EN 60079-10:2003

2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.Electrical apparatus for explosive gas atmospheres - Part 10: Classification of hazardous areas]GHOLWHYMatériel électrique pour atmosphères explosives gazeuses - Partie 10: Classement des emplacements dangereuxElektrische Betriebsmittel für gasexplosionsgefährdete Bereiche - Teil 10: Einteilung der explosionsgefährdeten Bereiche29.260.20Electrical apparatus for explosive atmospheresICS:SIST EN 60079-10:2003en,fr,deTa slovenski standard je istoveten z:EN 60079-10:200301-november-2003SIST EN 60079-10:2003SLOVENSKI
STANDARDSIST EN 60079-10:19971DGRPHãþD

EUROPEAN STANDARD
EN 60079-10 NORME EUROPÉENNE EUROPÄISCHE NORM
April 2003 CENELEC European Committee for Electrotechnical Standardization Comité Européen de Normalisation Electrotechnique Europäisches Komitee für Elektrotechnische Normung
Central Secretariat: rue de Stassart 35, B - 1050 Brussels
© 2003 CENELEC - All rights of exploitation in any form and by any means reserved worldwide for CENELEC members.
Ref. No. EN 60079-10:2003 E
ICS 29.260.20 Supersedes EN 60079-10:1996
English version
Electrical apparatus for explosive gas atmospheres Part 10: Classification of hazardous areas (IEC 60079-10:2002)
Matériel électrique pour atmosphères explosives gazeuses Partie 10: Classement des emplacements dangereux (CEI 60079-10:2002)
Elektrische Betriebsmittel für gasexplosionsgefährdete Bereiche Teil 10: Einteilung der explosionsgefährdeten Bereiche (IEC 60079-10:2002)
This European Standard was approved by CENELEC on 2002-12-01. CENELEC members are bound to comply with the CEN/CENELEC Internal Regulations which stipulate the conditions for giving this European Standard the status of a national standard without any alteration.
Up-to-date lists and bibliographical references concerning such national standards may be obtained on application to the Central Secretariat or to any CENELEC member.
This European Standard exists in three official versions (English, French, German). A version in any other language made by translation under the responsibility of a CENELEC member into its own language and notified to the Central Secretariat has the same status as the official versions.
CENELEC members are the national electrotechnical committees of Austria, Belgium, Czech Republic, Denmark, Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Luxembourg, Malta, Netherlands, Norway, Portugal, Slovakia, Spain, Sweden, Switzerland and United Kingdom.

at national level by publication of an identical
national standard or by endorsement
(dop) 2003-12-01 – latest date by which the national standards conflicting
with the EN have to be withdrawn
(dow) 2005-12-01
Annexes designated "normative" are part of the body of the standard. Annexes designated "informative" are given for information only. In this standard, Annex ZA is normative and Annexes A, B and C are informative. Annex ZA has been added by CENELEC. __________ Endorsement notice The text of the International Standard IEC 60079-10:2002 was approved by CENELEC as a European Standard without any modification. __________

- 3 - EN 60079-10:2003 Annex ZA
(normative)
Normative references to international publications
with their corresponding European publications This European Standard incorporates by dated or undated reference, provisions from other publications. These normative references are cited at the appropriate places in the text and the publications are listed hereafter. For dated references, subsequent amendments to or revisions of any of these publications apply to this European Standard only when incorporated in it by amendment or revision. For undated references the latest edition of the publication referred to applies (including amendments). NOTE When an international publication has been modified by common modifications, indicated by (mod), the relevant EN/HD applies. Publication Year Title EN/HD Year IEC 60050-426 1990 International Electrotechnical Vocabulary (IEV) Chapter 426: Electrical apparatus for explosive atmospheres
- - IEC 60079-4 1975 Electrical apparatus for explosive gas atmospheres Part 4: Method of test for ignition temperature
- - IEC 60079-4A 1970 Part 4: Method of test for ignition temperature – First supplement
- - IEC 60079-20 1996 Part 20: Data for flammable gases and vapours, relating to the use of electrical apparatus
- -
IEC 60079-10Edition 4.0 2002-06INTERNATIONAL STANDARD NORME INTERNATIONALEElectrical apparatus for explosive gas atmospheres – Part 10: Classification of hazardous areas
Matériel électrique pour atmosphères explosives gazeuses – Partie 10: Classement des emplacements dangereux
INTERNATIONAL ELECTROTECHNICAL COMMISSION COMMISSION ELECTROTECHNIQUE INTERNATIONALE XAICS 29.260.20 PRICE CODECODE PRIXISBN 2-8318-6379-1

