31J/234/CDV
COMMITTEE DRAFT FOR VOTE (CDV) PROJET DE COMITÉ POUR VOTE (CDV)  
Project number IEC 60079-10-1/Ed2  Numéro de projet 
IEC/TC or SC: SC 31J  
CEI/CE ou SC: 
Date of circulation Date de diffusion 2014-07-11
Closing date for voting (Voting mandatory for P-members) Date de clôture du vote (Vote obligatoire pour les membres (P)) 2014-10-17
Also of interest to the following committees Intéresse également les comités suivants  TC 18
Supersedes document Remplace le document   31J/221/CD and 31J/231/CC
Proposed horizontal standard Norme horizontale suggérée
Other TC/SCs are requested to indicate their interest, if any, in this CDV to the TC/SC secretary Les autres CE/SC sont requis d’indiquer leur intérêt, si nécessaire, dans ce CDV à l’intention du secrétaire du CE/SC
Functions concerned Fonctions concernées
MODIFICATION. IL NE PEUT SERVIR DE RÉFÉRENCE.
LES RÉCIPIENDAIRES DU PRÉSENT DOCUMENT SONT INVITÉS À
PRÉSENTER, AVEC LEURS OBSERVATIONS, LA NOTIFICATION DES
DROITS DE PROPRIÉTÉ DONT ILS AURAIENT ÉVENTUELLEMENT
CONNAISSANCE ET À FOURNIR UNE DOCUMENTATION EXPLICATIVE.
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.
Introductory note 
 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) for an International Standard is submitted for parallel voting. The CENELEC members are invited to vote through the CENELEC online voting system.
®
 
4 General 138
5.1 General 1514
5.4 Simplified methods 1617
6 Release of flammable substance 1719
6.1 General 1720
6.3.1 Gaseous release 1823
6.3.4 Aerosols 1926
6.3.5 Vapours 2027
6.5 Main types of ventilation 2130
6.5.1 Natural Ventilation 2131
6.5.2 Artificial Ventilation 2232
7 Type of zone 2334
7.1 Influence of grade of the source of release 2435
7.2 Influence of dilution 2436
7.3 Influence of availability of ventilation 2437
8 Extent of zone 2438
9 Documentation 2539
9.1 General 2540
 Annex A (informative) Suggested presentation of hazardous areas 2742
 
 Annex B (informative) Estimation of sources of release 3244
B.1 Symbols 32 45
B.2.1 Sources giving a continuous grade of release 32 47
B.2.2 Sources giving a primary grade of release 33 48
B.2.3 Sources giving a secondary grade of release 33 49
B.3 Assessment of grades of release 33 50
B.4 Summation of releases 33 51
B.5 Hole size and source radius 34 52
B.6 Forms of release 37 53
B.7 Release rate 38 54
B.7.1 Estimation of Release Rate 38 55
B.7.2 Release rate of evaporative pools 41 56
B.8 Release from openings in buildings 42 57
B.8.1 Openings as possible sources of release 42 58
B.8.2 Openings classification 43 59
 Annex C (informative) Ventilation guidance 4560
C.1 Symbols 45 61
C.2 Introduction 46 62
C.3 Assessment of ventilation and dilution and its influence on hazardous area 46 63
C.3.1 General 46 64
C.3.4 Assessment of ventilation velocity 48 67
C.3.5 Assessment of the degree of dilution 49 68
C.3.6 Dilution in a room 51 69
C.3.7 Criteria for availability of ventilation 52 70
C.4 Examples of ventilation arrangements and assessments 53 71
C.4.1 Introduction 53 72
C.4.2 Jet release in a large building 53 73
C.4.3 Jet release in a small naturally ventilated building 54 74
C.4.4 Jet release in a small artificially ventilated building 55 75
C.4.5 Release with low velocity 56 76
C.4.6 Fugitive emissions 56 77
C.4.7 Local ventilation-extraction 56 78
C.5 Natural Ventilation in buildings 57 79
C.5.1 Wind induced ventilation 57 80
C.5.2 Buoyancy induced ventilation 58 81
C.5.3 Combination of the natural ventilation induced by wind and82
buoyancy 59 83
D.1 General 61 85
 
D.3 Estimating the extent of the hazardous zone 62 87
 Annex E (Informative) Examples of hazardous area classification 6488
E.1 General 64 89
E.2 Examples 64 90
 Annex F (informative) Schematic approach to classif ication of hazardous areas - Sect ion91
F.2 8892
 Annex F (informative) Schematic approach to classification of hazardous areas – Section F.3 8993
 Annex F (informative) Schematic approach to classification of hazardous areas – Section F.4 9094
 Annex G (informative) Flammable mists 9195
 Annex H (informative) Hydrogen 9396
 Annex I (informative) Hybrid mixtures 9597
I.1 General 9598
 Annex J (informative) Useful equations in support to hazardous area classification 96101
 Annex K (informative) Industry codes and national standards 98102
K.1 General 98 103
Figure A.2 – Gas/vapour at low pressure 30107
Figure A.3 – Gas/vapour at high pressure 30108
Figure A.4.1 – Gas or vapour (liquefied under pressure or by refrigeration) 31 109
Figure A.4.2 – Gas or vapour (liquefied under pressure or by refrigeration) with spillage 31110
Figure A.5 – Flammable liquid (non boiling evaporative pool) 31111
Figure B.1 – Forms of rekease 37112
Figure B.2 – Volumetric evaporation rate of liquids 42113
Figure C.1 – Chart for determining the degree of dilution 50114
Figure C.2 – Self diffusion of an unimpeded high velocity jet release 54115
Figure C.3 – Supply only ventilation 55116
Figure C.4 – Supply and extraction ventilation 55117
Figure C.5 – Local extraction ventilation 57118
Figure C.6 – Volumetric flow rate of fresh air per m 2  of equivalent effective opening area 59119
Figure C.7 – Example of opposing ventilation driving forces 60120
Figure D.1 – Chart for estimating hazardous area distances 62121
Figure E.1 – Compressor facility handling natural gas 78122
Figure E.2 – Example of area classification for a compressor facility handling natural gas 85123
Figure E.2a – Example of area classification for a compressor facility handling natural gas 86124
125
Table A.1  Hazardous area classification data sheet – Part I: Flammable substance list and126
characteristics 28127
Table A.2  Hazardous area classification data sheet  Part II: List of sources of release 29128
Table B.1 – Suggested hole cross sections for secondary grade of releases 35129
Table B.2 – Effect of hazardous zones on openings as possible sources of release 44130
Table C.1 – Indicative outdoor ventilation velocities (uw) 49131
Table D.1 – Zones for grade of release and effectiveness of ventilation 61132
Table K.1 – Examples of codes and standards 98133
134
135
143
144
FOREWORD145
international co-operation on all questions concerning standardization in the electrical and electronic fields. To148
this end and in addition to other activities, IEC publishes International Standards, Technical Specifications,149
Technical Reports, Publicly Available Specifications (PAS) and Guides (hereafter referred to as “IEC150
Publication(s)”). Their preparation is entrusted to technical committees; any IEC National Committee interested151
in the subject dealt with may participate in this preparatory work. International, governmental and non -152
governmental organizations liaising with the IEC also participate in this preparation. IEC collaborates closely153
with the International Organization for Standardization (ISO) in accordance with conditions determined by154
agreement between the two organizations.155
2) The formal decisions or agreements of IEC on technical matters express, as nearly as possible, an international156
consensus of opinion on the relevant subjects since each technical committee has representation from all157 interested IEC National Committees.158
3) IEC Publications have the form of recommendations for international use and are accepted by IEC National159 Committees in that sense. While all reasonable efforts are made to ensure that the technical content of IEC160
Publications is accurate, IEC cannot be held responsible for the way in which they are used or for any161 misinterpretation by any end user.162
4) In order to promote international uniformity, IEC National Committees undertake to apply IEC Publications163
transparently to the maximum extent possible in their national and regional publications. Any divergence164
between any IEC Publication and the corresponding national or regional publication shall be clearly indicated in165
the latter.166
5) IEC itself does not provide any attestation of conformity. Independent certification bodies provide conformity167
assessment services and, in some areas, access to IEC marks of conformity. IEC is not responsible for any168 services carried out by independent certification bodies.169
6) All users should ensure that they have the latest edition of this publication.170
7) No liability shall attach to IEC or its directors, employees, servants or agents including individual experts and171
members of its technical committees and IEC National Committees for any personal injury, property damage or172
other damage of any nature whatsoever, whether direct or indirect, or for costs (including legal fees) and173
expenses arising out of the publication, use of, or reliance upon, this IEC Publication or any other IEC174
Publications.175
8) Attention is drawn to the Normative references cited in this publication. Use of the referenced publications is176
indispensable for the correct application of this publication.177
9) Attention is drawn to the possibility that some of the elements of this IEC Publication may be the subject of178
patent rights. IEC shall not be held responsible for identifying any or a ll such patent rights.179
International Standard IEC 60079-10-1 has been prepared by subcommittee 31J:180
Classification of hazardous areas and installation requirements, of IEC technical committee181
31: Equipment for explosive atmospheres.182
This second edition of IEC 60079-10-1 cancels and replaces the first edition, published in183
2009, and constitutes a technical revision.184
The significant technical changes with respect to the previous edition are as follows:185
a) Restructuring of main body of the text and dividing it into sections to identify possible186
methodologies for classifying hazardous areas and to provide further explanation on187
specific assessment factors;188
b) Introducing new terms and the definitions in section 3;189
c) Introducing clauses for alternative methods of area classification in section 5;190
d) Restructuring of the Annexes including:191
  Updating examples for presentation of hazardous area classification in Annex A;192
  Updating calculations for release rate in Annex B;193
 
