Protection PrinciplesEx

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Protection principles


BARTEC applies these protection principles to its different pieces of equipment according to theapplication for which they are going to be used. The industrial products of other manufacturers arealso equipped by BARTEC for use in potentially explosive atmospheres.

Overview

Protection principles are defined as principles which exclude systems, devices and components asignition sources.

Ignition sources which are caused by sparks from friction or impact or from electro-static charginghave to be prevented in explosion protected equipment by selecting appropriate materials and byconstructive measures, and this must be verified and confirmed by the appropriate tests.

Four protection principles can prevent equipment from becoming an ignition source. The types ofprotection listed as examples in the overview are discussed in a different section.

An important precondition for all the protection principles is that parts which are in unhindered contactwith the explosive atmosphere must not be able to reach non-permitted temperatures, which dependon the ignition temperature. This means that the ignition temperature is relevant for all protectionprinciples.

The protection principles can be equally applied to electrical and non-electrical devices and for gasesand for dusts. The principles allow for a design in various safety categories in accordance with theDirective 94/9/EC.

Category 1 - with maximum scope of protection and thus a very high degree of safetyCategory 2 - with increased scope of protection and therefore a high degree of safetyCategory 3 - with conventional scope of protection and therefore a conventional degree of safety

1. Explosive mixtures can penetrate the item ofequipment and be ignited. Measures are taken toensure that the explosion cannot spread to thesurrounding atmosphere.

2. The item of equipment is provided with anenclosure that prevents the ingress of anexplosive mixture and/or contact with sources ofignition arising from the normal.

3. Explosive mixtures can penetrate theenclosure but can not be ignited. Sparks andtemperatures capable of causing ignition must beprevented..

4. Explosive mixtures can penetrate theenclosure but can not be ignited. Sparks andtemperatures able to cause ignition may onlyoccur within certain limits.

Protection principles Flammable substances Types of protection Category

Gases Flammeproof enclosure 2Powder filling 2Enclosed break device 3

Gases and dust Pressurized apparatus 2Encapsulation 2

Gases Oil immersion 2Restricted breathing enclosure 3Non-incendive component 3Hermetically sealed device 3Sealed device 3Encapsulated device 3n-pressurization 3

Dust Protection by enclosure 2

Gases Increased safety 2Non-sparking device 3Protection by constructional safety 2

Gases and dust Energy limited circuit 3Gases Intrinsic safety 2

Protection by control of 3ignition sources

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Design regulations for explosion protected systems,devices and components - equipment

Hazards arising from the handling of flammable gases, vapours and dusts are caused by uniformchemical and physical processes. For this reason, the protection against these hazards must be carriedout in a uniform manner.Nearly universal uniform requirements have now been formulated by the International ElectrotechnicalCommission IEC, by the European Standardisation Committees CENELEC and CEN and by DKE andDIN.Manufacturers and operators are required to adhere to these, and where there are increased protectionrequirements, they are monitored by accredited test laboratories and the authorities.

Non-technical measuresThe requisite preconditions for the safe operation of electrical equipment in potentially explosiveatmospheres are created in a joint effort by the manufacturers of explosion protected equipment andthe constructors and operators of industrial plants. It is important that the operator of such plantsshould ensure that their personnel know how the danger of explosions is likely to arise and the measuresthat are to be taken to prevent it.

The employees should be regularly trained on the contents of the explosion protection document inaccordance with the Directive 1999/92/EC - in the Federal Republic of Germany implemented on thebasis of the "BetrSichV Betriebssicherheitsverordnung" (occupational safety regulations) - and informedby means of written corporate regulations which should be regularly updated. BARTEC as a specialistfor safety technology offers such consultation and training.

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RegulationsAn overview of the regulations for the determination of the parameters, the classification of zones, thedesign regulations for systems, devices and components as well as installation and operation in thearea where explosive gases, vapours and dusts are present, is shown in the table below whichcorresponds to the version as of April 2004 and may be subject to subsequent changes.