60079-10  IEC:2002 – 3 –CONTENTSFOREWORD.3INTRODUCTION.41 General .51.1 Scope.51.2 Normative references .62 Definitions and terms.63 Safety and area classification.103.1 Safety principles.103.2 Area classification objectives.104 Area classification procedure.114.1 General.114.2 Sources of release .124.3 Type of zone .124.4 Extent of zone .134.4.1 Release rate of gas or vapour.134.4.2 Lower explosive limit (LEL).144.4.3 Ventilation .144.4.4 Relative density of the gas or vapour when it is released.144.4.5 Other parameters to be considered.154.4.6 Illustrative examples.155 Ventilation .165.1 General.165.2 Main types of ventilation.165.3 Degree of ventilation .165.4 Availability of ventilation.166 Documentation .176.1 General.176.2 Drawings, data sheets and tables.17Annex A (informative)
Examples of sources of release.18Annex B (informative)
Ventilation.20Annex C (informative)
Examples of hazardous area classification.36Figure C.1 − Preferred symbols for hazardous area zones.57Figure C.2 – Schematic approach to the classification of hazardous areas.58– 2 –60079-10 © IEC:2002

60079-10  IEC:2002 – 5 –INTERNATIONAL ELECTROTECHNICAL COMMISSION____________ELECTRICAL APPARATUS FOR EXPLOSIVE GAS ATMOSPHERES –Part 10: Classification of hazardous areasFOREWORD1)The IEC (International Electrotechnical Commission) is a worldwide organization for standardization comprisingall national electrotechnical committees (IEC National Committees). The object of the IEC is to promoteinternational co-operation on all questions concerning standardization in the electrical and electronic fields. Tothis end and in addition to other activities, the IEC publishes International Standards. Their preparation isentrusted to technical committees; any IEC National Committee interested in the subject dealt with mayparticipate in this preparatory work. International, governmental and non-governmental organizations liaisingwith the IEC also participate in this preparation. The IEC collaborates closely with the InternationalOrganization for Standardization (ISO) in accordance with conditions determined by agreement between thetwo organizations.2)The formal decisions or agreements of the IEC on technical matters express, as nearly as possible, aninternational consensus of opinion on the relevant subjects since each technical committee has representationfrom all interested National Committees.3)The documents produced have the form of recommendations for international use and are published in the formof standards, technical specifications, technical reports or guides and they are accepted by the NationalCommittees in that sense.4)
In order to promote international unification, IEC National Committees undertake to apply IEC InternationalStandards transparently to the maximum extent possible in their national and regional standards. Anydivergence between the IEC Standard and the corresponding national or regional standard shall be clearlyindicated in the latter.5)
The IEC provides no marking procedure to indicate its approval and cannot be rendered responsible for anyequipment declared to be in conformity with one of its standards.6)
Attention is drawn to the possibility that some of the elements of this International Standard may be the subjectof patent rights. The IEC shall not be held responsible for identifying any or all such patent rights.International Standard IEC 60079-10 has been prepared by subcommittee 31J: Classificationof hazardous areas and installation requirements, of IEC technical committee 31: Electricalapparatus for explosive atmospheres.This fourth edition cancels and replaces the third edition published in 1995, and constitutes atechnical revision.The text of this standard is based on the following documents:FDISReport on voting31J/82/FDIS31J/84/RVDFull information on the voting for the approval of this standard can be found in the report onvoting indicated in the above table.This publication has been drafted in accordance with the ISO/IEC Directives, Part 3.Annexes A, B and C are for information only.The committee has decided that the contents of this publication will remain unchangeduntil 2007. At this date, the publication will be•reconfirmed;•withdrawn;•replaced by a revised edition, or•amended.– 3 –60079-10 © IEC:2002

60079-10  IEC:2002 – 7 –INTRODUCTIONIn areas where dangerous quantities and concentrations of flammable gas or vapour mayarise, protective measures are to be applied in order to reduce the risk of explosions. Thispart of IEC 60079 sets out the essential criteria against which the risk of ignition can beassessed, and gives guidance on the design and control parameters which can be used inorder to reduce such a risk.This standard can be used as a basis for the proper selection and installation of apparatus foruse in a hazardous area.60079-10 © IEC:2002– 4 –