60079-10-1/Ed2/CDV © IEC:2014 7
  Complete re-write of Annex C for ventilation assessment, with a new approach based194
upon the degree of dilution instead of the degree of ventilation;195
  Introduction of Annex D for zone extents:196
  Update of Annex E with the examples to explain the methodology set forth in Annexes197
 A, B, C and D;198
  Update of the flow chart in Annex F illustrating the area classification procedure by199
dividing it in four (4) sections;200
  Introduction of Annex H for hydrogen;201
  Introduction of Annex I for hybrid mixtures;202
  Introduction of Annex J for supplementary equations;203
  Introduction of Annex K for reference to national and industry codes with specific204
examples of hazardous area classification;205
The text of this standard is based on the following documents:206
FDIS Report on voting
207
Full information on the voting for the approval of this standard can be found in the report on208
voting indicated in the above table.209
This publication has been drafted in accordance with the ISO/IEC Directives, Part 2.210
 A lis t of all parts of the IEC 60079 series, under the general tit le Explosive atmospheres, can211
be found on the IEC website.212
The committee has decided that the contents of this publication will remain unchanged until213
the stability date indicated on the IEC web site under "http://webstore.iec.ch" in the data214
related to the specific publication. At this date, the publication will be215
reconfirmed,216
withdrawn,217
amended.219
220
The National Committees are requested to note that for this publication the stability date221
is 2020222
THIS TEXT IS INCLUDED FOR THE INFORMATION OF THE NATIONAL COMMITTEES AND WILL BE223
DELETED AT THE PUBLICATION STAGE.224
225
INTRODUCTION226
In areas where dangerous quantities and concentrations of flammable gas or vapour may227
arise, protective measures are to be applied in order to reduce the risk of explosions. This228
part of IEC 60079 sets out the essential criteria against which the ignition hazards can be229
assessed, and gives guidance on the design and control parameters which can be used in230
order to reduce such hazards.231
232
233
1 Scope236
This part of IEC 60079 is concerned with the classification of areas where flammable gas or237
vapour hazards may arise and may then be used as a basis to support the proper selection238
and installation of equipment for use in hazardous areas.239
It is intended to be applied where there may be an ignition hazard due to the presence of240
flammable gas or vapour, mixed with air, but it does not apply to:241
a) mines susceptible to firedamp;242
b) the processing and manufacture of explosives;243
c) areas where a hazard may arise due to the presence of combustible dusts or fibres (refer244
IEC 60079-10-2;245
d) catastrophic failures or rare malfunctions which are beyond the concept of abnormality246
dealt with in this standard (see 3.7.3 and 3.7.4);247
e) rooms used for medical purposes;248
f) commercial and industrial applications where only low pressure fuel gas is used for249
appliances e.g. for cooking, water heating and similar uses, where the installation is250
compliant with relevant gas codes;251
g) domestic premises.252
This standard does not take into account the consequences of ignition of an explosive253
atmosphere.254
Flammable mists may form or be present at the same time as flammable vapours. Liquids not255
considered to be hazardous in terms of this standard (due to the flash point), when released256
under pressure may also generate flammable mists. In such cases, the strict application of257
area classification for gases and vapours may not be appropriate as the basis for selection of258
equipment. Information on flammable mists is provided in Annex G.259
For the purpose of this standard, an area is a three-dimensional region or space.260
 Atmospheric conditions inc lude var iations above and below reference levels of 101,3 kPa261
(1 013 mbar) and 20 °C (293 K), provided that the variations have a negligible effect on the262
explosion properties of the flammable substances.263
In any process plant, irrespective of size, there may be numerous sources of ignition apart264
from those associated with equipment. Appropriate precautions will be necessary to ensure265
safety in this context. This standard may be used with judgement for other ignition sources.266
2 Normative references267
The following referenced documents are indispensable for the application of this document.268
For dated references, only the edition cited applies. For undated references, the latest edit ion269
of the referenced document (including any amendments) applies.270
IEC 60050-426,  International Electrotechnical Vocabulary (IEV)  –  Part 426: Equipment for271
explosive atmospheres272
vapour classification  – Test methods and data275
IEC 60079-10-2, Explosive atmospheres  –  Part 10-2; Classification of areas  –  Combustible276
dust atmospheres277
erection279
3 Terms and definitions280
For the purposes of this document, the terms and definitions given in IEC 60079-0 and the281
following apply.282
NOTE 1 to entry; Additional definitions applicable to explosive atmospheres can be found in the IEC 60050-426.283
3.1 Explosive atmosphere284
Mixture with air, under atmospheric conditions, of flammable substances in the form of gas,285
vapour, dust, fibres, or flyings, which, after ignition, permits self-sustaining flame propagation.286
IEC 60079-0 287
3.2 Explosive gas atmosphere288
Mixture with air, under atmospheric conditions, of flammable substances in the form of gas or289
vapour, which, after ignition, permits self-sustaining flame propagation.290
IEC 60079-0 291
NOTE 1 to entry;  
 Althou gh a mixture which has a concent ration above the upper flammable lim it (UEL) is not an292
explosive gas atmosphere, it can readily become so and, generally for area classification purposes, it is advisable293
to consider it as an explosive gas atmosphere.294
NOTE 2 to entry; There are some gases which are explosive with the concentration of 100% (e.g. acetylene, C 2H2;295
vinyl acetylene, C4H4; propyl nitrate vapour, CH3 (CH2)2 NO3; iso-propyl nitrate vapour, (CH3)2 CH ONO2; ethylene296 oxide vapour, (CH2)2 O; hydrazine, N2H4 vapour).297
3.3 Hazardous areas and zones298
3.3.1 Hazardous area (on account of explosive gas atmospheres)299
 An area in which an explosive gas atmosphere is or may be expected to be present, in300
quantities such as to require special precautions for the construction, installation and use of301
equipment.302
NOTE 1 to entry; The interior of many items of process equipment are commonly considered as a hazardous area303
even though a flammable atmosphere may not normally be present to account for the possibility of air entering the304 equipment. Where specific controls such as inerting are used the interior of process equipment may not need to be305
classified as a hazardous area.306
3.3.2 Non-hazardous area (on account of explosive gas atmospheres)307
 An area in which an explosive gas atmosphere is not expected to be present in quantities308
such as to require special precautions for the construction, installation and use of equipment.309
3.3.3 Zones310
Hazardous Area classification based upon the frequency of the occurrence and duration of an311
explosive atmosphere.312
3.3.4 Zone 0313
 An area in which an explosive gas atmosphere is present continuously or for long periods or314
frequently.315
NOTE 1 to entry; Both “long” and “frequently are the terms which are intended to describe a very high likelihood of316
a potentially explosive atmosphere to be present in the area. In that respect, those terms do not necessarily need317
to be quantified.318
3.3.5 Zone 1319
 An area in which an explosive gas atmosphere is likely to occur periodica lly or occasionally in320
normal operation.321
3.3.6 Zone 2322
 An area in which an explosive gas atmosphere is not likely to occur in normal operation but, if323
it does occur, it will exist for a short period only.324
IEC 60050-426 325
60079-10-1/Ed2/CDV © IEC:2014 11
NOTE 1 to entry; Indications of the frequency of the occurrence and duration may be taken from codes relating to326
specific industries or applications.327
3.3.7 Extent of zone328
Distance in any direction from the source of release where a gas/air mixture has been diluted329
by air to a concentration below the lower flammable limit.330
3.4 Releases331
3.4.1 Source of release332
 A point or location from which a gas, vapour, mist or liquid may be released into the333
atmosphere so that an explosive gas atmosphere could be formed.334
IEC 60050-426 335
3.4.2 Continuous grade of release336
Release which is continuous or is expected to occur f requently or  for long periods.337
NOTE 1 to entry; Both “long” and “frequently are the terms which are intended to describe a very high likelihood of338
a potential release . In that respect, those terms do not necessarily need to be quantified.339
3.4.3 Primary grade of release340
Release which can be expected to occur periodically or occasionally during normal operation.341
3.4.4 Secondary grade of release342
Release which is not expected to occur in normal operation and, if it does occur, is likely to do343
so only infrequently and for short periods.344
3.4.5 Release rate345
Quantity of flammable gas, vapour or mist emitted per unit time from the source of release.346
3.5 Ventilation and dilution347
3.5.1 Ventilation348
Movement of air and its replacement with fresh air due to the effects of wind, temperature349
gradients, or artificial means (for example, fans or extractors).350
3.5.2 Dilution351
The mixing of flammable vapour or gas with air which, over time, will reduce the flammable352
concentration. 353
3.5.3 Dilution volume354
The volume in the vicinity of a source of release where the concentration of flammable gas or355
vapour is not diluted to a safe level.356
NOTE to entry; In certain instances, the volumes under 3.5.3 and 3.5.5 could be the same.357
3.5.4 Background concentration358
The mean concentration of flammable substance within the volume under consideration359
outside of the release plume or jet.360
3.5.5 Volume under consideration361
The volume served by the actual ventilation in the vicinity of the release being considered.362
NOTE to entry; For an enclosed space this could be an entire room or part of a larger space where the considered363
ventilation will dilute the gas or vapour from a given source of release. Outdoors, this is the volume around a364
source of release where an explosive mixture could form. In congested outdoor places this volume could be365
 
