PRINCIPLES OF

EXPLOSIONPROTECTION

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Contact Details .......................................................................................................................................... 19Definitions....................................................................................................................................... 1Explosion proof apparatus................................................................................................................ 14Hazard zone classifications ................................................................................................................ 4Installations................................................................................................................................................ 15Introduction .................................................................................................................................... 1Maintenance ................................................................................................................................. 17Primary and secondary explosion protection.................................................................................. 2Temperature classes.......................................................................................................................... 13Types of protection for Zone............................................................................................................... 5Types of protection for Zone 2......................................................................................................... 11

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1. IntroductionWe associate the word explosion with an unintended event, the effects of which are beyondhuman control, and which may cause injury and damage. The danger of an explosion isalways present where combustible materials are being handled and where there is a sourceof ignition. Many industrial plants in which combustible materials are used have electricmotors to drive pumps, fans and conveyors, thermostats and pressure switches to controlprocesses, and electrical heating to heat products. Since every apparatus is a potentialdanger to employees and their environment, the authorities impose measures to prevent therisk of ignition. The fact that explosions seldom occur, despite countless explosion-hazardzones, proves that the safety measures taken are effective and successful.

2. Definitions2.1 CENELEC standardsStandards for explosion-proof electrical apparatus are issued by the European countries(Belgium, Denmark, Germany, Finland, France, Ireland, Italy, Holland, Norway, Austria,Portugal, Sweden, Spain and Great Britain). Members of CENELEC undertake to comply withthe conditions laid down in the standards, according to which the European standard isaccepted as national standard without alteration. This stipulation des not imply that theresults of the European testing institutes must be mutually recognized.

2.2 Electrical ApparatusAll items used as a whole or in part for the application of electrical energy. These include theitems for the generation, transmission, distribution, storage, measurement, regulation,conversion and consumption of electrical and items for telecommunications. Electricalapparatus for potentially explosive areas is divided into <<Group I>> and <<Group II>>.

2.2.1 Group IElectrical apparatus for mines susceptible to firedamp, with a maximum surface temperatureof 150°C, where coal dust can deposit to build up a layer. If there is no risk of depositsforming, a surface temperature of 450°C is permitted.

2.2.2 Group IIElectrical apparatus used in explosion-hazard zones (excluding Group I). Concerning surfacetemperature, the stipulations of section 5 <<Temperature classes>> apply.

2.2.3 Differences from normal designCompared with the electrical apparatus that is not explosion-proof, explosion-proof apparatusmust satisfy additional requirements such as:

Higher IP degree of protection; Use of tested junction boxes and cable entries (if plastic fittings are selected; Additional costly testing:

Testing of flameproof enclosure for explosion proofing and non transmission of internalignition.Temperature rise testing on junction boxes of the <<increased safety EExe>> type;

Additional monitoring devices to prevent excessively high temperatures, sparks andarcs.

2.3 <<Ex>>-componentsPart of an electrical apparatus for potentially explosive atmospheres that may not be usedalone in such atmospheres and which requires an additional certification in conjunction withany electrical apparatus together with which it was used.

2.4 Types of protection

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Modifications applied to electrical apparatus during manufacture in order to prevent theapparatus from igniting an ambient atmosphere that is potentially explosive.

2.5 Explosion-hazard zoneZone whose atmosphere could become explosive (the hazard is potential).

2.6 Explosive atmosphereA mixture with air, under atmospheric conditions, of flammable substances in the form gas,vapour or mist, in such proportions that it can be exploded by excessive temperature, arcs orsparks (the hazard is real).

2.7 Explosive mixtureA specific mixture used for testing electrical apparatus for explosion-hazard zones.

2.8 ExplosionIndependent propagation of the flame in an explosive mixture.

2.9 Maximum surface temperatureThe highest temperature attained under the most unfavourable conditions by any part orsurface of an electrical apparatus. The most unfavorable conditions include the admissibleoverloads and fault conditions specified in the standard for the type of protection concerned.The maximum surface temperature of an electrical apparatus must always be lower than theignition temperature of the gas or vapour mixture in which the apparatus is used.

If the heating surfaces are insulated, the applicable maximum surface temperature for theassessment is always the relevant heating surface. Normally the insulation is not gas-tight,and this might lead to an explosion in the insulation. Spaces in which steam pipes and thelike pass through the explosion-hazard zone are subject to the same requirements.

3. Primary and secondary explosion protection3.1 Primary explosion protectionA combustion reaction requires fuel, oxygen and, to trigger it, a certain amount of additionalenergy, known as the ignition source. Together, these three components form the hazardtriangle. If on of the three conditions is lacking, no combustion or explosion can occur.

