sample.

Clause 5.2 gives specific guidance for certain substances and mixtures such as coke oven gas, ethyl nitrite, carbon monoxide, and methane-hydrogen mixtures. These notes are useful where a substance has well-known classification behavior or where mixture composition changes the equipment group.

MESG test method and apparatus

Clause 6 sets out the MESG test method at normal conditions of 20 °C and 101.3 kPa. It uses two chambers with an adjustable 25 mm gap, ignites the internal mixture, and checks whether flame transmission reaches the outer chamber.

The test apparatus has defined chamber dimensions and material strength, and the gap has to be adjusted accurately. In practice, this means the result depends on a verified apparatus, not only on the sample.

Clause 6 also defines the test sequence:

• prepare the gas-air or vapour-air mixture carefully
• confirm the chamber conditions
• set the gap in small steps
• ignite with the specified spark source
• observe whether the outer chamber ignites
• determine the maximum safe gap from preliminary and confirmatory tests

Clause 6.5 requires verification of the apparatus with methane, propane, and hydrogen, and periodic rechecking of existing equipment. That makes the MESG data traceable and comparable between laboratories.

Auto-ignition temperature testing and reporting

Clause 7 determines AIT with a heated open Erlenmeyer flask and repeated tests at different temperatures and sample amounts. The document allows the IEC oven or the DIN oven described in Annex A, and it also allows automated apparatus if it meets the same conditions.

The test method requires controlled sample injection, precise thermocouple measurement, and observation of ignition within 5 min. The reported AIT is the lowest ignition temperature reduced by 3 % and rounded to the next 1 °C, with the ignition delay time and barometric pressure also recorded.

Clause 7.6 gives repeatability and reproducibility checks, so users can judge whether results are consistent enough for use. Clause 7.8 requires verification of the method with acetone, ethylene, and n-heptane, which is important for proving that a test set-up performs correctly.

Explosion characteristics in air

Clause 8 adds a method for measuring pmax, Kmax, and dp/dt for flammable gases or vapours in air in an enclosed space. It uses a spherical or cylindrical vessel, an ignition source at the center, pressure measurement, and testing over a range of concentrations.

This clause matters when the task is not just classification but explosion severity assessment. It supports design and evaluation of explosion protection measures and the testing of protection systems against a known explosion load.

What terms does prEN ISO/IEC 80079-20-1 define?

• Auto-ignition - A reaction shown by a visible flame or explosion, with ignition delay time not exceeding 5 min.
• Auto-ignition temperature (AIT) - The lowest surface temperature at which a gas or vapour-air mixture ignites under the specified test conditions.
• Maximum experimental safe gap (MESG) - The largest 25 mm joint gap that prevents flame transmission during the test.
• Minimum igniting current (MIC) - The smallest current that ignites the explosive test mixture in the specified spark apparatus.
• Flammable limits - The lower flammable limit (LFL) and upper flammable limit (UFL), which bound the concentration range in which a flammable mixture can form.
• Explosion characteristic - A numerical value that describes a contained explosion of a specified reactant concentration in a 1 m³ vessel.
• Equipment grouping - The classification system that assigns equipment to the intended explosive atmosphere, such as Group IIA, IIB, or IIC.

Who uses prEN ISO/IEC 80079-20-1?

prEN ISO/IEC 80079-20-1 is used by engineers, test laboratories, certification bodies, and manufacturers of equipment for explosive atmospheres. It is also relevant to quality managers and buyers who need to decide which equipment group or temperature class applies to a gas, vapour, or mixture.

In practice, users apply it when:

• selecting Ex equipment for hazardous areas
• classifying gases, vapours, and mixtures
• running MESG and AIT tests
• documenting safety data for product files, audits, or certification
• comparing laboratory results with tabulated values or verified apparatus performance

What changed in prEN ISO/IEC 80079-20-1 from the previous edition?

The foreword says this second edition cancels and replaces the first edition published in 2017. It identifies the following updates:

• the scope now includes determination of explosion characteristics of flammable gases and vapours with air in an enclosed space
• Clause 4.5 on classification of mixtures of gases is updated
• Clause 5 on data for flammable gases and vapours relating to the use of equipment is updated
• Clause 6 on the MESG test method is updated
• Clause 7 on the AIT test method is updated
• Clause 8 on determination of explosion characteristics is added
• Annex A, B, C, D, E, and F are added or updated to support the revised content

Which standards are used with prEN ISO/IEC 80079-20-1?