3.6.1 Flammable substance368
Substance which is itself flammable, or is capable of producing a flammable gas, vapour or369
mist.370
3.6.2 Flammable liquid371
Liquid capable of producing a flammable vapour under any foreseeable operating conditions.372
NOTE 1 to entry; An example of a foreseeable operating condition is one in which the flammable liquid is handled373
at temperatures close to or above its flash point.374
NOTE 2 to entry; This definition is used for the classification of hazardous areas and may be different from the375
definition of flammable liquids used for other purposes e.g. codes for classification of flammable liquids for376
transport.377
3.6.3 Liquefied flammable gas378
Flammable substance which is stored or handled as a liquid and which at ambient379
temperature and atmospheric pressure is a flammable gas.380
3.6.4 Flammable gas or vapour381
Gas or vapour which, when mixed with air in certain proportions, will form an explosive gas382
atmosphere.383
3.6.5 Flammable mist384
Droplets of liquid, dispersed in air so as to form an explosive atmosphere.385
3.6.6 Relative density of a gas or a vapour386
Density of a gas or a vapour relative to the density of air at the same pressure and387
temperature (air is equal to 1,0).388
3.6.7 Flashpoint389
Lowest liquid temperature at which, under certain standardized conditions, a liquid gives off390
vapours in a quantity such as to be capable of forming an ignitable vapour/air mixture.391
3.6.8 Boiling point392
Temperature of a liquid boiling at an ambient pressure of 101,3 kPa (1 013 mbar).393
NOTE 1 to entry; The initial boiling point that should be used for liquid mixtures is to indicate the lowest value of394
the boiling point for the range of liquids present, as determined in a standard laboratory distillation without395 fractionation.396
3.6.9 Vapour pressure397
Pressure exerted when a solid or liquid is in equilibrium with its own vapour398
NOTE 1 to entry; This is also, the partial pressure of the substance in the atmosphere above the liquid.  It is a399
function of the substance and of the temperature.400
3.6.10 Ignition temperature of an explosive gas atmosphere401
Lowest temperature of a heated surface which, under specified conditions (according to IEC402
80079-20-1), will ignite a flammable substance in the form of a gas or vapour mixture with air.403
IEC 60079-0]404
3.6.11 Lower flammable limit (LFL)405
The concentration of flammable gas, vapour or mist in air below which an explosive gas406
atmosphere will not be formed.407
IEC 60050-426 408
3.6.12 Upper flammable limit (UFL)409
The concentration of flammable gas, vapour or mist in air above which an explosive gas410
atmosphere will not be formed.411
IEC 60050-426 412
3.7.1 Normal operation414
Situation when the equipment is operating within its designed parameters .415
NOTE 1 to entry; Minor releases of flammable substance may be part of normal operation. For example, releases416 from seals which rely on wetting by the fluid being pumped are considered to be minor relea ses.417
NOTE 2 to entry; Failures (such as the breakdown of pump seals, flange gaskets or spillages caused by accidents418
which involve repair or shut-down are not considered to be part of normal operation.419
NOTE 3 to entry; Normal operation includes start-up and shut-down conditions and routine maintenance.420
3.7.2 Routine maintenance421
 Act ion recommended by the manufacturer to be performed occas ionally or periodically in422
normal operation to maintain proper performance of equipment.423
3.7.3 Rare malfunction424
Type of malfunction which may happen only in rare instances.  425
NOTE 1 to entry; Rare malfunctions in the context of this standard include failure of separate and independent426
process controls, that may be either automated or manual, that could trigger a chain of events that would lead to427 major release of flammable substance. `428
NOTE 2 to entry; Rare malfunctions could also include unanticipated conditions that are not covered by the plant429
design such as unexpected corrosion that results in a release. Where releases due to corrosio n or similar430
conditions may or could reasonably be expected as part of the plant operations then this is not considered as a431
rare malfunction.432
3.7.4 Catastrophic failure433
 An occurrence which exceeds the design parameters of the process plant and control system434
resulting in a major release of flammable substance.  435
NOTE 1 to entry; Catastrophic failures in the context of this standard include, for example, major accidents such as436
the rupture of a process vessel, or large scale failures of equipment or piping such as total breakdown of a flange437
or seal.438
4 General439
4.1 Safety principles440
Installations in which flammable substances are handled or stored should be designed,441
constructed, operated and maintained so that any releases of flammable substance, and442
consequently the extent of hazardous areas, are kept to a minimum, whether in normal443
operation or otherwise, with regard to frequency, duration and quantity of a release.444
It is important to examine those parts of process equipment and systems from which a release445
of flammable substance may arise and to consider modifying the design to minimize the446
likelihood and frequency of such releases and the quantity and rate of release of substance.447
These fundamental considerations should be examined at an early stage of the design448
development of any process plant and should also receive prime attention in carrying out the449
area classification study.450
In the case of activities other than those of normal operation, e.g. commissioning or non-451
routine maintenance, the area classification may not be valid. It is expected that this would be452
dealt with by a safe system of work.The area classification should take into account any453
routine maintenance.454
60079-10-1/Ed2/CDV © IEC:2014 14
In a situation in which there may be an explosive gas atmosphere, the following steps should455
be taken:456
a) eliminate the likelihood of an explosive gas atmosphere occurring around the source of457
ignition, or458
b) eliminate the source of ignition.459
Where this is not possible, protective measures, process equipment, systems and procedures460
should be selected and prepared so the likelihood of the coincidence of a) and b) is so small461
as to be accepted as low as reasonably practicable. Such measures may be used singularly,462
if they are recognized as being highly reliable or in combination to achieve the required level463
of safety.464
4.2 Area classification objectives465
 Area classification is a method of ana lys ing and class ifying the environment where explosive466
gas atmospheres may occur, so as to facilitate the proper selection and installation of467
equipment to be used safely in that environment. The classification also takes into account468
the ignition characteristics of the gas or vapour such as ignition energy and ignition469
temperature. Area classification has two components, the determination of the type of any470
hazardous zone, and the extent of the zone (see 7, 8 and 9).471
NOTE Selected characteristics may be designated for equipment e.g. ignition energy and temperature ratings,472
see IEC 80079-20-1473
In most practical situations where flammable substances are used, it is difficult to ensure that474
an explosive gas atmosphere will never occur. It may also be difficult to ensure that475
equipment will never give rise to a source of ignition. Therefore, in situations where an476
explosive gas atmosphere has a high likelihood of occurring, reliance is placed on using477
equipment which has a low likelihood of creating a source of ignition. Conversely, where the478
likelihood of an explosive gas atmosphere occurring is reduced, equipment constructed with479
less rigorous requirements may be used.480
In particular, zone 0 or zone 1 areas should be minimized in number and extent by design or481
suitable operating procedures. In other words, plants and installations should be mainly482
zone 2 or non-hazardous. Where release of flammable substance is unavoidable, process483
equipment items should be limited to those which give secondary grade releases or, failing484
this (that is where primary or continuous grade releases are unavoidable), the releases should485
be of very limited quantity and rate. In carrying out area classification, these principles should486
receive prime consideration. Where necessary, the design, operation and location of process487
equipment should ensure that, even when it is operating abnormally, the amount of flammable488
substance released into the atmosphere is minimized, so as to reduce the extent of the489
hazardous area.490
Once a plant has been classified and all necessary records made, it is important that no491
modification to equipment or operating procedures is made without reference to those492
responsible for the area classification. A description should be added to update the493
classification for any plant or operational changes. Reviews should be carried out during the494
life of the plant.495
4.3 Explosion risk assessment496
Subsequent to the completion of the area classification, a risk assessment may be carried out497
to assess whether the consequences of ignition of an explosive atmosphere requires the use498
of equipment of a higher equipment protection level (EPL) or may justify the use of equipment499
with a lower equipment protection level than normally required.500
In some cases a zone of negligible extent (NE) may arise and may be treated as non501
hazardous. Such a zone implies that an explosion, if it takes place, will have negligible502
consequences. Experiments have shown that a flammable cloud that is less than 0,1 m 3   or503
1,0% of the enclosed space concerned (whichever is smaller) could be considered as zone504
NE. The zone NE concept can be applied irrespective of any other adjustments for risk505
assessment to determine EPL.506
60079-10-1/Ed2/CDV © IEC:2014 15
The EPL requirements may be recorded, as appropriate, on the area classification documents507
and drawings to allow proper selection of equipment.508
NOTE IEC 60079-0 describes EPLs and IEC 60079-14 defines the application of EPLs to an installation .509
4.4 Competence of Personnel510
The area classification should be carried out by those who understand the relevance and511
significance of the properties of the flammable materials, principles of gas/vapour dispersion512
and those who are familiar with the process and the equipment. It may be beneficial for other513
engineering disciplines, e.g. electrical and mechanical engineers, and personnel with specific514
responsibility for safety to be part of and have an input to the area classification process. The515
competency of the person shall be relevant to the nature of the plant and methodology used516
for carrying out the area classification. Appropriate continuing education or training should be517
undertaken by personnel on a regular basis where required.518
NOTE Competency can be demonstrated in accordance with a training and assessment framework relevant to519
national regulations or standards or user requirements.520
5 Area classification methodology521
It is rarely possible by a simple examination of a plant or plant design to decide which parts of522
the plant can be equated to the three zonal definitions (zones 0, 1 and 2). A more detailed523
approach is therefore necessary and this involves the analysis of the basic possibility of an524
explosive gas atmosphere occurring.525
In determining where a release of flammable gas or vapour may occur, the likelihood and526
duration of the release should be assessed in accordance with the definitions of continuous,527
primary and secondary grades of release. Once the grade of release, the release rate,528
concentration, velocity, ventilation and other factors are assessed there is then a firm basis529
on which to assess the likely presence of an explosive gas atmosphere in the surrounding530
areas and determine the type and/or extent of the hazardous zones.531
This approach therefore requires detailed consideration to be given to each item of process532
equipment which contains a substance flammable by itself or due to process conditions, and533
which could therefore be a source of release.534
For more details, see section 6, 7 and 8.535
5.1 General536
The following sub clauses give guidance on options for classifying areas in which there may537
be an explosive gas atmosphere. An example of a schematic approach to the classification of538
hazardous areas is given in Annex F.539
The area classification should be carried out when the initial process and instrumentation line540
diagrams and initial layout plans are available, and should be confirmed before plant start-up.541
Consideration should always be given to the type, number and location of various potential542
points of release so that relevant zone and boundary conditions are assigned in the overall543
assessment. Control systems designed and installed to a Functional Safety standard may544
reduce the potential for a source of release and/or the quantity of a release (e.g. batch545
sequence controls, inerting systems). Such controls may therefore be considered where546
relevant to the hazardous area classification.547
When classifying buildings, consideration should also be given to a careful evaluation of prior548
experience with the same or similar installations. It is not enough to identify only a potential549
source of the combustible or flammable material within the building and proceed immediately550
to defining the extent of Zone 1 or Zone 2 classified areas. Where experience indicates that a551
particular design concept is sound, a more hazardous classification for similar installations552
 