60079-10  IEC:2002 – 9 –ELECTRICAL APPARATUS FOR EXPLOSIVE GAS ATMOSPHERES –Part 10: Classification of hazardous areas1 General1.1 ScopeThis part of IEC 60079 is concerned with the classification of hazardous areas whereflammable gas or vapour risks may arise, in order to permit the proper selection andinstallation of apparatus for use in such hazardous areas.It is intended to be applied where there may be a risk of ignition due to the presence offlammable gas or vapour, mixed with air under normal atmospheric conditions (see note 2),but it does not apply toa)mines susceptible to firedamp;b)the processing and manufacture of explosives;c)areas where a risk may arise due to the presence of ignitable dusts or fibres;d)catastrophic failures which are beyond the concept of abnormality dealt with in thisstandard (see note 3);e)rooms used for medical purposes;f)areas where the presence of flammable mist may give rise to an unpredictable risk andwhich require special consideration (see note 5);g)domestic premises.This standard does not take into account the effects of consequential damage.Definitions and explanations of terms are given together with the main principles andprocedures relating to hazardous area classification.For detailed recommendations regarding the extent of the hazardous areas in specificindustries or applications, reference may be made to the codes relating to those industries orapplications.NOTE 1
For the purpose of this standard, an area is a three-dimensional region or space.NOTE 2
Atmospheric conditions include variations above and below reference levels of 101,3 kPa (1 013 mbar)and 20 °C (293 K), provided that the variations have a negligible effect on the explosion properties of theflammable materials.NOTE 3
Catastrophic failure in this context is applied, for example, to the rupture of a process vessel or pipelineand events that are not predictable.NOTE 4
In any process plant, irrespective of size, there may be numerous sources of ignition apart from thoseassociated with electrical apparatus. Appropriate precautions will be necessary to ensure safety in this context.This standard may be used with judgement for other ignition sources.NOTE 5
Mists may form or be present at the same time as flammable vapours. This may affect the way flammablematerial disperses and the extent of any hazardous areas. The strict application of area classification for gasesand vapours may not be appropriate because the flammability characteristics of mists are not always predictable.Whilst it can be difficult to decide upon the type and extent of zones, the criteria applicable to gases and vapourswill, in most cases, give a safe result. However, special consideration should always be given to the danger ofignition of flammable mists.– 5 –60079-10 © IEC:2002

60079-10  IEC:2002 – 11 –1.2 Normative referencesThe following referenced documents are indispensable for the application of this document.For dated references, only the edition cited applies. For undated references, the latest editionof the referenced document (including any amendments) applies.IEC 60050(426):1990, International Electrotechnical Vocabulary (IEV) – Chapter 426:Electrical apparatus for explosive atmospheresIEC 60079-4:1975, Electrical apparatus for explosive gas atmospheres – Part 4: Method oftest for ignition temperatureIEC 60079-4A:1970, First supplement to IEC 60079-4 (1966), Electrical apparatus forexplosive gas atmospheres – Part 4: Method of test for ignition temperatureIEC 60079-20:1996, Electrical apparatus for explosive gas atmospheres – Part 20: Data forflammable gases and vapours, relating to the use of electrical apparatus2 Definitions and termsFor the purpose of this part of IEC 60079, the following definitions and terms apply.NOTE
Where a definition appears in this clause and in IEC 60050(426), the definition given in this clause isapplicable.2.1explosive atmospheremixture with air, under atmospheric conditions, of flammable substances in the form of gas,vapour, mist or dust, in which after ignition, combustion spreads throughout the unconsumedmixture[IEV 426-02-02, modified]2.2explosive gas atmospheremixture with air, under atmospheric conditions, of flammable substances in the form of gas orvapour in which, after ignition, combustion spreads throughout the unconsumed mixture[IEV 426-02-03, modified]NOTE
Although a mixture which has a concentration above the upper explosive limit (UEL) is not an explosivegas atmosphere, it can readily become so and, in certain cases for area classification purposes, it is advisable toconsider it as an explosive gas atmosphere.2.3hazardous areaarea in which an explosive gas atmosphere is present, or may be expected to be present, inquantities such as to require special precautions for the construction, installation and use ofapparatus[IEV 426-03-01, modified]2.4non-hazardous areaarea in which an explosive gas atmosphere is not expected to be present in quantities suchas to require special precautions for the construction, installation and use of apparatus[IEV 426-03-02, modified]60079-10 © IEC:2002– 6 –