• IEC 60050-426 - Provides the electrical apparatus for explosive atmospheres vocabulary used in the definitions.
• IEC 60079-11 - Provides the intrinsic safety spark test apparatus referenced for MIC and the MIC ratio definition.
• IEC 60079-14 - Provides installation and selection guidance, including the temperature-class framework used with AIT.
• IEC 60584-1 - Gives thermocouple EMF specifications and tolerances for the temperature measurements in AIT testing.
• EN 15967:2022 - Provides the reference basis for maximum explosion pressure and maximum rate of pressure rise verification.
• ISO 1773 - Defines the 200 ml narrow-necked boiling flask used as the AIT test vessel.
• JIS C 1602 - Provides the thermocouple class used in Annex A and Annex C.
• IEC 60079-0:2026 - Defines the equipment-group framework cited in the equipment grouping notes.

What does the prEN ISO/IEC 80079-20-1 document contain?

The document includes a pressure-time curve for explosion characterization, a schematic MESG apparatus, and a concentration-based method for deriving pmax and dp/dt. It also includes detailed test setups for AIT, including the IEC and DIN oven arrangements in Annex A and the additional thermocouple arrangement for cool flame detection in Annex C.

Table 2 maps AIT to temperature classes, and Tables 3 and 4 are verification tables for MESG and AIT apparatus performance. Annex D illustrates how AIT changes with vessel volume, while Annex F summarizes filtering and smoothing approaches for sensor data.

Annex B points readers to tabulated values in maintained databases, and Annex E gives the verification data used for maximum explosion pressure and maximum rate of pressure rise. Together, these parts support laboratory testing, equipment classification, and explosion-protection design.

SLOVENSKI STANDARD
01-oktober-2026
Eksplozivne atmosfere - 20-1. del: Lastnosti materiala in razvrstitev za pline in
hlape - Preskusne metode in podatki (ISO/IEC DIS 80079-20-1:2026)
Explosive atmospheres - Part 20-1: Material characteristics for gas and vapour
classification - Test methods and data (ISO/IEC DIS 80079-20-1:2026)
Explosionsfähige Atmosphären - Teil 20-1: Stoffliche Eigenschaften zur Klassifizierung
von Gasen und Dämpfen - Prüfverfahren und Daten (ISO/IEC DIS 80079-20-1:2026)
Atmosphères explosives - Partie 20-1: Caractéristiques des produits pour le classement
des gaz et des vapeurs - Méthodes et données d'essai (ISO/IEC DIS 80079-20-1:2026)
Ta slovenski standard je istoveten z: prEN ISO/IEC 80079-20-1
ICS:
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.

DRAFT
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Explosive atmospheres —
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ISO/IEC DIS 80079-20-1:2026(en)

DRAFT
ISO/IEC DIS 80079-20-1:2026(en)
International
Standard
ISO/IEC
DIS
80079-20-1
ISO/TMBG
Explosive atmospheres —
Secretariat: ISO
Part 20-1:
Voting begins on:
Material characteristics for gas
2026-08-21
and vapour classification — Test
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methods and data
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© ISO/IEC 2026 – All rights reserved
ii
ISO/IEC CDV 80079-20-1 © IEC 2026