60079-10-1/Ed2/CDV © IEC:2014 16
When classifying hazardous areas, consideration should also be given to a careful evaluation555
of prior experience with the same or similar installations. Where documented evidence556
indicates that a particular design concept is sound this experience may be used to support the557
classification chosen.558
5.2 Classification by calculation of sources of releases559
Classification may be approached by calculation, considering appropriate statistical and560
numerical assessments for the factors concerned, for each source of release.561
Formulae relevant to determining the release rates under specified conditions can be found in562
 Annex B. These formulae are generally accepted as providing a good basis for calculating563
release rates for the conditions provided.564
Guidance on the assessment of ventilation and dispersion is provided in Annex C. This565
guidance is not intended to be universally applicable and may not be reliable in some566
situations.567
Other forms of assessment, e.g. computational fluid dynamics (CFD), may be used and may568
provide a good basis for assessment in some situations. Computer modelling is also an569
appropriate tool when assessing the interaction of multiple factors.570
In all cases the assessment method and tools used should be validated as suitable or used571
with appropriate caution. Those carrying out the assessment should also understand the572
limitations or requirements of any tools and adjust the input conditions or results accordingly573
to ensure appropriate conclusions.574
5.3 Use of industry codes and national standards575
Industry codes and national standards may be used where they provide guidance or examples576
appropriate to the application and comply with the general principles of this standard.577
 Annex K identif ies some relevant industry codes and national standards that may provide578
further detail as well as examples.579
5.4 Simplified methods580
Where it is not practicable to make required assessments from individual sources of release,581
a simplified method may be used.582
Simplified methods shall identify sources for each of the zone types, zone 0, 1 and 2 that are583
suitably conservative to allow for potential sources of release without individual detail. The584
 judgement is best made by reference to a set of cri teria based on indus try ex per ience and585
appropriate to the particular plant.586
It is not necessary to carry out a detailed assessment of all items in a plant where an587
assessment for one item or condition would be adequate to provide a conservative588
classification for all other similar items or conditions on the plant.589
Larger zone areas are characteristic of simplified methods, stemming from the approach and590
the necessity to apply more conservative zonal classification where doubt exists as to the591
hazards involved. This approach shall err on the side of safety.592
To arrive at less conservative or more accurate figures of the boundaries of the classified593
area, reference to illustrative examples or more detailed assessment of point sources of594
release, as applicable should be used.595
5.5 Combination of methods596
The use of different methods may be appropriate for classification of a plant at various stages597
of its development or for various parts of the plant.598
For example, at the initial conceptual stage of a plant the simplified method may be599
appropriate to set out the equipment separations, plant layout and plant boundaries. This600
 