60079-10  IEC:2002 – 13 –2.5zoneshazardous areas are classified into zones based upon the frequency of the occurrence andduration of an explosive gas atmosphere, as follows:2.5.1zone 0place in which an explosive atmosphere consisting of a mixture with air of flammablesubstances in the form of gas, vapour or mist is present continuously or for long periods orfrequently[IEV 426-03-03, modified]2.5.2zone 1place in which an explosive atmosphere consisting of a mixture with air of flammablesubstances in the form of gas, vapour or mist is likely to occur in normal operationoccasionally[IEV 426-03-04, modified]2.5.3zone 2place in which an explosive atmosphere consisting of a mixture with air of flammablesubstances in the form of gas, vapour or mist is not likely to occur in normal operation but, if itdoes occur, will persist for a short period only[IEV 426-03-05, modified]NOTE 1
In this definition, the word "persist" means the total time for which the flammable atmosphere will exist.This will normally comprise the total of the duration of the release, plus the time taken for the flammableatmosphere to disperse after the release has stopped. (The term "persistence time" as used in annex B refersspecifically to only one part of the total time for which the flammable atmosphere will exist.)NOTE 2
Indications of the frequency of the occurrence and duration may be taken from codes relating to specificindustries or applications.2.6source of releasepoint or location from which a flammable gas, vapour, or liquid may be released into theatmosphere in such a way that an explosive gas atmosphere could be formed[IEV 426-03-06, modified]2.7grades of releasethere are three basic grades of release, as listed below in order of decreasing frequency andlikelihood of the explosive gas atmosphere being present:a) continuous grade;b) primary grade;c) secondary grade.A source of release may give rise to any one of these grades of release, or to a combinationof more than one2.7.1continuous grade of releaserelease which is continuous or is expected to occur frequently or for long periods– 7 –60079-10 © IEC:2002

60079-10  IEC:2002 – 15 –2.7.2primary grade of releaserelease which can be expected to occur periodically or occasionally during normal operation2.7.3secondary grade of releaserelease which is not expected to occur in normal operation and, if it does occur, is likely to doso only infrequently and for short periods2.8release ratequantity of flammable gas or vapour emitted per unit time from the source of release2.9normal operationsituation when the equipment is operating within its design parametersNOTE 1
Minor releases of flammable material may be part of normal operation. For example, releases from sealswhich rely on wetting by the fluid which is being pumped are considered to be minor releases.NOTE 2
Failures (such as the breakdown of pump seals, flange gaskets or spillages caused by accidents) whichinvolve urgent repair or shut-down are not considered to be part of normal operation nor are they considered to becatastrophic.NOTE 3
Normal operation includes start-up and shut-down conditions.2.10ventilationmovement of air and its replacement with fresh air due to the effects of wind, temperaturegradients, or artificial means (for example, fans or extractors)2.11explosive limitsNOTE
The terms "explosive limit" and "flammable limit" are equivalent. IEC 60079-20 and IEC 61779-1 use theterm "flammable limit" whilst all the other standards use the more widely accepted term "explosive limit".2.11.1lower explosive limit (LEL)concentration of flammable gas or vapour in air, below which the gas atmosphere is notexplosive[IEV 426-02-09, modified]2.11.2upper explosive limit (UEL)concentration of flammable gas or vapour in air, above which the gas atmosphere is notexplosive[IEV 426-02-10, modified]2.12relative density of a gas or a vapourdensity of a gas or a vapour relative to the density of air at the same pressure and at thesame temperature (air is equal to 1,0)60079-10 © IEC:2002– 8 –

60079-10  IEC:2002 – 17 –2.13flammable material (flammable substance)material which is flammable of itself, or is capable of producing a flammable gas, vapour ormist2.14flammable liquidliquid capable of producing a flammable vapour under any foreseeable operating conditions2.15flammable gas or vapourgas or vapour which, when mixed with air in certain proportions, will form an explosive gasatmosphere2.16flammable mistdroplets of flammable liquid, dispersed in air so as to form an explosive atmosphere2.17flashpointlowest liquid temperature at which, under certain standardized conditions, a liquid gives offvapours in a quantity such as to be capable of forming an ignitable vapour/air mixture[IEV 426-02-14]2.18boiling pointtemperature of a liquid boiling at an ambient pressure of 101,3 kPa (1 013 mbar)NOTE
The initial boiling point that should be used for liquid mixtures is to indicate the lowest value of the boilingpoint for the range of liquids present, as determined in a standard laboratory distillation without fractionation.2.19vapour pressurepressure exerted when a solid or liquid is in equilibrium with its own vapour. It is a function ofthe substance and of the temperature2.20ignition temperature of an explosive gas atmospherelowest temperature of a heated surface at which, under specified conditions, the ignition of aflammable substance in the form of a gas or vapour mixture with air will occur[IEV 426-02-01, modified]NOTE
IEC 60079-4 and IEC 60079-4A standardize a method for the determination of this temperature.2.21extent of zonedistance in any direction from the source of release to the point where the gas/air mixture hasbeen diluted by air to a value below the lower explosive limit2.22liquefied flammable gasflammable material which is stored or handled as a liquid and which at ambient temperatureand atmospheric pressure is a flammable gas– 9 –60079-10 © IEC:2002