CONTENTS
FOREWORD . 4
1 Scope . 8
2 Normative references . 8
3 Terms and definitions . 8
4 Classification of gases and vapours . 12
4.1 General . 12
4.2 Classification according to the MESG . 13
4.3 Classification according to the minimum igniting current ratio (MIC ratio) . 13
4.4 Classification according to the similarity of chemical structure . 13
4.5 Classification of mixtures of gases . 14
5 Data for flammable gases and vapours, relating to the use of equipment . 15
5.1 Determination of the properties . 15
5.1.1 General . 15
5.1.2 Equipment group . 15
5.1.3 Flammable limits. 15
5.1.4 Flash point (FP) . 15
5.1.5 Temperature class . 16
5.1.6 Minimum igniting current (MIC) . 16
5.1.7 Auto-ignition temperature (AIT) . 16
5.2 Properties of particular gases and vapours . 16
5.2.1 Coke oven gas . 16
5.2.2 Ethyl nitrite (CAS No. 109-95-5) . 16
5.2.3 MESG of carbon monoxide . 17
5.2.4 Methane, Equipment Group IIA // Methane hydrogen mixture . 17
6 Method of test for the MESG . 17
6.1 Outline of method . 17
6.2 Test apparatus . 17
6.2.1 General . 17
6.2.2 Material and mechanical strength . 18
6.2.3 Exterior chamber . 18
6.2.4 Interior chamber . 19
6.2.5 Gap adjustment . 19
6.2.6 Injection of mixture . 19
6.2.7 Position of ignition source . 19
6.3 Procedure . 19
6.3.1 Preparation of gas mixtures . 19
6.3.2 Temperature and pressure . 19
6.3.3 Gap adjustment . 19
6.3.4 Ignition . 20
6.3.5 Observation of the ignition process . 20
6.4 Determination of MESG . 20
6.4.1 General . 20
6.4.2 Preliminary tests . 20
6.4.3 Confirmatory tests . 20
6.4.4 Reproducibility of MESG . 20
ISO/IEC CDV 80079-20-1 © IEC 2026

6.4.5 Tabulated values . 21
6.5 Verification of the MESG determination method . 21
7 Test method for auto-ignition temperature . 21
7.1 Outline of method . 21
7.2 Apparatus . 21
7.2.1 General . 21
7.2.2 Test vessel and support . 22
7.2.3 Thermocouples . 22
7.2.4 Oven . 22
7.2.5 Metering devices . 23
7.2.6 Mirror . 23
7.2.7 Timer . 23
7.2.8 Equipment for purging the test vessel with air . 23
7.2.9 Automated apparatus. 23
7.3 Sampling, preparation and preservation of test samples . 24
7.3.1 Sampling . 24
7.3.2 Preparation and preservation . 24
7.4 Procedure . 24
7.4.1 General . 24
7.4.2 Sample injection . 25
7.4.3 Determination of the auto-ignition temperature (AIT) . 25
7.5 Reported auto-ignition temperature (AIT) . 27
7.6 Validity of results . 27
7.6.1 Repeatability . 27
7.6.2 Reproducibility . 27
7.7 Data . 27
7.8 Verification of the auto-ignition temperature determination method . 27
8 Determination of explosion characteristics of flammable gases or vapours in air. 28
8.1 General . 28
8.2 Test methods . 29
8.2.1 Apparatus . 29
8.2.2 Procedure . 30
8.3 Interpretation of test results . 31
8.4 Test report . 31
Annex A (normative) Ovens of test apparatus for the tests of auto-ignition temperature . 33
A.1 General . 33
A.2 “IEC oven” . 33
A.3 “DIN oven” . 33
Annex B (informative) Tabulated values . 42
Annex C (informative) Determination of cool flames . 43
Annex D (informative) Volume dependence of auto-ignition temperature . 45
Annex E (normative) Verification of maximum explosion pressure values and
maximum rates of explosion pressure rise . 46
Annex F (informative) Noise cancelling and data smoothing techniques filtering . 47
F.1 General considerations . 47
F.2 Sampling frequency of the sensor system for Data Smoothing . 47
F.3 Considerations for data evaluation . 47
ISO/IEC CDV 80079-20-1 © IEC 2026

Bibliography . 54

Figure 1 – pressure-time curve . 11
Figure 2 – Test apparatus . 18
Figure 3 – Determination of explosion characteristics from measurements at a range of
flammable gas concentrations . 31
Figure A.1 – Test apparatus: assembly . 34
Figure A.2 – Section A-A (flask omitted) . 35
Figure A.3 – Base heater (board made of refractory material) . 35
Figure A.4 – Flask guide ring (board made of refractory material) . 36
Figure A.5 – Neck heater (board made of refractory material) . 36
Figure A.6 – Oven . 38
Figure A.7 – Lid of steel cylinder . 39
Figure A.8 – Lid of steel cylinder . 40
Figure A.9 – Injection of gaseous sample . 41
Figure C.1 – Additional thermocouple to detect cool flames . 43
Figure C.2 – ‘Negative temperature coefficient’ shown for butyl butyrate as an example . 44
Figure D.1 – Volume dependence of auto-ignition temperature [12][14][13] . 45
Figure F.1 – High pass filter detrend . 48
Figure F.2 – Lowpass filter smoothing . 49
Figure F.3 – Moving average . 50
Figure F.4 – Median method . 51
Figure F.5 – Savitzky-Golay-Filter . 52
Figure F.6 – Douglas-Peucker-Algorithmus . 53