60079-10-1/Ed2/CDV © IEC:2014 17
release. As the plant design proceeds and detailed data is available on the potent ial sources602
of release, the classification should be upgraded using method more detailed assessment.603
In some cases the simplified method can be applied to a group of similar equipment in604
sections of plant (e.g. sections of piping with flanges, such as pipe racks) while applying a605
more detailed assessment to the more significant potential sources of release (e.g. relief606
valves, vents, gas compressors, pumps and the like).607
In many cases the classification examples provided in relevant national or industry cod es can,608
where appropriate, be used to classify some components of larger plants.609
6 Release of flammable substance610
6.1 General611
The release rate of flammable substance is the most important factor that affects the extent of612
a zone. The higher the release rate the larger the extent of a zone.613
For a given release volume, the lower the  LFL the greater will be the extent of the zone.614
NOTE Experience has shown that a release of ammonia, with a  LFL  of 15 % by volume, will often dissipate rapidly615
in the open air, so an explosive gas atmosphere will, in most cases be of negligible extent.616
 An int roduction to the nature of releases that should be considered when approaching617
classification of potentially explosive areas is provided in the following clauses.618
6.2 Sources of release619
The basic elements for establishing the hazardous zone types are the identification of the620
source of release and the determination of the grade or grades of the release.621
Since an explosive gas atmosphere can exist only if a flammable gas or vapour is present622
with air, it is necessary to decide if any flammable substances can exist in the area623
concerned. Generally speaking, such gases and vapours (and flammable liquids or solids624
which may give rise to them) are contained within process equipment that may or may not be625
totally enclosed. It is necessary to identify where a flammable atmosphere can exist inside626
process equipment, or where a release of flammable substances can create a flammable627
atmosphere outside process equipment.628
Each item of process equipment (for example, tank, pump, pipeline, vessel, etc.) should be629
considered as a potential source of release of flammable substance. If the item cannot630
foreseeably contain flammable substance, it will clearly not give rise to a hazardous area631
around it. The same will apply if the item contains a flammable substance but cannot release632
it into the atmosphere (for example, a full y welded pipeline is not considered to be a source of633
release).634
If it is established that the item may release flammable substance into the atmosphere, it is635
necessary, first of all, to determine the grade or grades of release in accordance with the636
definitions, by establishing the likely frequency and duration of the release. It should be637
recognized that the opening-up of parts of enclosed process systems (for example, during638
filter changing or batch filling) should also be considered as sources of release when639
developing the area classification. By means of this procedure, each release will be graded640
either ‘continuous’, ‘primary’ or ’secondary’.  641
NOTE 1 Releases may form part of process, e.g. taking samples, or may occur as part of a routine maintenance642
procedure. These forms of release are generally classified as continuous or primary grades of release. Other643 accidental releases are generally classified as a secondary grade of release.644
NOTE 2 One item may give rise to more than one grade of release. For example, there may be a small primary645
grade release, but a larger release could occur under abnormal operation; thus giving rise to a secondary grade646
release. In this situation, both release conditions (both grades of release) need full consideration as described in647
this standard.648
Having established the grade or grades of the release, it is necessary to determine the649
 
60079-10-1/Ed2/CDV © IEC:2014 18
If the total quantity of flammable substance available for release is ‘small’, for example, labo -651
ratory use, whilst a potential explosion condition may exist, it may not be appropriate to use652
this area classification procedure. In such cases, account shall be taken of the particular653
factors involved.654
The area classification of process equipment in which flammable substance is burned, for655
example, fired heaters, furnaces, boilers, gas turbines etc., should take into account purge656
cycle, start-up and shut-down conditions.657
In some cases the construction of closed systems where specific construction codes are met658
can be accepted as effectively preventing and/or limiting releases of flammable substances to659
a negligible leakage hazard. The hazardous area classification of such equipment or660
installations requires a complete assessment to verify the full compliance of the installation to661
the relevant constructional and operating standards. Verification of compliance should662
consider design, installation, operation, maintenance and monitoring activities.663
Mists which form through leaks of pressurized liquid can be flammable even though the liquid664
temperature is below the flash point. It is important therefore to ensure that clouds of mist do665
not occur (see Annex G).666
6.3 Forms of release667
The characteristic of any release depends upon the physical state of the flammable668
substance, its temperature and pressure. The physical states include:669
  a gas, which may be at an elevated temperature or pressure;670
  a gas liquefied by the application of pressure, e.g. LPG;671
  a gas which can only be liquefied by refrigeration, e.g. methane;672
  a liquid with an associated release of flammable vapour.673
Releases from such plant items as pipe connections, pumps and compressor seals and valve674
packings often start with a low flow rate. However, if the release is not stopped erosion of the675
source of the release can greatly increase the rate of release and hence the extent of the676
hazard.677
 A release of flammable substance above its flashpoint wil l give rise to a flammable vapour or678
gas cloud which may initially be less or more dense than the surrounding air or may be679
neutrally buoyant. The forms of release and the pattern of behaviour at various conditions are680
displayed as a flow chart in Figure B.1.681
Every form of release will eventually end as a gaseous or vapour release and the gas or682
vapour may appear as buoyant, neutrally buoyant or heavy. This characteristics will affect the683
extent of the zone generated by a particular form of release (see Figure B.1).684
The horizontal extent of the zone at ground level will generally increase with increasing685
relative density and the vertical extent above the source will generally increase with686
decreasing relative density.687
6.3.1 Gaseous release688
 A gas release wil l produce a gas jet or plume at the release source depending on the689
pressure at the point of release, e.g. pump seal, pipe connection or evaporative pool area.690
The relative density of the gas, the degree of turbulent mixing and the prevailing air691
movement will all influence the subsequent movement of any gas cloud.692
In calm conditions low velocity releases of a gas that is significantly less dense than air will693
tend to move upwards, e.g. hydrogen and methane. Conversely, a gas that is significantly694
denser than air will tend to accumulate at ground level or in any pits or depressions, e.g.695
 
60079-10-1/Ed2/CDV © IEC:2014 19
with air and become neutrally buoyant. A gas or vapour with density that is not significantly697
different to air is regarded as neutrally buoyant.698
Higher pressure releases will initially produce jets of released gas which will mix turbulently699
with the surrounding air and entrain a ir in the jet.700
 At high pressures , a thermodynamic effect due to expans ion can come into play. As the gas701
escapes, it expands and cools down and may initially behave as heavier than air. However,702
the cooling due to the Joule-Thomson effect is eventually offset by the heat supplied by the703
air. The resulting gas cloud will eventually become neutrally buoyant. The transition from704
heavier than air to neutrally bouyant behaviour may occur at any time depending on the705
nature of the release and may occur after the cloud has been diluted to below the LFL .706
NOTE Hydrogen demonstrates a reverse Joule-Thomson effect.707
6.3.2 Liquefied under pressure708
Some gases can be liquefied by the application of pressure alone, e.g. propane and butane,709
and are usually stored and transported in this form.710
When a pressurized liquefied gas leaks from its containment the most likely scenario is that711
the substance will escape as a gas from any vapour space or gas lines. The rap id evaporation712
produces significant cooling at the point of release and icing due to the condensation of water713
vapour from the atmosphere may occur.714
 A liquid leak wil l par tiall y evaporate at the point of release. This is known as flash715
evaporation. The evaporating liquid pulls energy from itself and the surrounding atmosphere716
and in turn cools down the leaking fluid. The cooling of the fluid prevents total evaporation717
and therefore an aerosol is produced. If the leak is large enough then cold pools of fluid can718
accumulate on the ground which will evaporate over time to add to the gas release.719
The cold aerosol cloud will act like a dense gas. A pressurized liquid release can often be720
seen as the cooling effect of evaporation will condense ambient humidity to produce a vapour721
cloud.722
6.3.3 Liquefied by refrigeration723
Other gases, the so-called permanent gases, can only be liquefied by refrigeration e.g.724
methane and hydrogen. Small leaks of refrigerated gas will evaporate quickly without forming725
a pool of liquid by drawing heat from the environment. If the leak is large a cold pool of liquid726
may form.727
 As the cold liquid pulls energy f rom the ground and surrounding atmosphere the liquid wil l boi l728
generating a cold dense gas cloud. As with liquids, dikes or bund walls can be used to direct729
or hold the flow of leakages.730
NOTE Care needs to be taken when classifying areas containing cryogenic flammable gases such as liquefied731
natural gas. Vapours emitted will generally be heavier than air at low temperatures but will become neutrally732
buoyant on approaching ambient temperature.733
6.3.4 Aerosols734
 An aerosol is not a gas , but consists of small droplets of liquid suspended in air. The droplets735
are formed from vapours or gases under certain thermodynamic conditions or by flash736
evaporation of pressurized liquids. The scattering of light within an aerosol cloud frequently737
makes the cloud visible to the naked eye. The dispersion of an aerosol may vary between the738
behaviour of a dense gas or a neutrally buoyant gas. Aerosol droplets can coalesce and rain739
 