Title/Content Document ident. Document ident. Document ident.IEC CEN/CENELEC DINPublication date Publication date Publication date

Basics

Explosive atmospheres - Explosion prevention and protection - EN 1127-1 DIN EN 1127-1Part 1: Basic concepts and methodology 2007-11-00 2008-02-00

Explosive atmospheres - Explosion prevention and protection - EN 1127-2 DIN EN 1127-2Part 2: Basic concepts and methodology for mining 2002-04-00 2002-07-00

EN 1127-2/prA1 DIN EN 1127-2/A12007-11-00 2008-01-00

Explosive atmospheres - Explosion prevention and protection - EN 13237 DIN EN 13237Part 1: Terms and definitions for equipment and protective systems 2003-06-00 2003-11-00

intended for use in explosive atmospheres

Characteristics of combustible gases and vapours

Determination of the maximum explosion pressure and maximum rate - EN 13673-1 DIN EN 13673-1of pressure rise of gases and vapours - 2003-04-00 2003-09-00Part 1: Determination of the maximum explosion pressure

Determination of the maximum explosion pressure and maximum rate - EN 13673-2 DIN EN 13673-2und des maximalen zeitlichen Druckanstieges für Gas und Dämpfe 2005-09-00 2005-12-00Part 2: Determination of the maximum explosion pressure DIN EN 13673-2

Corrigendum 12007-08-00

Electrical apparatus for explosive gas atmosphere IEC 60079-1-1 - -Part 1-1: Flameproof enclosures "d"; Method of test for ascertainment 2002-07-00

for maximum experimental safe gap

Electrical apparatus for explosive gas atmosphere IEC/TR 60079-12 - -Part 12: Classification of mixtures of gases or vapours with air according to 1978-00-00

their maximum experimental safe gaps and minimum igniting currents

Electrical apparatus for explosive gas atmospheres IEC/TR3 60079-20 - -Part 20: Data for flammable gases and vapours, 1996-10-00

relating to the use of electrical apparatus

Electrical apparatus for explosive gas atmospheres IEC 60079-4 - -Part 4: Method of test for ignition temperature 1975-00-00

Electrical apparatus for explosive gas atmosphere IEC 60079-4 AMD1 - -Part 4: Method of test for ignition temperature; Amendment 1 1995-06-00

Testing of mineral oil hydrocarbons - - DIN 51794Determination of ignition temperature 2003-05-00

Characteristics of combustible dusts

Electrical apparatus for use in the presence of combustible dust IEC 61241-2-1 - -Part 2: Test methods - Section 1: Methods for determining 1994-12-00

the minimum ignition temperatures of dust

Electrical apparatus for use in the presence of combustible dust - EN 50281-2-1 DIN EN 50281-2-1Part 2-1: Test methods; Methods for determining 1998-09-00 1999-11-00

the minimum ignition temperatures of dust

Electrical apparatus for use in the presence of combustible dust; IEC/TR 61241-2-2 EN 61241-2-2 DIN EN 61241-2-2Part 2: Test methods - Section 2: method for determining the 1993-08-00 1995-08-00 1996-04-00

electrical resistivity of dust in layers IEC/TR 61241-2-2 Cor. 11994-05-00

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Types of protection for explosion protected electrical apparatus - electrical apparatus -

■ combustible gases and vapours

Electrical apparatus for explosive gas atmospheres IEC 60079-0EN 60079-0 DIN EN 60079-0Part 0: General requirements 2007-10-002006-07-00 2007-05-00

Electrical apparatus for explosive gas atmospheres IEC 60079-1 EN 60079-1DIN EN 60079-1Part 1: Flameproof enclosure "d" 2007-04-00 2007-07-002004-12-00DIN EN 60079-1Berichtigung 12006-09-00

Electrical apparatus for explosive gas atmospheres IEC 60079-2 EN 60079-2DIN EN 60079-2Part 2: Pressurised enclosure "p" 2007-02-00 2007-11-002005-02-00DIN EN 60079-2Berichtigung 12006-09-00