Table 1 – . 5
Table 2 – Classification of temperature class and range of auto-ignition temperatures . 16
Table 3 – Values for verification of the apparatus . 21
Table 4 – Values for verification of the apparatus . 27

ISO/IEC CDV 80079-20-1 © IEC 2026

INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
Explosive atmospheres -
Part 20-1: Material characteristics for gas and vapour classification - Test
methods and data
FOREWORD
1) ISO (the International Organization for Standardization) and IEC (the International
Electrotechnical Commission) form the specialized system for worldwide standardization.
National bodies that are members of ISO or IEC participate in the development of International
Standards through technical committees established by the respective organization to deal with
particular fields of technical activity. ISO and IEC technical committees collaborate in fields of
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liaison with ISO and IEC, also take part in the work
2) The formal decisions or agreements of IEC and ISO 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 and ISO National bodies.
3) IEC and ISO documents have the form of recommendations for international use and are
accepted by IEC and ISO National bodies in that sense. While all reasonable efforts are made
to ensure that the technical content of IEC and ISO documents is accurate, IEC and ISO 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 and ISO National bodies undertake to apply
IEC and ISO documents transparently to the maximum extent possible in their national and
regional publications. Any divergence between any IEC and ISO document and the
corresponding national or regional publication shall be clearly indicated in the latter.
5) IEC and ISO do not provide any attestation of conformity. Independent certification bodies
provide conformity assessment services and, in some areas, access to IEC and ISO marks of
conformity. IEC and ISO are not responsible for any services carried out by independent
certification bodies.
6) All users should ensure that they have the latest edition of this document.
7) No liability shall attach to IEC and ISO or their directors, employees, servants or agents
including individual experts and members of its technical committees and IEC and ISO National
bodies for any personal injury, property damage or other damage of any nature whatsoever,
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publication, use of, or reliance upon, this ISO/IEC document or any other IEC and ISO
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8) Attention is drawn to the Normative references cited in this document. Use of the referenced
publications is indispensable for the correct application of this document.
9) IEC and ISO draw attention to the possibility that the implementation of this document may
involve the use of (a) patent(s). IEC and ISO take 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 and ISO [had/had not] received notice of (a) patent(s), which may be
ISO/IEC CDV 80079-20-1 © IEC 2026

required to implement this document. 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 and www.iso.org/patents. IEC and ISO shall not be held responsible for
identifying any or all such patent rights.
IEC 80079-20-1 has been prepared by subcommittee 31M, Non-electrical equipment and
protective systems for explosive atmospheres, of IEC technical committee 31, Equipment for
explosive atmospheres. It is an International Standard.
This second edition cancels and replaces the first edition published in 2017. This edition
constitutes a technical revision.
The significance of changes between ISO Standard, ISO/IEC 80079-20-1, Edition 2 xxxx and
ISO/IEC 80079-20-1, Edition 1 2017 are as listed below:
Table 1 –
Type
Changes Clause Minor and editorial Extension Major technical
changes changes
Scope 1 X
The addition of the
determination of
explosion
characteristics of
flammable gases
and vapours with air
in an enclosed
space pmax, Kmax,
and dp/dt
Classification of X
4.5
mixtures of gases
Data for flammable X
gases and vapours,
relating to the use
of equipment
Method of test for
X
the MESG
Test method for
X
auto-ignition 7
temperature
Determination of X
explosion
characteristics of 8
flammable gases or
vapours in air.
Ovens of test X
apparatus for the
Annex A
tests of auto-
ignition temperature
Tabulated values X
Annex B
Determination of X
Annex C
cool flames
Volume X
dependence of
Annex D
auto-ignition
temperature
ISO/IEC CDV 80079-20-1 © IEC 2026