60079-10-1/Ed2/CDV © IEC:2014 20
surrounding environment, evaporate and add to the gas/vapour cloud (for more details see741
 Annex G).742
6.3.5 Vapours743
Liquids at equilibrium with their environment will generate a layer of vapour above their744
surface. The pressure this vapour exerts in a closed system is known as the vapour pressure,745
which increases in a non-linear function with temperature.746
The process of evaporation uses energy which may come from a variety of sources, for747
example from the liquid or the surrounding environment. The evaporation process may748
decrease the temperature of the liquid and may tend to balance the heat input to the liquid to749
limit temperature rise. However, changes in liquid temperature due to increased evaporation750
from normal environmental conditions are considered too marginal to affect the hazardous751
area classification. The concentration of the generated vapour is not easy to predict as it is a752
function of the evaporation rate, temperature of the liquid and the surrounding air flow.753
6.3.6 Liquid releases754
The release of flammable liquids will normally form a pool on the ground, with a vapour cloud755
at the liquid’s surface unless the surface is absorbent. The size of the vapour cloud will756
depend on the properties of the substance and its vapour pressure at the ambient757
temperature (see B.7.2).758
NOTE The vapour pressure is an indication of a liquid's evaporation rate. A substance with a high vapour759
pressure at normal temperatures is often referred to as volatile. As a general rule, vapour pressure of liquid at760
ambient temperatures increases with decreasing boiling point. As the temperature rises so does the vapour761
pressure.762
Release may also occur on water. Many flammable liquids are less dense than water and are763
often not miscible. Such liquids will spread on the surface of water, whether it is on the764
ground, in plant drains, pipe trenches or on open waters (sea, lake or river), forming a thin765
film and increasing the evaporation rate due to the increased surface area. In these766
circumstances the calculations in Annex B may not be suitable.767
6.4 Ventilation (or air movement) and dilution768
Gas or vapour released into the atmosphere may dilute through turbulent  mixing with air, and769
to a lesser extent by diffusion driven by concentration gradients, until the gas disperses770
completely and the concentration is essentially zero. Air movement due to natural or artificial771
ventilation will promote dispersion. Increased air movement may also increase the rate of772
release of vapour due to increased evaporation on an open liquid surface.773
Suitable ventilation rates can reduce persistence of an explosive gas atmosphere thus774
influencing the type of zone.775
 A structure with suff icient openings to allow free passage of air through all parts of the776
building is considered in many cases to be well ventilated and should be treated as an open777
air area, e.g. a shelter with open sides and rooftop ventilation openings.778
Dispersion or diffusion of a gas or vapour into the atmosphere is a key factor in reducing the779
concentration of the gas or vapour to below the lower flammable limit.780
Ventilation and air movement has two basic functions:781
a) To increase the rate of dilution and promote dispersion to limit the extent of a zone;782
b) To avoid the persistence of an explosive atmosphere that may influence the type of a783
zone.784
With increased ventilation or air movement the extent of a zone will normally be reduced.785
 
60079-10-1/Ed2/CDV © IEC:2014 21
On the other hand, some obstacles, for example, dykes, walls and ceilings, which limit the787
extent of vapour or gas movement, may also limit the extent of the zone.788
NOTE Increased air movement may also increase the release rate of vapour due to increased evaporation from789
open liquid surfaces. However the benefits of increased air movement normally outweigh the increase in release790
rate.791
For low velocity releases the rate of gas or vapour dispersion in the atmosphere increases792
with wind speed, but in stable atmospheric conditions layering of the gas or vapour may occur793
and the distance for safe dispersal can be greatly increased.794
NOTE 1 In plant areas with large vessels and structures, even at low wind speeds eddies may be formed behind795
vessels and structures thus forming pockets of gas or vapour, despite sufficient turbulence that could otherwise796
promote dispersion.797
NOTE 2  In normal practice, the tendency of layering is not taken into account in area classification because the798
conditions which give rise to this effect are rare and occur for short periods only. However, if prolonged periods of799
low wind speed are expected for the specific circumstance then the extent of the zone should take account of the800
additional distance required to achieve dispersion.801
6.5 Main types of ventilation802
The two main types of ventilation are:803
a) natural ventilation;804
b) artificial (or forced) ventilation, either general to the area or local to the source of release.805
6.5.1 Natural Ventilation806
Natural ventilation in buildings arises from pressure differences induced by the wind and/or by807
temperature gradients (buoyancy induced ventilation). Natural ventilation may be effective in808
certain indoor situations (for example, where a building has openings in its walls and/or roof)809
to dilute releases safely.810
Examples of natural ventilation:811
  an open building which, having regard to the relative density of the gases and/or vapours812
involved, has openings in the walls and/or roof so dimensioned and located that the813
ventilation inside the building, for the purpose of area classification, can be regarded as814
equivalent to that in an open-air situation;815
  a building which is not an open building but which has natural ventilation (generally less816
than that of an open building) provided by permanent openings made for ventilation817
purposes.818
Consideration of natural ventilation in buildings should recognise that gas or vapour buoyancy819
may be a significant factor and should be arranged to promote dispersion and dilution.820
Ventilation rates arising from natural ventilation are inherently very variable. Where dilution of821
releases is by natural ventilation, the worst case scenario shall   preferably be  considered to822
determine the degree of ventilation. Such a scenario will then lead to a higher level of823
availability even though the degree of the ventilation is reduced. Generally, with any natural824
ventilation, a lower degree of ventilation leads to a higher level of availability and vice versa825
which will compensate for overly optimistic assumptions made in estimating the degree of826
ventilation.827
There are some situations which require special care. This is particularly the case where the828
ventilation openings are limited to mainly one side of the enclosure. Under certain829
unfavourable ambient conditions, such as windy days when the wind is blowing onto the830
ventilated face of the enclosure, the external air movement may prevent the operation of the831
thermal buoyancy mechanism. Under these circumstances the level of ventilation and the832
 
6.5.2.1 General835
 Air movement required for ventilation may also be provided by artif icial means, for example,836
fans or extractors. Although artificial ventilation is mainly applied inside a room or enclosed837
space, it can also be applied to situations in the open air to compensate for restricted or838
impeded air movement due to obstacles.839
The artificial ventilation may be either general (e.g. a whole room) or local (e.g. extraction840
near a point of release) and for both of these, differing degrees of air movement and841
replacement can be appropriate.842
With the use of artificial ventilation it is sometimes possible to achieve:843
  reduction in the type and/or extent of zones;844
  shortening of the time of persistence of an explosive gas atmosphere;845
  prevention of the generation of an explosive gas atmosphere .846
6.5.2.2 Ventilation considerations847
 Artif icial venti lat ion can provide an effective and reliable venti lation sys tem in an indoor848
situation. The following considerations should be included for artificial ventilation systems:849
a) classification of the inside of the extraction system and immediately outside the extraction850
system discharge point and other openings of the extraction system;851
b) for ventilation of a hazardous area the ventilation air should normally be drawn from a852
non-hazardous area taking into account the suction effects on the surrounding area;853
c) before determining the dimensions and design of the ventilation system, the location,854
grade of release, release velocity and release rate should be defined.855
In addition, the following factors will influence the quality of an artificial ventilation system:856
a) flammable gases and vapours usually have densities other than that of air, thus they may857
accumulate near to either the floor or ceiling of an enclosed area, where air movement is858
likely to be reduced;859
b) proximity of the artificial ventilation to the source of release; artificial ventilation close to860
the source of release will normally be more effective and may be needed to adequately861
control gas or vapour movement;862
c) changes in gas density with temperature;863
d) impediments and obstacles may cause reduced, or even no, air movement, i.e. no864
ventilation in certain parts of the area;865
e) turbulence and circulating air patterns.866
For more details see Annex C.867
Consideration should be given to the possibility or need for recirculation of air in the868
ventilation arrangement. This may impact the background concentration and effectiveness of869
the ventilation system in reducing the hazardous area. In such cases the classification of the870
hazardous area may need to be modified accordingly. Recirculation of air may also be871
necessary in some applications e.g. for some processes or to provide for the needs of872
personnel or equipment in high or low ambient temperatures where supplemental cooling or873
heating of the air is required. Where recirculation of air is needed then additional controls for874
safety may also be required. e.g. a gas analyzer with dampers controlling fresh air intake.875
6.5.2.3 Examples of artificial ventilation876
General artificial ventilation may include a building which is provided with fans in the walls877
and/or in the roof to improve the general ventilation in the building.878
The role of fans may be twofold. They can increase the air flow through a building, helping to879
 