Electrical apparatus for explosive gas atmospheres IEC 60079-5 EN 60079-5DIN EN 50017Part 5: Powder filling "q" 2007-03-00 2007-11-002000-02-00

Electrical apparatus for explosive atmospheres IEC 60079-6 EN 60079-6 DIN EN 60079-6Part 6: Oil immersion "o" 2007-03-00 2007-05-00 2008-02-00

Electrical apparatus for explosive gas atmospheres IEC 60079-7 EN 60079-7 DIN EN 60079-7Part 7: Increased safety "e" 2006-07-00 2007-01-00 2007-08-00

Electrical apparatus for explosive gas atmospheres IEC 60079-11 EN 60079-11 DIN EN 60079-11Part 11: Intrinsic safety "i" 2006-07-00 2007-01-00 2007-08-00

Explosion-proof electro-technical apparatus IEC/TR 60079-13* - -Part 13: Construction and use of rooms or buildings 1982-00-00

protected by pressurization (report)

Electrical apparatus for potentially explosive gas atmospheres IEC 60079-15 EN 60079-15 DIN EN 60079-15Part 15: Type of protection "n" 2005-03-00 2005-10-00 2006-05-00

Electrical apparatus for potentially explosive atmospheres IEC/TR 60079-16* - -Part 16: Artificial ventilation for the protection of analyser(s) houses 1990-04-00

Electrical apparatus for potentially explosive atmospheres IEC 60079-18 EN 60079-18 DIN EN 60079-18Part 18: Encapsulation "m" 2004-03-00 2004-04-00 2005-01-00

Electrical apparatus for explosive atmospheres IEC 60079-25 EN 60079-25 DIN EN 50039Part 25: Intrinsically safe systems 2003-08-00 2004-01-00 2004-09-00

Electrical apparatus for potentially explosive atmospheres IEC 60079-26 EN 60079-26 DIN EN 60079-26Part 26: Special requirements for the construction, test and 2006-08-00 2007-03-00 2007-10-00

marking for electrical apparatus for use in Zone 0


Characteristics of combustible dusts

Electrical apparatus for use in the presence of combustible dust IEC 61241-2-3 - -Part 2: Test methods - Section 3: Method for determining 1994-09-00

minimum ignition energy of dust/air mixtures

Electrical apparatus for use in presence of combustible dust - prEN 61241-2-4 -Part 2: Test methods - Section 4: Method for determining 1993-05-00

the low explosive limit of dust/air mixtures

Classification of hazardous areas with combustible gases and vapours

Electrical apparatus for exlosive gas atmosphere IEC 60079-10 EN 60079-10 DIN EN 60079-10Part 10: Classification of hazardous areas 2002-06-00 2003-04-00 2004-08-00

Classification of hazardous areas with combustible dusts

Electrical apparatus for use in the presence of combustible dust IEC 61241-10 EN 61241-10 DIN EN 61241-10Part 3: Classification of areas where combustible dusts are or 2004-06-00 2004-07-00 2005-04-00

may be present

* Changes to the registration number can arise due to the comparison between IEC or ISO and CENELEC or CEN.

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Type of protection (explosion protection) - Non-electrical equipment -

■ combustible gases, vapours as well as dusts

Non-electrical equipment for potentially explosive atmospheres - EN 13463-1 DIN EN 13463-1Part 1: Basic methodology and requirements 2001-11-00 2002-04-00

EN 13463-1/AC DIN EN 13463-12002-07-00 Corrigendum 1pr EN 13463-1 2003-06-002007-03-00 DIN EN 13463-1

2007-05-00

Non-electrical equipment for potentially explosive atmospheres - EN 13463-2 DIN EN 13463-2Part 2: Protection by flow restricting enclosure "fr" 2004-11-00 2005-02-00

Non-electrical equipment for potentially explosive atmospheres - EN 13463-3 DIN EN 13463-3Part 3: Protection by flameproof enclosure "d" 2005-04-00 2005-07-00

Non-electrical equipment for potentially explosive atmospheres - - -Part 4: Protection by inherent safety

Non-electrical equipment for potentially explosive atmospheres - EN 13463-5 DIN EN 13463-5Part 5: Protection by constructional safety "c" 2003-12-00 2004-03-00

Non-electrical equipment for potentially explosive atmospheres - EN 13463-6 DIN EN 13463-6Part 6: Protection by control of ignition sources "b" 2005-04-00 2005-07-00

Non-electrical equipment for potentially explosive atmospheres - - -Part 7: Protection by ...