Type
Changes Clause Minor and editorial Extension Major technical
changes changes
Annex for X
verification of
maximum explosion
pressure values and Annex E
maximum rates of
explosion pressure
rise
Noise Cancelling
X
and Data
Smoothing
Annex F
Techniques
Filtering or noise
cancelling
NOTE: The technical changes referred to include the significance of technical changes in the
revised IEC Standard, but they do not form an exhaustive list of all modifications from the
previous version. More guidance may be found by referring to the Redline Ver sion of the
standard.
Explanations:
A) Definitions
Minor and editorial changes clarification decrease of technical requirements minor technical
change editorial corrections
These are changes which modify requirements in an editorial or a minor technical way. They
include changes of the wording to clarify technical requirements without any technical change,
or a reduction in level of existing requirement.
Extension addition of technical options
These are changes which add new or modify existing technical requirements, in a way that new
options are given, but without increasing requirements for equipment that was fully compliant
with the previous standard. Therefore, these will not have to be cons idered for products in
conformity with the preceding edition.
Major technical changes addition of technical requirements increase of technical requirements
These are changes to technical requirements (addition, increase of the level or removal) made
in a way that a product in conformity with the preceding edition will not always be able to fulfil
the requirements given in the later edition. These changes have to be considered for products
in conformity with the preceding edition.
Note: These changes represent current technological knowledge. However, these changes
should not normally have an influence on equipment already placed on the market.
B) Information about the background of Changes
The text of this International Standard is based on the following documents:
Draft Report on voting
ISO/IEC CDV 80079-20-1 © IEC 2026

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.
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.
A list of all parts in the ISO/IEC 80079 series, published 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
– reconfirmed,
– withdrawn, or
– revised.
Users of this document are advised that interpretation sheets clarifying the interpretation of this
document can be published. Interpretation sheets are available from the IEC web store and can
be found in the “history” tab of the page for each document.
ISO/IEC CDV 80079-20-1 © IEC 2026

1 Scope
This part of ISO/IEC 80079 provides guidance on classification of gases and vapours. It
specifies a test method intended for the measurement of the maximum experimental safe gaps
(MESG) for gas-air mixtures or vapour-air mixtures under normal conditions of temperature and
pressure (20 °C, 101,3 kPa) so as to allow the selection of an appropriate group of equipment.
This document also specifies a test method intended for use in the determ ination of the auto-
ignition temperature (AIT) of a vapour-air mixture or gas-air mixture at atmospheric pressure,
so as to allow the selection of an appropriate temperature class of equipment.
The materials and the characteristics included in a table (see Annex B ) have been selected
with particular reference to the use of equipment in hazardous areas. The data in this table
have been taken from a number of references which are given in the bibliography. The methods
for determining the MESG or the AIT can also be used for gas-air-inert mixtures or vapour-air-
inert mixtures. However, data on air-inert mixtures are not tabulated.
Values of chemical properties of materials are provided to assist in the selection of equipment
to be used in hazardous areas. Further data might be added as the results of validated tests
become available.
This part of the ISO/IEC 80079 specifies a method for the determination of explosion
characteristics of flammable gases and vapours with air in an enclosed space p , K , and
max max
dp/dt. It gives the criteria by which results obtained using other test procedures can be
correlated to yield explosion characteristics as determined by the method specified in this part
of ISO/IEC 80079.
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 60050-426, International Electrotechnical Vocabulary − Part 426: Electrical apparatus for
explosive atmospheres (available at http://www.electropedia.org/)
IEC 60079-11, Explosive atmospheres − Part 11: Equipment protection by intrinsic safety "i"
IEC 60079-14, Explosive atmospheres − Part 14: Electrical installations design, selection and
erection
IEC 60584-1, Thermocouples - Part 1: EMF specifications and tolerances
EN 15967:2022, Determination of maximum explosion pressure and the maximum rate of
pressure rise of gases and vapours
ISO 1773, Laboratory glassware - Narrow-necked boiling flasks
JIS C 1602, Thermocouples
3 Terms and definitions
For the purposes of this document, the terms and definitions given in IEC 60050 -426 and the
following apply.
ISO/IEC CDV 80079-20-1 © IEC 2026