3.1.1 FuelThe rate at which the combustion reaction is independently propagated in the mixturedepends on the concentration of the vapour/air or gas/air mixture. A distinction is madebetween an explosion and a deflagration on the basis of the rate of combustion. The ignitionrange covers the mixture concentration span within which an explosion can be triggered bymeans of an ignition source. Primary explosion proofing is a matter of keeping below thelower ignition limit. In this region the mixture is too weak for an explosion to be triggered.

The ignition range is between a lower and an upper ignition limit. Below the lower ignition limitthe concentration of combustible gases and vapours (in %vol or g/m

3) is too low to cause

combustion or explosion. Above the upper limit the mixture is too rich in combustible fractionsfor an explosion to take place.

3.1.2 OxygenOxygen is a gas that must be present in sufficient quantity for an explosion or a fire to takeplace.

3.2 Protective measuresPrimary explosion proofing includes the following protective measures:

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3.2.1 Replacing the combustible substanceIt often possible to replace the combustible substance with a material that is eitherincombustible or incapable of forming a potentially explosive atmosphere. The main suitablesubstitutes are:

Aqueous solut ions Incombustible halogenated hydrocarbons, and Incombustible materials

3.2.2 Inserting the apparatusInserting the apparatus means replacing part of the oxygen in the air in a restricted volumewith inert gases. Most fuel/air mixtures are no longer capable of ignition when the oxygencontent is below 8%vol (for hydrogen and carbon monoxide below 4%vol). The commerciallyavailable gases nitrogen and carbon dioxide are generally used for inserting. The oxygen isdisplaced in two stages:1.) Purging of the container or installation before the operation or process begins, for

example by evacuation followed by replacing the vacuum with nitrogen;2.) Maintaining the low oxygen concentration achieved by purging by making up any losses

of inert gas during the operation or process.

Unless the inerting of the production equipment or tanks is ensured by the process condition,it must be checked and monitored with oxygen measuring devices, for example.

3.2.3 Use of sealed systemsInstallations designed as sealed systems, in which combustible materials are present, havethe advantage that no gases and vapours an escape.Measures for achieving sealed systems are:

Continuous processes Gas compensation pipe Pressure equalization at a safe place in the open Entry through air locks.

3.2.4 Ventilation measuresVentilation measures can have the effect of greatly reducing the range of potentiallyexplosive atmosphere in the vicinity of installations, apparatus and the like.Ventilation can be provided in various ways:

Natural ventilation Artificial ventilation, e.g. room ventilation and Extraction at source

Artificial ventilation is required: When handling or processing combustible materials that may form a potentially

explosive atmosphere in a non-enclosed system and For the storage of combustible liquids with a flash point below 30°C and the

heavier-than-air combustible gases in underground rooms.Artificial ventilation is necessary because it provides a higher throughput and a more focusedairflow and natural ventilation.

Extraction at source is generally preferable to artificial room ventilation because it is moreeffective and less costly. This applies especially when dealing with combustible dusts.

Ventilation measures that are necessary to protect health often also satisfy the requirementsof explosion proofing.

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3.2.5 Concentration monitoringDepending on local conditions, leaks can be detected at an early stage by monitoring theconcentration of combustible gases, vapous and mists in the vicinity o possible sources ofrisk (leakage points), so that the necessary safety measures can be implementedimmediately, such as emergency ventilation, fail-safe shutdown of the plant etc.

3.3 Secondary explosion protectionGenerally, primary explosion protection cannot be completely achieved at all. Steps musttherefore be taken to prevent the ignition of potentially explosive atmospheres. Ignitionsources may results from, among other things, excessive surface temperatures, electricsparks or exothermic reactions (heat released during chemical reactions). Sparks andexcessive temperatures can be avoided by design measures.4. Hazard zone classifications

All areas n which, because of the local and operational conditions, potentially explosiveatmospheres may occur in hazardous quantity are deemed explosion-hazard zones.According to the probability, in terms of time and location, of the presence of potentiallyexplosive atmospheres, potentially explosive areas are divided into zones which allowdifferential evaluation of the explosion hazard. It is the responsibility of the relevantauthorities to apply the appropriate rules. The rules cannot be applied to combustible dusts orfibers, nor to the premises used for medical purposes.

Since the concentrations decrease with increasing distance from the source of risk thelocation of the source of risk is of the greatest importance for the zone classification. Theterm <<source of risk>> is understood to mean the place at which the combustible gases,vapours, mists and liquids arise or emerge, or at which potentially explosive mixtures mayform.

4.1 Zone 0Area in which hazardous, potentially explosive atmospheres occur continuously, for longperiods, or for short periods that recur frequently.

4.2 Zone 1Area in which hazardous, potentially explosive atmospheres are likely to occur occasionallyor periodically. This the classical field of application for explosion proof apparatus.