60079-10  IEC:2002 – 19 –3 Safety and area classification3.1 Safety principlesInstallations in which flammable materials are handled or stored should be designed, operatedand maintained so that any releases of flammable material, and consequently the extent ofhazardous areas, are kept to a minimum, whether in normal operation or otherwise, withregard to frequency, duration and quantity.It is important to examine those parts of process equipment and systems from which releaseof flammable material may arise and to consider modifying the design to minimize thelikelihood and frequency of such releases and the quantity and rate of release of material.These fundamental considerations should be examined at an early stage of the designdevelopment of any process plant and should also receive prime attention in carrying out thearea classification study.In the case of maintenance activities other than those of normal operation, the extent of thezone may be affected but it is expected that this would be dealt with by a permit-to-worksystem.In a situation in which there may be an explosive gas atmosphere, the following steps shouldbe taken:a)eliminate the likelihood of an explosive gas atmosphere occurring around the source ofignition, orb)eliminate the source of ignition.Where this is not possible, protective measures, process equipment, systems and proceduresshould be selected and prepared so the likelihood of the coincidence of a) and b) is so smallas to be acceptable. Such measures may be used singly, if they are recognized as beinghighly reliable, or in combination to achieve an equivalent level of safety.3.2 Area classification objectivesArea classification is a method of analysing and classifying the environment where explosivegas atmospheres may occur so as to facilitate the proper selection and installation of appa-ratus to be used safely in that environment, taking into account gas groups and temperatureclasses.In most practical situations where flammable materials are used, it is difficult to ensure that anexplosive gas atmosphere will never occur. It may also be difficult to ensure that apparatuswill never give rise to a source of ignition. Therefore, in situations where an explosive gasatmosphere has a high likelihood of occurring, reliance is placed on using apparatus whichhas a low likelihood of creating a source of ignition. Conversely, where the likelihood of anexplosive gas atmosphere occurring is reduced, apparatus constructed to a less rigorousstandard may be used.It is rarely possible by a simple examination of a plant or plant design to decide which parts ofthe plant can be equated to the three zonal definitions (zones 0, 1 and 2). A more detailedapproach is therefore necessary and this involves the analysis of the basic possibility of anexplosive gas atmosphere occurring.60079-10 © IEC:2002– 10 –

60079-10  IEC:2002 – 21 –The first step is to assess the likelihood of this, in accordance with the definitions of zone 0,zone 1 and zone 2. Once the likely frequency and duration of release (and hence the grade ofrelease), the release rate, concentration, velocity, ventilation and other factors which affectthe type and/or extent of the zone have been determined, there is then a firm basis on whichto determine the likely presence of an explosive gas atmosphere in the surrounding areas.This approach therefore requires detailed consideration to be given to each item of processequipment which contains a flammable material, and which could therefore be a source ofrelease.In particular, zone 0 or zone 1 areas should be minimized in number and extent by design orsuitable operating procedures. In other words, plants and installations should be mainlyzone 2 or non-hazardous. Where release of flammable material is unavoidable, processequipment items should be limited to those which give secondary grade releases or, failingthis (that is where primary or continuous grade releases are unavoidable), the releases shouldbe of very limited quantity and rate. In carrying out area classification, these principles shouldreceive prime consideration. Where necessary, the design, operation and location of processequipment should ensure that, even when it is operating abnormally, the amount of flammablematerial released into the atmosphere is minimized, so as to reduce the extent of thehazardous area.Once a plant has been classified and all necessary records made, it is important that nomodification to equipment or operating procedures is made without discussion with thoseresponsible for the area classification. Unauthorized action may invalidate the areaclassification. It is necessary to ensure that all equipment affecting the area classificationwhich has been subjected to maintenance is carefully checked during and after re-assemblyto ensure that the integrity of the original design, as it affects safety, has been maintainedbefore it is returned to service.4 Area classification procedure4.1 GeneralThe area classification should be carried out by those who have knowledge of the propertiesof flammable materials, the process and the equipment, in consultation, as appropriate, withsafety, electrical, mechanical and other engineering personnel.The following subclauses give guidance on the procedure for classifying areas in which theremay be an explosive gas atmosphere and on the extent of zones 0, 1 and 2. An example of aschematic approach to the classification of hazardous areas is given in figure C.1.The area classification should be carried out when the initial process and instrumentation linediagrams and initial layout plans are available and confirmed before plant start-up. Reviewsshould be carried out during the life of the plant.– 11 –60079-10 © IEC:2002