ISO and IEC maintain terminological databases for use in standardization at the following
addresses:
– IEC Electropedia: available at http://www.electropedia.org/
– ISO Online browsing platform: available at http://www.iso.org/obp
3.1
auto-ignition
reaction which is evidenced by a clearly perceptible flame or explosion, and for which the
ignition delay time does not exceed 5 min
Note 1 to entry: See Clause 7 for a test method for auto-ignition temperature (AIT).
3.2
ignition delay time
time between the completed injection of the flammable material and the ignition
3.3
auto-ignition temperature
AIT
lowest temperature (of a surface) at which under specified test conditions an ignition of a
flammable gas or vapour in mixture with air or air-inert gas occurs
Note 1 to entry: See Clause 7 for a test method.
3.4
maximum experimental safe gap
MESG
maximum gap of a joint of 25 mm in length which prevents any transmission of an explosion
during tests made under the specified conditions
Note 1 to entry: See Clause 6 for a test method.
3.5
minimum igniting current
MIC
minimum current in a specified test circuit that causes the ignition of the explosive test mixture
in the spark test apparatus according to IEC 60079-11 [1]
Note 1 to entry: See 5.1.6 for the test circuit.
3.6
flammable limits
lower flammable limit (LFL) (3.6.1) and upper flammable limit (UFL) (3.6.2) of gas in a gas-air
mixture, between which a flammable mixture is formed
Note 1 to entry: The term “explosive limits” is used especially in European standardization and regulations
interchangeably to describe these limits.
Note 2 to entry: The concentration can be expressed as either a volume fraction or a mass per unit volume.
3.6.1
lower flammable limit
LFL
concentration of flammable gas or vapour in air, below which an explosive gas atmosphere
does not form
Note 1 to entry: For the purposes of Ex Equipment, this was previously referred to as the lower explosive limit
(LEL).
Note 2 to entry: The concentration can be expressed as either a volume fraction or a mass per unit volume.
ISO/IEC CDV 80079-20-1 © IEC 2026

3.6.2
upper flammable limit
UFL
concentration of flammable gas or vapour in air, above which an explosive gas atmosphere
does not form
Note 1 to entry: For the purposes of Ex Equipment, this was previously referred to as the upper explosive limit
(UEL).
Note 2 to entry: The concentration can be expressed as either a volume fraction or a mass per unit volume.
3.7
equipment grouping
classification system of equipment related to the explosive atmosphere for which they are
intended to be used
Note 1 to entry: IEC 60079-0 identifies three equipment groups:
a) Group I − equipment for mines susceptible to fire damp;
b) Group II, which is sub-divided into groups IIA, IIB, and IIC − equipment for all places with an explosive gas
atmosphere other than mines susceptible to fire damp;
c) Group III, which is sub-divided into groups IIIA, IIIB, and IIIC − equipment for all places with an explosive dust
atmosphere other than mines susceptible to fire damp.
Note 2 to entry: Examples of transportable equipment include laptops, fans, temporary luminaires on a stand,
equipment on casters
[SOURCE: IEC 60079-0:2026 [2] , 3.34]
3.8
flash point
FP
lowest liquid temperature at which, under specified test conditions, a liquid gives off vapours in
quantity such as to be capable of forming an ignitable vapour-air mixture
3.9
gas
gaseous phase of a substance that cannot reach equilibrium with its liquid or solid state in the
temperature and pressure range of interest
Note 1 to entry: This is a simplification of the scientific definition, and merely requires that the substance is above
its boiling point or sublimation point at the ambient temperature and pressure.
3.10
vapour
gaseous phase of a substance that can reach equilibrium with its liquid or solid state in the
temperature and pressure range of interest
Note 1 to entry: This is a simplification of the scientific definition, and merely requires that the substance is below
its boiling point or sublimation point at the ambient temperature and pressure.
3.11
explosion
sudden increase of pressure and temperature, due to oxidation or other exothermic reaction
3.12
explosion characteristic
numerical term, determined in accordance with the test methods specified in this part of ISO/IEC
80079 which characterises the contained explosion of a specified concentration of reactants in
a vessel having a volume of 1m

ISO/IEC CDV 80079-20-1 © IEC 2026

Note 1 to entry: Figure 1 shows the pressure-time curve, expressed in MPa and seconds respectively, of a typical
explosion.
Key
p pressure
p explosion pressure
ex
t time
a maximum rate of pressure rise
Figure 1 – pressure-time curve