4.3 Zone 2Area in which hazardous, potentially explosive atmospheres occur only seldom, and thenonly for short periods (max. 2 hours).

4.4 Factors affecting zone classificationNo generally applicable definition of hazardous areas can be given, since they are affectedby a variety of factors. The following factors affect the zone classification of a potentiallyexplosive area:1.) The properties of the combustible materials2.) The quantity of emerging gases, vapours, mists and liquids3.) The nature of the source of the danger4.) The nature and effectiveness of the ventilation5.) The prevention of the propagation of gases mists and vapours.6.) The promotion of the propagation of gases, mists and vapours by a current of heated air.

7.) Past experience8.) Maintenance and the internal organization (trained personnel)

5. Types of protection for Zone 0

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In zone 0, only apparatus may be used that is specially approved for this zone. Normallythese pieces of apparatus have two mutually independent protection types (such asflameproof enclosures and intrinsically safe power supply). Intrinsically safe circuits mustcomply with the category <<ia>>. The reference to zone 0 must appear on the nameplate(EEx d [ia] IIC T6 Zone 0).

5.1 Intrinsic safety EN 50 020A piece of electrical apparatus is intrinsically safe if all circuits are intrinsically safe. A circuit isintrinsically safe if its amperage and voltage are limited I such a way (function of inductance,capacitance and resistance) that no sparks or thermal effects can occur in it. The energy ofsuch a circuit is lower than the minimal ignition energy required igniting an explosive mixture.For this type of protection, classes A, B and C are prescribed on the basis of theexperimentally established minimum ignition current (MIC [see section 6.4.1])

In the case of intrinsic safety <<i>>, a distinction is made between categories <<ia>> and<<ib>>. Category <<ia>> is required to meet more stringent requirements in the event ofdefects occurring and combinations of such defects.The following safety factors apply:

1.5 In norm al operation and with one defect1.0 With two defects

If assignment to temperature classes is done according to surface temperature, a safetyfactor of 1.0 is applied.Intrinsically safe connections and conductors must be marked (if color-coded light blue only,RAL 5012).

6. Types of protection for Zone 1

Explosion proof apparatus can be designed according to the various types of protection.Those most frequently used are:1.) Flameproof enclosure EEx d

(Heaters, switches, motors, contactors etc),2.) <<increased safety>> EEx e (Terminals,

connection housing, motors, some heaters)3.) Intrinsic safety EEx i

(Measurement and control apparatus, monitoring devices)

Explosion proof apparatus often has several degrees of protection. The most frequentcombination is an enclosure in the form of a flameproof enclosure, and a junction box of<<increased safety>> design. The advantage of this combination is that unauthorizedpersons cannot tamper wit the flameproof part, since the connections are separatelyarranged.

6.1 Basic reconditions of EN 50 014The conditions and stipulations laid down in CENELEC Standard EN 50 014 apply withoutrestriction to all permitted types of ignition protection.

6.2 Flameproof enclosure <<d> EN 50 018EEx d IIC, EEx d IIB and EEx d IIA

Apparatus and components that may ignite a potentially explosive atmosphere are housed inan enclosure which, in the event of the explosion of an imitable mixture inside the enclosure,withstands the pressure and prevents propagation of the explosion to the potentiallyexplosive atmosphere surrounding the enclosure (spark ignition). It is assumed that apotentially explosive mixture cannot be prevented from entering.

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The concept of a flameproof enclosure thus takes into account the possibility of an explosioninside the enclosure, but it is essential that the explosion be restricted to the enclosure space.The following requirements guarantee the necessary safety:a.) The enclosure must withstand 1.5 times the explosion pressure, andb.) There must be no spark ignition.

6.2.1 Explosion pressureThe strength of the enclosure is checked by determining the explosion pressure. In practice,the maximum explosion pressure is never reached, since losses arise owing to thermalconduction and radiation. In addition, the losses are highly dependent on the shape of theenclosure, on any apparatus contained within the enclosure and on the ignition location. Inorder to obtain corresponding safety factor for the strength of the enclosure, the initialpressure of the gas mixture is raised and the explosion pressure rises proportionally.The explosion pressure must be determined experimentally since those loses cannot bedetermined by calculation. Besides, explosions vary in so many ways it is impossible togeneralize about them. However, measurement technology makes it possible to measure andrecord the rapidly changing pressures. The free volume of a symmetrical enclosure only hasa slight influence on the explosion pressure that may be expected. This effect ceases to existaltogether for enclosures with a volume of more than 5000cm

3.