60079-10  IEC:2002 – 23 –4.2 Sources of releaseThe basic elements for establishing the hazardous zone types are the identification of thesource of release and the determination of the grade of release.Since an explosive gas atmosphere can exist only if a flammable gas or vapour is presentwith air, it is necessary to decide if any of these flammable materials can exist in the areaconcerned. Generally speaking, such gases and vapours (and flammable liquids and solidswhich may give rise to them) are contained within process equipment which may or may notbe totally enclosed. It is necessary to identify where a flammable atmosphere can exist insidea process plant, or where a release of flammable materials can create a flammableatmosphere outside a process plant.Each item of process equipment (for example, tank, pump, pipeline, vessel, etc.) should beconsidered as a potential source of release of flammable material. If the item cannot containflammable material, it will clearly not give rise to a hazardous area around it. The same willapply if the item contains a flammable material but cannot release it into the atmosphere (forexample, an all-welded pipeline is not considered to be a source of release).If it is established that the item may release flammable material into the atmosphere, it isnecessary, first of all, to determine the grade of release in accordance with the definitions, byestablishing the likely frequency and duration of the release. It should be recognized that theopening-up of parts of enclosed process systems (for example, during filter changing or batchfilling) should also be considered as sources of release when developing the areaclassification. By means of this procedure, each release will be graded either "continuous","primary" or "secondary".Having established the grade of the release, it is necessary to determine the release rate andother factors which may influence the type and extent of the zone.If the total quantity of flammable material available for release is "small", for example, labo-ratory use, whilst a potential hazard may exist, it may not be appropriate to use this areaclassification procedure. In such cases, account shall be taken of the particular risks involved.The area classification of process equipment in which flammable material is burned, forexample, fired heaters, furnaces, boilers, gas turbines etc., should take into account purgecycle, start-up and shut-down conditions.4.3 Type of zoneThe likelihood of the presence of an explosive gas atmosphere and hence the type of zonedepends mainly on the grade of release and the ventilation.NOTE 1
A continuous grade of release normally leads to a zone 0, a primary grade to zone 1 and a secondarygrade to zone 2 (see annex B).NOTE 2
Where zones created by adjacent sources of release overlap and are of different zonal classification, thehigher risk classification will apply in the area of overlap. Where overlapping zones are of the same classification,this common classification will normally apply.However, care needs to be taken where the overlapping zones relate to flammable materials which have differentapparatus groups and/or temperature class. So, for example, if a zone 1 IIA T3 area overlapped a zone 2 IIC T1area, then classifying the overlap as zone 1 IIC T3 may be over-restrictive but classifying it as zone 1 IIA T3 orzone 1 IIC T1 would not be acceptable. In this situation, the area classification should be recorded as zone 1 IIA T3and zone 2 IIC T1.60079-10 © IEC:2002– 12 –