3.13
explosion pressure
p
ex
highest pressure occurring in a closed vessel during the explosion of a specific mixture of
flammable substances with air or air and inert gases determined under specified test condition
Note 1 to entry: Figure 1 shows the pressure-time curve for an explosion with p marked
ex
Note 2 to entry: p is expressed as absolute pressure with gases and vapour and as overpressure with dusts.
ex
[SOURCE: EN 15967:2022 [3], 3.1, modified - Note 1 to entry has been modified and Note 2 to
entry has been added]
3.14
maximum explosion pressure
p
max
ISO/IEC CDV 80079-20-1 © IEC 2026

maximum value of the explosion pressure (p ) determined by tests over a wide range of reactant
ex
concentrations
3.15
rate of explosion pressure rise
dp/dt
slope of the pressure-time curve in the straight portion of the pressure rise
Note 1 to entry: shows the pressure-time curve for an explosion with the rate of explosion pressure rise marked

[SOURCE: ISO 6772:2012 [4], 3.1 modified - Note deleted and term changed to "rate of
explosion pressure rise"]
3.16
maximum rate of explosion pressure rise
(dp/dt)
max
maximum value of the rate of explosion pressure rise determined by tests over a wide range of
reactant concentrations
3.17
K
max
normalised characteristic defining the maximum rate of pressure rise with time (dp/dt) of an
max
explosion in a volume V, according to the Formula (1). The violence of an explosion is
evaluated from the value of K
max
𝑑𝑝
1⁄3
(1)
𝐾 = ( ) × 𝑉
𝑚𝑎𝑥
𝑑𝑡
𝑚𝑎𝑥
Note 1 to entry: Under certain circumstances, Formula (1) is not valid for vessels with a length to diameter ratio
greater than 2:1 or with a volume of less than 1m .

Note 2 to entry: This standard restricts itself to quiescent conditions. For further information on turbulent conditions
the reader is referred to the explosion protection guidelines (for example EN 14797 [5] or NFPA 69 [6] ).
4 Classification of gases and vapours
4.1 General
Equipment Group I addresses mines susceptible to firedamp.
NOTE Firedamp consists mainly of methane, but always contains small quantities of other gases, such as nitrogen,
carbon dioxide, and hydrogen, and sometimes ethane and carbon monoxide. The terms firedamp and methane are
used frequently in mining practice as synonyms.
Equipment Group II addresses flammable gases and vapours other than in mines susceptible
to firedamp. Equipment Group II gases and vapours are classified according to their MESG or
MIC ratio into Equipment Groups IIA, IIB, and IIC.
All flammable materials are classified according to their AIT into temperature classes.
ISO/IEC CDV 80079-20-1 © IEC 2026

4.2 Classification according to the MESG
Gases and vapours may be classified according to their MESG into Equipment Groups IIA, IIB,
or IIC, based on the determination method described in this document. In order to ensure
standardized results the MESG apparatus is dimensioned to avoid the possibl e effects of
obstruction on the safe gaps.
NOTE 1 The standard method for determining MESG is described in Clause 6, but where determinations have been
undertaken only in an 8 l spherical vessel with ignition close to the flange gap these can be accepted provisionally.
NOTE 2 It is possible that the design of the test apparatus for safe gap determination, other than that used for
selecting the appropriate equipment group of enclosure for a particular gas, will need to be different to the one
described in this document. For example, the volume of the enclosure, flange width, gas concentrations and the
distance between the flanges and any external wall or obstruction can be varied. As the design depends on the
particular investigation which is to be undertaken, it is impracticable to recommend specific design requirements, but
for most applications the general principles and precautions indicated in this document will still apply.
NOTE 3 In IEC 60079-14 minimum distances of obstruction from the flameproof flange joints related to the
equipment group of the hazardous area are given.
For the purpose of classification the MESG limits are:
– Equipment Group IIA: MESG ≥ 0,90 mm;
– Equipment Group IIB: 0,50 mm < MESG < 0,90 mm;
– Equipment Group IIC: MESG ≤ 0,50 mm.
Determination of both the MESG and MIC ratio is required when 0,50 < MESG < 0,55. Then the
equipment group is determined by MIC ratio.
NOTE 4 For gases and highly volatile liquids, the MESG is determined at 20 °C.
NOTE 5 If it was necessary to do the MESG determination at temperatures higher than ambient temperature, a
temperature 5 K above that needed to give the necessary vapour pressure or 50 K above the flash point is used and
this value of MESG is given in the table and the classification of the equipment group is based on this result.
4.3 Classification according to the minimum igniting current ratio (MIC ratio)
Gases and vapours may be classified according to the ratio of their minimum igniting currents
(MIC) to the ignition current of laboratory methane into Equipment Groups IIA, IIB, or IIC. The
purity of laboratory methane shall be not less than 99,9 % by volume.
NOTE The standard method of determining MIC ratios is with the apparatus described in IEC 60079 -11, but where
determinations have been undertaken in other apparatus these can be accepted provisionally.
For the purpose of classification the MIC ratios are:
Equipment Group IIA: MIC > 0,80;
Equipment Group IIB: 0,45 ≤ MIC ≤ 0,80;
Equipment Group IIC: MIC < 0,45.
Determination of both the MESG and MIC ratio is required when 0,70 < MIC < 0,90 or 0,40 <
MIC < 0,50. Then the equipment group is determined by MESG.
4.4 Classification according to the similarity of chemical structure
When a gas or vapour is a member of a homologous series of compounds, the classification of
the gas or vapour can provisionally be inferred from the data of the neighbouring members of
the series.
ISO/IEC CDV 80079-20-1 © IEC 2026