Non-electrical equipment for potentially explosive atmospheres - EN 13463-8 DIN EN 13463-8Part 8: Protection by liquid immersion "k" 2003-09-00 2004-01-00

■ combustible gases and vapours

Electrical apparatus for potentially explosive atmospheres IEC 60079-27EN 60079-27 DIN EN 60079-27Part 27: Fieldbus intrinsically safe concept (FISCO) and 2008-01-002004-03-00 2006-10-00fieldbus non-incendive concept (FNICO)

Part 28: Protection of equipment and transmission systems IEC 60079-28 EN 60079-28 DIN EN 60079-28using optical radiation 2006-08-00 2007-03-00 2007-10-00

Part 29-1: Gas detectors – Performance requirements of detectors IEC 60079-29-1 EN 60079-29-1DIN EN 60079-29-1for flammable gases 2007-08-00 2007-11-002006-07-00

Part 29-2: Gas detectors - Selection, installation, use and maintenance of detectors IEC 60079-29-2 EN 60079-29-2DIN EN 60079-29-2for flammable gases and oxygen 2007-08-00 2007-11-002007-11-00

Part 30-1: Electrical resistance trace heating IEC 60079-30-1 EN 60079-30-1 DIN EN 60079-30-1General and testing requirements 2007-01-00 2007-04-00 2007-12-00

Part 30-2: Electrical resistance trace heating IEC 60079-30-2 EN 60079-30-2 DIN EN 60079-30-2Application guide for design, installation and maintenance 2007-01-00 2007-04-00 2007-12-00

■ combustible dusts

Electrical apparatus for use in the presence of combustible dust IEC 61241-0 EN 61241-0 DIN EN 61241-0Part 0: General requirements 2004-07-00 2006-12-00 2007-07-00

IEC 61241-0 Cor. 12005-11-0

Electrical apparatus for use in the presence of combustible dust IEC 61241-1 EN 61241-1 DIN EN 61241-1Part 1: Protection by enclosures "tD" 2004-05-00 2004-06-00 2005-06-00

Electrical apparatus for use in the presence of combustible dust IEC 61241-4 EN 61241-4 DIN EN 61241-4Part 4: Type of protection "p" 2001-03-00 2006-12-00 2007-07-00

Electrical apparatus for use in the presence of combustible dust IEC 61241-11 EN 61241-11 DIN EN 61241-11Part 11: Protection by intrinsic safety "iD" 2005-10-00 2006-12-00 2007-07-00

IEC 61241-11 Cor. 12006-02-00

Electrical apparatus for use in the presence of combustible dust IEC 61241-18 EN 61241-18 DIN EN 61241-18Part 18: Protection by encapsulation "mD" 2004-08-00 2004-11-00 2005-07-00

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With the help of this table, the following contents listed in the title/contents column can be correlatedto the regional and national equivalents. The regional and national title does not need to correspondto the "world" title. (Please also note the footnote for the table!)

At BARTEC the design regulations are consistently applied for electrical equipment. Conformityis - after the completion of the development at BARTEC - checked by notified bodies, test laboratoriesof the IEC Ex-scheme as well as test laboratories of the European Community or by national testlaboratories, and compliance is monitored and realised using a quality assurance system for everypiece of equipment produced. During the routine test, safety-relevant requirements are checkedaccording to the specifications and confirmed by means of a marking.

BARTEC also supports its customers with non-electrical equipment using the knowledge it hasaccumulated over decades of experience.