60079-10  IEC:2002 – 25 –4.4 Extent of zoneThe extent of the zone depends on the estimated or calculated distance over which anexplosive atmosphere exists before it disperses to a concentration in air below its lowerexplosive limit. When assessing the area of spread of gas or vapour before dilution to belowits lower explosive limit, expert advice should be sought.Consideration should always be given to the possibility that a gas which is heavier than airmay flow into areas below ground level (for example, pits or depressions) and that a gaswhich is lighter than air may be retained at high level (for example, in a roof space).Where the source of release is situated outside an area or in an adjoining area, the pene-tration of a significant quantity of flammable gas or vapour into the area can be prevented bysuitable means such as:a) physical barriers;b) maintaining a sufficient overpressure in the area relative to the adjacent hazardous areas,so preventing the ingress of the explosive gas atmosphere;c) purging the area with sufficient flow of fresh air, so ensuring that the air escapes from allopenings where the flammable gas or vapour may enter.The extent of the zone is mainly affected by the following chemical and physical parameters,some of which are intrinsic properties of the flammable material; others are specific to theprocess. For simplicity, the effect of each parameter listed below assumes that the otherparameters remain unchanged.4.4.1 Release rate of gas or vapourThe greater the release rate, the larger the extent of the zone. The release rate depends itselfon other parameters, namelya) Geometry of the source of releaseThis is related to the physical characteristics of the source of release, for example, anopen surface, leaking flange, etc. (see annex A).b) Release velocityFor a given source of release, the release rate increases with the release velocity. In thecase of a product contained within process equipment, the release velocity is related tothe process pressure and the geometry of the source of release. The size of a cloud offlammable gas or vapour is determined by the rate of flammable vapour release and therate of dispersion. Gas and vapour flowing from a leak at high velocity will develop a cone-shaped jet which will entrain air and be self-diluting. The extent of the explosive gasatmosphere will be almost independent of wind velocity. If the material is released at lowvelocity or if its velocity is reduced by impingement on a solid object, it will be carried bythe wind and its dilution and extent will depend on wind velocity.c) ConcentrationThe release rate increases with the concentration of flammable vapour or gas in thereleased mixture.d) Volatility of a flammable liquidThis is related principally to the vapour pressure, and the enthalpy ("heat") of vaporization.If the vapour pressure is not known, the boiling point and flashpoint can be used as aguide.– 13 –60079-10 © IEC:2002

60079-10  IEC:2002 – 27 –An explosive gas atmosphere cannot exist if the flashpoint is above the relevant maximumtemperature of the flammable liquid. The lower the flashpoint, the greater may be theextent of the zone. If a flammable material is released in a way that forms a mist (forexample, by spraying) an explosive atmosphere may be formed below the flashpoint of thematerial, for example.NOTE 1
Flashpoints of flammable liquids are not precise physical quantities, particularly where mixtures areinvolved.NOTE 2
Some liquids (for example, certain halogenated hydrocarbons) do not possess a flashpoint althoughthey are capable of producing an explosive gas atmosphere. In these cases, the equilibrium liquid temperaturewhich corresponds to the saturated concentration at the lower explosive limit should be compared with therelevant maximum liquid temperature.e) Liquid temperatureThe vapour pressure increases with temperature, thus increasing the release rate due toevaporation.NOTE
The temperature of the liquid after it has been released may be increased, for example, by a hotsurface or by a high ambient temperature.4.4.2 Lower explosive limit (LEL)For a given release volume, the lower the LEL the greater will be the extent of the zone.Experience has shown that a release of ammonia, with an LEL of 15 % by volume, willdissipate rapidly in the open air, so an explosive gas atmosphere will normally be of negligibleextent.4.4.3 VentilationWith increased ventilation, the extent of the zone will normally be reduced. Obstacles whichimpede the ventilation may increase the extent of the zone. On the other hand, someobstacles, for example, dykes, walls or ceilings, may limit the extent. A compressor shelterwith a large roof-ventilator and with the sides open sufficient, to allow free passage of airthrough all parts of the building is considered well ventilated and should be treated as anoutdoor area (i.e. "medium" degree and "good" availability).4.4.4 Relative density of the gas or vapour when it is releasedIf the gas or vapour is significantly lighter than air, it will tend to move upwards. If significantlyheavier, it will tend to accumulate at ground level. The horizontal extent of the zone at groundlevel will increase with increasing relative density and the vertical extent above the source willincrease with decreasing relative density.NOTE 1
For practical applications, a gas or vapour which has a relative density below 0,8 is regarded as beinglighter than air. If the relative density is above 1,2, it is regarded as being heavier than air. Between these values,both of these possibilities should be considered.NOTE 2
With gases or vapours lighter than air, an escape at low velocity will disperse fairly rapidly upwards; thepresence of a roof will, however, inevitably increase the area of spread under it. If the escape is at high velocity ina free jet the action of the jet, although entraining air which dilutes the gas or vapour, may increase the distanceover which the gas/air mixture remains above its lower flammable limit.NOTE 3
With gases or vapours heavier than air, an escape at low velocity will tend to flow downward and maytravel long distances over the ground before it is safely dispersed by atmospheric diffusion. Special regardtherefore needs to be paid to the topography of any site under consideration and also to surrounding areas in orderto determine where gases or vapours might collect in hollows or run down inclines to lower levels. If the escape isat high velocity in a free jet the jet-mixing action by entraining air may well reduce the gas/air mixture to below itslower flammable limit in a much shorter distance than in the case of a low-velocity escape.NOTE 4
Care needs to be taken when classifying areas containing cryogenic flammable gases such as liquefiednatural gas. Vapours emitted can be heavier than air at low temperatures and become lighter than air onapproaching ambient temperature.60079-10 © IEC:2002– 14 –