The classification according to the similarity of chemical structure is not allowed if the
classification of one of the neighbouring members is based on MESG and the other on MIC
ratio.
4.5 Classification of mixtures of gases
Mixtures of gases should generally be allocated to an equipment group only after a special
determination of MESG or MIC ratio. One method to estimate the equipment group is to
calculate the MESG of the mixture by using the Brandes-Redeker [2] modification to Le
Châtelier’s principle which incorporates not just the fraction of fuel required but also the relative
amount of oxygen required for stoichiometric combustion.
𝑀𝐸𝑆𝐺 =
mix
𝑖
𝑋
(2)
𝑂
𝑛 2
∑ ( )
𝑖=1
𝑀𝐸𝑆𝐺
𝑖
Where:
MESG is the MESG of component
i
and
𝑦 𝑆
1 𝑖
(𝑖)
𝑋 =
𝑂 𝑛
(3)
∑ ( )
𝑦 𝑆
1 𝑖
𝑖=1
where
Y is the mole fraction of component i in the fuel mixture, and
S is the oxygen to fuel ratio for stoichiometric combustion of component i in the fuel mixture
i
This method should not be applied to mixtures or streams that have
a) experimental data already available;
b) carbon monoxide or carbon disulfide;
c) acetylene or its equivalent hazard (for example self-decomposition properties).
For mixtures that have been diluted with nitrogen or have enhanced oxygen content consider
the method of Rodgers [1] where
𝑀𝐸𝑆𝐺
𝑓𝑢𝑒𝑙−𝑖𝑛−𝑎𝑖𝑟
(4)
𝑀𝐸𝑆𝐺 =
𝑑𝑖𝑙𝑢𝑡𝑒𝑑
1,938
𝐹𝑢𝑒𝑙 𝑟𝑎𝑡𝑖𝑜
Diluted refers to non-standard ratios of nitrogen to oxygen, which means nitrogen diluted, but
also refers to enhanced oxygen mixes. The MESG for the fuel in air can be from a measurement
or calculation such as by the Brandes-Redeker method. ·
𝑦 + 4,76 · 𝑆 · 𝑦
𝐹 𝐹
𝐹𝑢𝑒𝑙 𝑟𝑎𝑡𝑖𝑜 =
(5)
𝑦 + 4,76 · 𝑆 · 𝑦 + 𝑦 − 3,76 · 𝑦
𝐹 𝑁 𝑂
2 2
ISO/IEC CDV 80079-20-1 © IEC 2026

Where
y is the mole fraction of flammable components in the stream or mixture
F
𝑦 is the mole fraction of nitrogen in the stream or mixture
𝑁
𝑦 is the mole fraction of oxygen in the stream or mixture
𝑂
𝑛
∑
𝑆 = (𝑦 · 𝑆 ) which is the oxygen to fuel ratio for stoichiometric combustion of the
𝑖=1 𝑖 𝑖
flammable fraction of the stream or mixture and the other terms are defined above.
– For mixtures containing less than 5% oxidizer by volume in total, use a MESG of infinity.
– For mixtures containing a fuel in concentrations of 5% or less by volume, use a MESG of 2
mm.
– For mixtures containing an inert gas such as nitrogen resulting in concentrations of oxygen
of less than 5% by volume use an MESG of infinity.
NOTE 1 An alternate method that includes sto
...