60079-10-1/Ed2/CDV © IEC:2014 23
dilution of a cloud which is much smaller than the room which contains it, even if no gas is881
transported out of the room. Fans may also enhance dilution by increasing turbulence in some882
outdoor situations.883
Local artificial ventilation may be:884
a) An air/vapour extraction system applied to an item of process equipment which885
continuously or periodically releases flammable vapour.886
b) A forced or extraction ventilation system applied to a local area where it is expected that887
an explosive gas atmosphere may otherwise occur.888
For more details see C.4.889
6.5.3 Degree of dilution890
The effectiveness of the ventilation in controlling dispersion and persistence of the explosive891
atmosphere will depend upon the ‘degree of dilution’, the availability of ventilation and the892
design of the system. For example, ventilation may not be sufficient to prevent the formation893
of an explosive atmosphere but may be sufficient to avoid its persistence.894
The degree of dilution is defined to correspond with the ability of a given release to dilute895
down to a safe level within defined ventilation or atmospheric conditions. Therefore a larger896
release corresponds with a lower degree of dilution for a given set of ventilation / atmospheric897
conditions, and a lower ventilation rate corresponds with a lower degree of dilution for a given898
size of release.899
If other forms of ventilation, e.g. cooling fans are taken into account, then care should be900
exercised as to ventilation availability. Ventilation for other purposes may also affect dilution901
in either a positive or negative manner.902
The degree of dilution will also affect the dilution volume.   The dilution volume is903
mathematically equal to the hazardous volume but the boundary of the hazardous area904
additionally takes into account other factors such as possible movement of the release due to905
the direction and velocity of the release and of the surrounding volume of air.906
‘Degrees of dilution’ depend not only on the ventilation, but also on the nature and the type of907
the expected release of gas. Some releases, e.g. release with low velocity, will be amenable908
to mitigation by enhanced ventilation with others much less so, e.g. release with high velocity.909
The following three degrees of dilution are recognized:910
a) High dilution911
b) Medium dilution913
The concentration is controlled resulting in a stable zone boundary, whilst the release is in914
progress and the explosive gas atmosphere does not persist unduly after the release has915
stopped. The type and extent of zone are limited to the design parameters.916
c) Low dilution917
There is significant concentration whilst release is in progress and/or significant persistence918
of a flammable atmosphere after the release has stopped.919
7 Type of zone920
The likelihood of the presence of an explosive gas atmosphere depends mainly on the grade921
of release and the ventilation. This is identified as a zone. Zones are recognized as: zone 0,922
zone 1, zone 2 and the non-hazardous area.923
Where zones created by adjacent sources of release overlap and are of different zonal924
classification, the more severe classification criteria will apply in the area of overlap. Where925
overlapping zones are of the same classification, this common classification will normally926
apply.927
7.1 Influence of grade of the source of release928
There are three basic grades of release, as listed below in order of decreasing frequency of929
occurrence and/or duration of release of flammable substance:930
a) continuous grade;931
b) primary grade;932
c) secondary grade.933
 A source of re lease may give ri se to any one of these grades of re lease, or to a combination934
of more than one.935
The grade of release generally determines type of the zone. In an adequately ventilated area936
(typical open air plant) a continuous grade of release generally leads to a zone 0937
classification, a primary grade to zone 1 and a secondary grade to zone 2. This general rule938
may be modified by considering the degree of dilution and availability of ventilation which may939
result in a more or less severe classification (refer to 7.2 and 7.3).940
7.2 Influence of dilution941
The effectiveness of ventilation or degree of dilution shall be considered when estimating the942
type of zone classification. A medium degree of dilution will generally result in the943
predetermined types of the zones based upon the types of the sources of release. A high944
degree of dilution will allow a less severe classification, e.g. zone 1 instead of zone 0, zone 2945
instead of zone 1 and even zone of negligible extent in some cases. On the other hand a low946
degree of dilution will require a more severe classification (see Annex D).947
7.3 Influence of availability of ventilation948
The availability of ventilation has an influence on the presence or formation of an explosive949
gas atmosphere and thus also on the type of zone. As availability, or reliability, of the950
ventilation decreases, the likelihood of not dispersing flammable atmospheres increases. The951
zone classification will tend to be more severe, i.e. a zone 2 may change to a zone 1 or even952
zone 0. Guidance on availability is given in Annex D.953
NOTE Combining the concepts of the efficiency of ventilation and the availability of ventilation results in a954
qualitative method for the evaluation of the zone type. This is further explained in Annex D.955
8 Extent of zone956
The extent of the zone depends on the estimated or calculated distance over which an957
explosive atmosphere exists before it disperses to a concentration in air below its lower958
flammable limit. The extent of the zone should consider the level of uncertainty in the959
assessment by the application of a safety factor. When assessing the area of spread of gas or960
vapour before dilution to below its lower flammable limit, expert advice should be sought.961
Consideration should always be given to the possibility that a gas which is heavier than air962
may flow into areas below ground level (for example, pits or depressions) and that a gas963
which is lighter than air may be retained at high level (for example, in a roof space).964
Where the source of release is situated outside an area or in an adjoining area, the pene-965
tration of a significant quantity of flammable gas or vapour into the area can be prevented by966
suitable means such as:967
a) physical barriers;968
b) maintaining a sufficient overpressure in the area relative to the adjacent hazardous969
areas, so preventing the ingress of the explosive gas atmosphere;970
c) purging the area with sufficient flow of fresh air, so ensuring that the air escapes from all971
openings where the flammable gas or vapour may enter.972
The extent of the zone requires assessment of a number of physical and chemical973
 
60079-10-1/Ed2/CDV © IEC:2014 25
For releases of limited volume a lesser distance may be accepted to an on-going release.976
Under some conditions heavier than air gases and vapours can behave like a spilled liquid spreading 977
down terrain slopes, through plant drains or pipe trenches and can be ignited at a point remote from978
the original leakage, therefore putting at risk a large area of plant (see B.6). The layout of the plant,979
where possible, should be designed to aid the rapid dispersal of explosive gas atmospheres.  980
 An area with restricted ventilation (for example, in pits or trenches) that would otherwise be zone 2981
may require zone 1 classification; on the other hand, wide shallow depressions used for pumping982
complexes or pipe reservations may not require such rigorous treatment.983
9 Documentation984
9.1 General985
It is recommended that the steps taken to carry out area classification and t he information and986
assumptions used are fully documented. The area classification document should be a living987
document and should include the method used for area classification and should be revised988
during any plant changes. All relevant information used should be referenced. Examples of989
such information, or of a method used, would be:990
a) recommendations from relevant codes and standards;991
b) gas and vapour dispersion characteristics and calculations;992
c) a study of ventilation characteristics in relation to flammable substance release993
parameters so that the effectiveness of the ventilation can be evaluated.994
d) the properties of all process substances used on the plant (see IEC 60079-20-1), which995
may include:996
  molar mass997
  flash point998
  boiling point999
  gas group and temperature class1004
 A suggested format for the substances lis ting is given in Table A.1 and a format for recording1005
the results of the area classification study and any subsequent alterations is given in Table1006
 A.2. 1007
The source of information (code, national standard, calculation) needs to be recorded so that,1008
at subsequent reviews, the philosophy which was adopted is clear to the area classification1009
team.1010
9.2 Drawings, data sheets and tables1011
 Area class ification documents may be in hard copy or electronic form and should inc lude1012
plans and elevations or three dimensional models, as appropriate, which show both the type1013
and extent of zones, gas group, ignition temperature and/or temperature class.1014
Where the topography of an area influences the extent of the zones, this should be1015
documented.1016
The documents should also include other relevant information such as:1017
a) The location and identification of sources of release. For large and complex plants or1018
process areas, it may be helpful to itemize or number the sources of release so as to1019
 
60079-10-1/Ed2/CDV © IEC:2014 26
b) The position of openings in buildings (for example, doors, windows and inlets and outlets1021
of air for ventilation).1022
The area classification symbols which are shown in Figure A.1 are the preferred ones. A1023
symbol key shall always be provided on each drawing. Different symbols may be necessary1024
where multiple equipment groups and/or temperature classes are required within the same1025
 
1031
1032
1033
 
60079-10-1/Ed2/CDV © IEC:2014 28
Table A.1 Hazardous area classification data sheet – Part I: Flammable substance list and characteristics1037
Plant:  Reference
drawing:
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
Flammable substance  Volatility a   LFL Ex characteristics 
Name Composi
and remarks
 