Note about how to use the tableThe information is based on the IEC titles, in cases where there is no IEC document available, the EN titles have been used.The format of the date has been standardised. It refers to the state on 2008-03-31 which was accessible to the author. This style seems to be becoming universallyaccepted, but has not been introduced in all the documents.The table is to provide an information overview of the standard. For concrete work with the standards and their procurement, the latest update should be requested fromthe publisher or from the standardisation committee.


Explosion protection in plants: Installation, maintenance and repair

Electrical apparatus for explosive gas atmospheres IEC 60079-14EN 60079-14 DIN EN 60079-14Part 14: Electrical installations in hazardous areas (other than mines) 2007-12-002003-08-00 2004-07-00

Electrical apparatus for use in the presence of combustible dust IEC 61241-14 EN 61241-14 DIN EN 61241-14Part 14: Selection and installation 2004-07-00 2004-09-00 2005-06-00

Electrical apparatus for explosive gas atmospheres IEC 60079-17 EN 60079-17 DIN EN 60079-17Part 17: Inspection and maintenance of electrical installations in 2007-08-00 2007-09-00 2004-06-00

hazardous areas (other than mines)

Electrical apparatus for explosive gas atmospheres IEC 60079-19 EN 60079-19 DIN EN 60079-19Part 19: Repair and overhaul for apparatus used in explosive 2006-10-00 2007-07-00 2008-02-00

atmospheres (other than mines or explosives)

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Relevance and advantage of the classification in zones

The practice has been established of dividing potentially explosive atmospheres into zones. Thisclassification takes the different dangers from explosive atmospheres into account and allows explosionprotection measures to be taken which reflect the situation both from the point of view of safetyengineering and of economic efficiency. For the European community, the zone definitions are uniformlyprovided in Directive 1999/92/EC. It must be applied with technical understanding of the specificsituation.

IEC 60079-10 assumes an approximately similar classification for gases and vapours which will alsoapply to future facilities constructed in accordance with the US standard NEC 505. IEC 61241-10provides support for the zone classification with dusts.

Potentially explosive atmospheres are classified depending on the frequency and duration of theexplosive atmosphere.

This classification provides the scope of the measures to be taken according to Annex II section A inthe Directive 1999/92/EC in conjunction with Annex I of the Directive 94/9/EC.

Classification of hazardous places

Notes:1. Layers, deposits and heaps of combustible must be considered as any other source which can form an explosive

atmosphere.2. 'Normal operation' means the situation when installations are used within their design parameters.3. The definitions comply with the EC directive. In the HSE occupational safety regulations, hazardous explosive

atmospheres are defined.In according with EN 1127-1Section 3.17 defines potentially explosive atmospheres as follows: Mixture of air and flammable gases, vapours,fogs and dusts under atmospheric conditions due to a transmission of the burning process to the entire unburnedmixture after inflammation.Section 3.19 defines hazardous potentially explosive atmospheres as follows: Potentially explosive atmospherewhich will lead to damage in case of explosion.

Zone 0 A place in which an explosive atmosphere consisting of a mixture with air of flammable substances inthe form of gas, vapour or mist is present continuously or for long periods or frequently.

Zone 1 A place in which an explosive atmosphere consisting of a mixture with air of flammable substances inthe form of gas, vapour or mist is likely to occur in normal operation occasionally.

Zone 2 A place in which an explosive atmosphere consisting of a mixture with air of flammable substances inthe form of gas, vapour or mist is not likely to occur in normal operation but, if it does occur, willpersist for a short period only.

Zone 20 A place in which an explosive atmosphere in the form of a cloud of combustible dust in air is presentcontinuously, or for long periods or frequently.

Zone 21 A place in which an explosive atmosphere in the form of a cloud of combustible dust in air is likelyto occur in normal operation occasionally.

Zone 22 A place in which an explosive atmosphere in the form of a cloud of combustible dust in air is notlikely to occur in normal operation but, if it does occur, will persist for a short period only.

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In places of work the potentially explosive atmospheres are normally classified at most as zone 1 and2 and/or 21 and 22. Zones 0 and 20 are restricted to very small inaccessible areas in work places orare usually restricted to the inside of technical equipment.