60079-10  IEC:2002 – 29 –4.4.5 Other parameters to be considereda)Climatic conditionsThe rate of gas or vapour dispersion in the atmosphere increases with wind speed butthere is a minimum speed of 2 m/s – 3 m/s required to initiate turbulent diffusion; belowthis, layering of the gas or vapour occurs and the distance for safe dispersal is greatlyincreased. In plant areas sheltered by large vessels and structures, the speed of airmovement may be substantially below that of the wind; however, obstruction of airmovement by items of equipment tends to maintain turbulence even at low wind speeds.NOTE 1
In annex B (clause B.4), 0,5 m/s wind speed is considered to be appropriate for determining therates at which ventilation in an outdoor situation dilutes a flammable release. This lower value of wind speed isappropriate for that purpose, in order to maintain a conservative approach, even though it is recognized thatthe tendency of layering may compromise the calculation.NOTE 2
In normal practice the tendency of layering is not taken into account in area classification becausethe conditions which give rise to this tendency are rare and occur for short periods only. However, if prolongedperiods of low wind speed are expected for the specific circumstance then the extent of the zone should takeaccount of the additional distance required to achieve dispersion.b)TopographySome liquids are less dense than water and do not readily mix with water: such liquids canspread on the surface of water (whether it be on the ground, in plant drains or in pipetrenches) and then be ignited at a point remote from the original spillage, therefore puttingat risk a large area of plant.The layout of the plant, where possible, should be designed to aid the rapid dispersal ofexplosive gas atmospheres. An area with restricted ventilation (for example, in pits ortrenches) that would otherwise be Zone 2 may require Zone 1 classification; on the otherhand, wide shallow depressions used for pumping complexes or pipe reservations may notrequire such rigorous treatment.4.4.6 Illustrative examplesSome ways in which the above-mentioned parameters affect the vapour or gas release rateand hence the extent of the zone are demonstrated in the examples in annex C.a) Source of release: open surface of liquidIn most cases, the liquid temperature will be below the boiling point and the vapourrelease rate will depend principally on the following parameters:−liquid temperature;−vapour pressure of the liquid at its surface temperature;−dimensions of the evaporation surface;−ventilation.b) Source of release: virtually instantaneous evaporation of a liquid (for example, from a jetor spray)Since the discharged liquid vaporizes virtually instantaneously, the vapour release rate isequal to the liquid flow rate and this depends on the following parameters:−liquid pressure;−geometry of the source of release.Where the liquid is not instantaneously vaporized, the situation is complex becausedroplets, liquid jets and pools may create separate sources of release.– 15 –60079-10 © IEC:2002

60079-10  IEC:2002 – 31 –c) Source of release: leakage of a gas mixtureThe gas release rate is affected by the following parameters:−pressure within the equipment which contains the gas;−geometry of the source of release;−concentration of flammable gas in the released mixture.For examples, of sources of release, see clause A.2.5 Ventilation5.1 GeneralGas or vapour released into the atmosphere can be diluted by dispersion or diffusion into theair until its concentration is below the lower explosive limit. Ventilation, i.e. air movementleading to replacement of the atmosphere in a (hypothetical) volume around the source ofrelease by fresh air, will promote dispersion. Suitable ventilation rates can also avoidpersistence of an explosive gas atmosphere thus influencing the type of zone.5.2 Main types of ventilationVentilation can be accomplished by the movement of air due to the wind and/or bytemperature gradients or by artificial means such as fans. So two main types of ventilation arethus recognized:a) natural ventilation;b) artificial ventilation, general or local.5.3 Degree of ventilationThe most important factor is that the degree or amount of ventilation is directly related to thetypes of sources of release and their corresponding release rates. This is irrespective of thetype of ventilation, whether it be wind speed or the number of air changes per time unit. Thusoptimal ventilation conditions in the hazardous area can be achieved, and the higher theamount of ventilation in r
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