60079-10-1/Ed2/CDV © IEC:2014 29
Table A.2 Hazardous area classification data sheet Part II: List of sources of release1038
Plant:
Area: 
Refer ence drawing:
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
Source of release Flammable substance Ventilation Hazardous area
Descrip
tion
Loca
tion
b Quote the number of list in Part I
c  G  – Gas; L  – Liquid; LG  – Liquefied gas; S  – Solid
d  N  – Natural; AG  – Artificial General; AL  – Artificial Local.
e See IEC 60079-10-1 Annex C
f   Indicate code reference if used, or calculation reference  
 
A.1 Hazardous area suggested shapes1039
The following figures show some suggested hazardous area shapes based on the form of1040
release described in B.6, which may be useful in the preparation of hazardous area1041
classification drawings. The effects of impingement of the release on obstacles and the1042
influence of topography are not considered. The hazardous area generated by a release may1043
result in the combination of different shapes.1044
Key1045
SR Source of release1046
r Main extent of the hazardous area to be defined taking into consideration the estimated1047
hazardous distance1048
r  ’ , r”   Secondary extents of the hazardous area to be defined taking into account release1049
behaviour1050
h Distances between the source of release and ground level or surface below the release1051
or 
 1052
(or at high pressure in case of unknown release direction)1054
1055
or 
 1056
 
1058
Figure A.4.1 – Gas or vapour (liquefied under pressure or by refrigeration)1059
NOTE Liquid pool should not be formed in case of dripping.1060
1061
Figure A.4.2 – Gas or vapour (liquefied under pressure or by refrigeration) with spillage1062
NOTE Liquid pool could be formed in case of spillage. In this case, an additional source of release could be1063
considered1064
1065
 
1072
d C   
    polytropic index of adiabatic expansion or ratio of specific heats;1077
 M    molar mass of gas or vapour (kg/kmol);1078
 p   pressure inside the container (Pa);1079
 p  
a p   atmospheric pressure (Pa);1081
c p   critical pressure (Pa);1082
v p   vapour pressure of the liquid at temperature T (kPa);1083
 R   universal gas constant (8314 J/kmol K);1084
     liquid density (kg/m 3 );1085
 g      gas or vapour density (kg/m
3 );
 1086
S    cross section of the opening (hole), through which the fluid is released (m 2 );1087
T    absolute temperature of the fluid, gas or liquid (K);1088
aT    absolute ambient temperature (K);1089
wu   wind speed over the liquid pool surface (m/s);1090
W    release rate of liquid (mass per time, kg/s);1091
eW    evaporation rate of liquid (kg/s);1092
 g W    mass release rate of gas (kg/s);1093
 Z    compressibility factor.1094
B.2 Examples of grade of release1095
The following examples are not intended to be rigidly applied and may need to be varied to1096
suit particular process equipment and the situation. It needs to be recognised that some1097
equipment may exhibit more than one grade of release.1098
B.2.1 Sources giving a continuous grade of release1099
a) The surface of a flammable liquid in a fixed roof tank, with a permanent vent to the atmo -1100
sphere.1101
b) The surface of a flammable liquid which is open to the atmosphere continuously or for1102
long periods.1103
B.2.2 Sources giving a primary grade of release1105
a) Seals of pumps, compressors or valves if release of flammable substance during normal1106
operation is expected.1107
b) Water drainage points on vessels which contain flammable gases or liquids, which may1108
release flammable substance into the atmosphere while draining off water during normal1109
operation.1110
c) Sample points which are expected to release flammable substance into the atmosphere1111
during normal operation.1112
d) Relief valves, vents and other openings which are expected to release flammable1113
substance into the atmosphere during normal operation  1114
B.2.3 Sources giving a secondary grade of release1115
a) Seals of pumps, compressors and valves where release of flammable substance during1116
normal operation of the equipment is not expected.1117
b) Flanges, connections and pipe fittings, where release of flammable substance is not1118
expected during normal operation.1119
c) Sample points which are not expected to release flammable substance during normal1120
operation.1121
d) Relief valves, vents and other openings which are not expected to release flammable1122
substance into the atmosphere during normal operation.1123
B.3 Assessment of grades of release1124
 A wrong assessment of grades of release may compromise the outcome of the whole1125
procedure. Although the grades of release are defined (see 3.4.2, 3.4.3 and 3.4.4), in practice1126
it is not always easy to distinguish one grade of release from the other.1127
For example, it is usually considered that every release that does not occur in normal1128
operation is a secondary release and the anticipated duration of the release is usually1129
neglected. However, the concept of a secondary grade of release is also based upon the1130
assumption that the release will only last for short periods. This implies that a potentially1131
ongoing release will be detected soon after the beginning of the release and that remedial1132
action will be taken as soon as possible. Such assumption leads to the issue of regular1133
monitoring and maintenance of the equipment and installation.1134
Obviously, if there is no regular monitoring and the maintenance is poor, the releases may1135
last for hours if not days before being detected. Such delay in detection does not mean that1136
the sources of the release should therefore be declared as primary or continuous. There are1137
many unattended remote installations where a release may occur without being noticed for1138
long time, but even such installations should be monitored and inspected on a reasonably1139
regular basis. So, any assessment of the release grade must be based upon careful1140
considerations and the assumption that monitoring and inspection of the equipment and1141
installations will be performed in a reasonable way according to the any manufacturer’s1142
instructions, relevant regulations and protocols and sound engineering practice. Area1143
classification should not be a cover for a poor maintenance practice but the user must be1144
aware that poor practices may compromise the established basis for area classification.1145
There are many cases of release which may apparently fit comfortably with the definition of a1146
primary grade of release. However when scrutinizing the nature of the release it may be1147
revealed that the release could happen so frequently and so unpredictably that one cannot be1148
reasonably assured that an explosive atmosphere will not exist near the source of release. In1149
such cases the definition of continuous grade of release may be more suitable. Therefore the1150
definition of a continuous grade of release implies not only continuous releases but releases1151
with a high frequency as well (see 3.4.2).1152
B.4 Summation of releases1153
In indoor areas with more than one source of release, in order to determine the type and1154
 
background concentration is determined. Since continuous grade releases, by definition, can1156
be expected to be releasing most if not all of the time, then all continuous grade releases1157
should be included.1158
Primary grade releases occur in normal operation but it is unlikely that all of these sources1159
will be releasing at the same time. Knowledge and experience of the installation should be1160
used to determine the maximum number of primary grade releases that may release1161
simultaneously under worst conditions.1162
Secondary grade releases are not expected to release in normal operation so, given that it is1163
unlikely that more than one secondary source would release at any one time, only the largest1164
secondary release should be considered.1165
The summation of sources of release with regular (i.e. well predictable) activity should be1166
based on detailed analysis of operating procedures. In the determination of the summated1167
releases (both mass and volumetric):1168
  the overall continuous release is the sum of all the individual continuo us releases,1169
  the overall primary release is the sum of some of the individual primary releases1170
combined with the overall continuous release,1171
  the overall secondary release is the largest individual secondary release combined with1172
the overall primary release.1173
Where the same flammable substance is released from all of the release sources then the1174
release rates (both mass and volumetric) can be summated directly. However, when the1175
releases are of different flammable substances, the situation is more complex. In the1176
determination of the degree of dilution (see Figure C.1), the release characteristics  need to be1177
determined for each flammable substance before any summation takes place. The secondary1178
release with the highest value should be used.1179
In the determination of the background concentration (see equation C.1) the volumetric1180
release rates can be summated directly. The critical concentration with which the background1181
concentration is compared is a proportion of the LFL (typically 20%). Since there are a number1182
of different flammable substances being released the combined LFL   should be used as the1183
comparator.1184
In general, continuous and primary sources of release should preferably not be located in1185
areas with a low degree of dilution. Either sources of release should be relocated, ventilation1186
should be improved or the grade of release should be reduced.1187
B.5 Hole size and source radius1188
The most significant factor to be estimated in a system is the hole size. It determines the1189
release rate of the flammable substance and thus eventually the type of zone and the extent1190
of the zone.1191
Release rate is proportional to the square of the hole radius. A modest underestimate of the1192
hole size will therefore lead to a gross underestimate of the calculated value for release rate,1193
which should be avoided. Overestimate of the hole size will lead to a conservative calculation1194
which is acceptable for safety reasons, however, the degree of conservatism should also be1195
limited because it eventually results with overlarge zone extents. A carefully balanced1196
approach is therefore needed when estimating the hole size.1197
NOTE While the term ‘hole radius’ is used, most unintended holes are not round. In such cases the coefficient of1198
di