Explosion parameters

In order to allow a combination of measures for explosion protection, which is optimised with respectto the chemical-physical properties of the flammable gases, vapours or dusts, to be made, and thereforea standardisation of the types of protection to be possible for the manufacturer, a system of explosionparameters has been created. These are determined using an application-orientated test method.

Before flammable substances can react with the atmospheric oxygen in an explosion, energy must beprovided.

This energy may, for example, be exchanged on a surface. A heated surface increases the energycontent of the explosive mixture in contact with it. If the surface temperature is sufficiently high, thisincreased energy content can lead to the explosive reaction. However, the energy may also be suppliedthrough a spark or a hot gas jet flowing out of a gap into the explosive mixture. Both types lead todifferent explosion parameters being defined.

Zone 0

Zone 1

Zone 2

Zone 20

Zone 21

Zone 22

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Ignition temperature - temperature class

Many factors such as size, shape, type and surface quality have an influence on the ignition temperature.IEC, CENELEC and other standardisation committees have agreed on a method for gases and vapoursdefined in IEC 60079-4 "Method of test ignition temperature". This method is defined in such a way,that a value very close to the lowest practically possible, is determined.

By means of this method, gases and vapours are divided into temperature classes. According to thesetemperature classes, the surface temperatures in explosion protected equipment and other technologicalobjects is designed in such a way that ignition by the surface is not possible. In the standard, permissibleexcess values and necessary safety margins below these standard values are defined in detail.

For different types of dust, the method for determining the ignition temperature has also been unifiedand coded in document IEC 61241-2-1. Please note that dust in its deposited form - determined usingprocedure A - has a different ignition temperature than in its stirred form - determined as a cloudusing procedure B.

The permissible surface temperature for those parts of the systems, devices and components accessibleto the dust is determined by subtracting 75 K from the value determined using procedure A and bymultiplying by 2/3 the value determined using procedure B. The smaller of the 2 values determined inthis way corresponds to the lowest permissible surface temperature of the equipment. The surface isthe area accessible to the dust, temperature classes are not defined for dust, so that a concrete type ofdust must always be assumed. The parameters are made available in comprehensive tables, laboratoriesdetermine the values on request, and a small, non-official overview is contained in the followingtable.


Temperature Ignition temperature Permissible surface temperatureclasses range of the mixture of the electrical equipment

T1 > 450 °C 450 °C

T2 > 300 ... < 450 °C 300 °C

T3 > 200 ... < 300 °C 200 °C

T4 > 135 ... < 200 °C 135 °C

T5 > 100 ... < 135 °C 100 °C

T6 > 85 ... < 100 °C 85 °C

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Examples of the ignition temperatures of different types of dust

Designation A values B values Permissible limiting temperatureof the solid ignition temperature ignition temperature lowest value of the calculation (A-75K) and 2/3*Bmaterial IEC 61241-2-1 IEC 61241-2-1

procedure A procedure B 450... 300... 280... 260... 230... 215... 200... 180... 165... 160...deposit (°C) cloud (°C) > 300 > 280 > 260 > 230 > 215 > 200 > 180 > 165 > 160 > 135

Dust from natural materials (examples)

Cotton 350 560 275Brown coal 225 380 150Cellulose 370 500 295Cereals 290 420 215Wood resin 290 500 215Sawdust (wood) 300 400 225Cocoa 460 580 385Copra 290 470 215Cork 300 470 225Fodder concentrate 295 525 220Linen 230 440 155Milk powder 340 440 265Paper 300 540 225Pectin sugar 380 410 273Soya 245 500 170Starch 290 440 215Hard coal 245 590 170Tobacco 300 450 225Tapioca 290 450 215Tea 300 510 225Peat 295 360 220Wheat flour 450 480 320Sugar beet 290 460 215

Dust of chemical technical products (examples)

Cellulose ether 275 330 200Isosorbide dinitrate 240 220 146Unvulcanised rubber 220 460 145Petroleum coke 280 690 205Polysaccharide deriv. 270 580 195Polyvinyl acetate 340 500 265Polyvinyl chloride 380 530 305Soot 385 620 310Laminated plastic 330 510 255Sulphur 280 280 186

