AS NZS 80079.20.1:2019 pdf download.Explosive atmospheres Part 20.1: Material characteristics for gas and vapour classification – Test methods and data (ISO/IEC 80079-20-1:2017, (ED.1.0)/COR1:2018, MOD).
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 and/or MIC ratio into equipment groups IIA, IIB and IIC.
All flammable materials are classified according to their AlT into temperature classes.
4.2 Classification according to the maximum experimental safe gap (MESG)
Gases and vapours may be classified according to their MESG into Equipment Groups IIA, IIB or lIC, based on the determination method described in this document. In order to ensure standardized results the MESG apparatus is dimensioned to avoid the possible effects of obstruction on the safe gaps.
NOTE 1 The standard method for determining MESG is described in 6.2, but where determinations have been undertaken only in an 8 I spherical vessel with ignition close to the flange gap these can be accepted provisionally.
NOTE 2 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, may 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 may have to 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 llA: MESG ≥ 0,90 mm.
Equipment Group lIB: 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, lIB or lIC. The purity of laboratory methane shall be not less than 99,9 % by volume.
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.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.
5 Data for flammable gases and vapours, relating to the use of equipment Determination of the properties
5.1.1 General
The compounds listed in this document are in accordance with Clause 4, or have physical
properties similar to those of other compounds in that list.
5.1.2 Equipment group
The equipment groups are the result of MESG or MIC ratio determination except where there is no value listed for MESG or MIC ratio. For these, the equipment group is based on chemical similarity (see Clause 4).
NOTE 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. This value of MESG is given in Table B.1 and the classification of the equipment group is based on this result.
5.1.3 Flammable limits
Determinations have been made by a number of different methods, but the preferred method is with a low energy ignition at the bottom of a vertical tube. The values (in percentage by volume and mass per volume) are listed in Table B.1.AS NZS 80079.20.1 pdf download.

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