Metal dusts (examples)

Aluminium 280 530 205Bronze 260 390 185Iron 300 310 206Copper silicon alloy 305 690 230Magnesium 410 610 335Manganese 285 330 210Zinc 440 570 365

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Minimum ignition current ratio - Maximum experimentalsafe gap – Explosion sub-groupIgnition on a hot surface occurs in a relatively large „macroscopic“ part of the mixture. In contrast,the ignition from a spark spreads in a relatively small „microscopic“ part of the volume. The dischargefrom a capacitor or the interruption of a predefined resistive/inductive electric circuit can be used forclassifying gases and vapours or dusts according to their ease of ignition in the microscopic part ofthe mixture volume.

For the assessment of the ignition of gases and vapours in a circuit using a device defined inIEC 60079-11, a comparative value with methane as reference in a standardised circuit is used. Thiscomparative value is the minimum ignition current ratio, MIC. It is the means used for classifyinggases and vapours within explosion group II in the subgroups IIA, IIB and IIC.

An analogous grading is done when the ignitability of a hot gas jet escaping from a gap is used for theclassification. In IEC 60079-1-1 „Method of test for ascertainment of the experimental safe gap“, atest apparatus is agreed in which a spherical gas volume of 20 cm³ is formed by two hemispheres.These have a 25-mm wide flange. This ball-shaped object is placed into a larger vessel and bothspaces are filled with the mixture for which the safe gap is to be determined. The gap between the25 mm wide flanges for which ten ignitions inside the ball volume just fail to ignite the mixture in theouter vessel is a value specific to the mixture and is called the maximum experimental safe gap,MESG.

The processes involved in the prevention or spread of the explosion in the gap are very complex.Classifying the gases and vapours by the safe gap results approximately - with a small overlap - in thesame classification as that obtained with the minimum ignition current ratio. IEC/TR 60079-12 providesan overview of the classification using the two measuring methods MESG and MIC.

The safe gap value is of considerable importance for designs of protection type „Flameproof enclosure“;the value for the minimum ignition current ratio is important for those of protection type „Intrinsicsafety“. For these two types of protection, the subgroups IIA, IIB and IIC for gases and vapours arerelevant. The information on gases and vapours can also be applied approximately to mists.

For the assessment of conditions concerning electrostatic discharge, the minimum ignition energyof gases and vapours from the assignment to sub-group IIA, IIB or IIC can be assumed:

IIA approx. 300 µWsIIB approx. 50 µWsIIC < 50 µWs

For dust, standards for the determination of parameters are already available or are in preparation.

The minimum ignition energy, a parameter similar to the minimum ignition current, is determined inaccordance with IEC 61241-2-3 for flammable dusts.

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The following table shows examples of the assignment of gases and vapoursto the respective temperature classes and explosion sub-groups:

Explosion groups Classification according to MESG

IIA IIB IIC> 0.9 mm ≤ 0.9... ≥ 0,5 mm < 0.5 mm

T1 > 450 °C Acetone Town gas HydrogenAmmoniaBenzene - pureAcetic acidEthaneEthyl acetateEthyl chlorideCarbon monoxideMethaneMethanolMethylene chlorideNaphthalenePhenolPropaneToluene

T2 > 300 ... ≤ 450 °C Ethyl alcohol Ethylene Ethine (acetylene)i amyl acetate Ethylene oxiden butanen butyl alcoholCyclohexane

Acetic anhydride

T3 > 200 ... ≤ 300 °C Petroleum spirit - gen. Ethylene glycolDiesel fuel Hydrogen sulphideJet propulsion fuelHeating fuel DIN 51603n hexane

T4 > 135 ... ≤ 200 °C Acetaldehyde Ethyl ether

T5 > 100 ... ≤ 135 °C

T6 > 85 ... ≤ 100 °C Carbon bisulphide

Temperatureclass

Ignitiontemperature

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