6.2.2 Ignition penetrationThe gap lengths <<l>> and widths <<w>> occurring at the joints of an enclosure is decisivefor its resistance to ignition penetration. The gap length can easily be determined bymeasurement or by the difference in diameter. It is not affected by the explosion pressure.The gap widths, on the other hand, may be influenced by the explosion pressure. The safetyof the design is not only a matter of maintaining and checking the gaps of an enclosurewithout pressure; in addition, the largest gap occurring in the event of an explosion inside theenclosure must be determined. Deflection of enclosure walls, flanges, etc., may cause thegap to exceed the permitted values.

The safe gap widths are known for the usual gases. By using one of these usual gases it ispossible to determine whether the gap occurring under explosion pressure is exceeded.On the basis of the experimentally determined safe gap (MESG), enclosures are divided intogroups A, B and C. This subdivision determines the allocation of the individual media (gasmixtures) the requirements to be satisfied by the design increase in the order of the letters.most manufactures of flameproof equipment design for the strictest requirements in order tocover all media with one design, since it is uneconomic to have different apparatus in seriesproduction.

Current international standards distinguish between design in which gaps occur on the onehand without threads and on the other hand with threads.

Gap without threads for enclosures with volume up to 2000 cm3:

Group IIA IIB IIC

Gap length l 12.5 mm 12.5 mm 12.5 mmGap width w 0.3 mm 0.2 mm 0.15mm

For thread gaps, there are minimum requirements as to pitch, quality, number of active turnsof thread and screw-in depth. Only metric ISO threads are permitted.

6.2.3 Tests

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In type testing, an explosive mixture is ignited inside the enclosure, and the resultingexplosion pressure is measured. The number of tests and the gas mixtures are laid down inEN 50 018 and are mandatory.

For electrical apparatus of group IIA:3 tests with (4.6 ± 0.3%) propane;

For electrical apparatus of group IIB:3 tests with (8.0 ± 0.5%) ethylene;

For electrical apparatus of group IIC: 5tests with (14 ± 1%) acetylene and 5tests with (31 ± 1%) hydrogen;

The manufacturer must subject apparatus with <<flameproof enclosure>> protection toroutine test. The routine tests laid down for the flameproof enclosure include an overpressuretest carried out in accordance with the prescribed procedure. The purpose of the routine testis to ensure that, on the one hand, the enclosures, pipes, etc., withstand the pressure, and onthe other hand, that there are no holes cracks that provide an uncontrolled connection to theoutside.

6.3 <<Increased safety e>> protection EN 50 019(EEx e II)

With this degree of protection, special measures are taken to ensure an increased degree ofsafety and to prevent the occurrence of unacceptably high surface temperatures and ofsparks r arcs inside or on the external parts of electrical apparatus. The basic differencebetween <<increased safety>> protection and <<flameproof enclosure>> is that the formertotally precludes any ignition sources and thus an explosion. Particular importance isattached to compliance with the maximum surface temperature requirement (see section 8Temperature classes) and to clearances and creepage paths.

Enclosures and junction boxes that contain bare live parts must conform to protection type IP54. Enclosures and junction boxes that contain only insulated parts may be made toprotection type IP 44.

6.3.1 Creepage pathsThe creepage path is the shortest distance between two conducting paths along an insulatingsurface. The creepage paths between conducting parts of different potential must satisfy therequirements listed in the table below. Grooves in the surface of insulating parts may only beincluded in measurement of creepage distances if they are at least 2.5mm deep and at least2.5mm wide.

The dimensioning of creepage paths depends on the service voltage, the tracking resistanceof the insulation and the insulation’s contours. Electrical insulating materials are ratedaccording to their comparative tracking index (CTI) under IEC 112. Because inorganicinsulating materials such as glass and ceramics do not leave any creepage current traces,determinations of the CTI can be omitted. These materials are normally assigned to MaterialClass I.

Minimum creepage path [mm]Service voltage, U[Volts] Materials Class

I II IIIaU ~ 15 1.6 1.6 1.6

15 < U ~ 30 1.8 1.8 1.830 < U ~ 60 2.1 2.6 3.460 < U ~ 110 2.5 3.2 4.0

110 < U ~ 175 3.2 4.0 5.0

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175 < U ~ 275 5.0 6.3 8.0275 < U ~ 420 8. 10.0 12.5420 < U ~ 550 10.0 12.5 16.0550 < U ~ 750 12.0 16.0 20.0

Table: Creepage paths for insulating materials

6.3.2 ClearancesThe clearance is the shortest distance in the air between two bare conducting parts. Mainsvoltage is taken as the basis for determining the clearances between live parts and earth(ground); this also applies for use in systems with solidly earthed neutral point. The tablebelow shows the minimum clearances as a function of rated insulation voltage.Particularly when components with bare live parts are retrofitted, attention must be given toconformance with the minimum clearance requirement.

Service voltage, U[Volts]

Minimum Clearance[mm]

U ~ 15 1.615 < U ~30 1.830 < U ~60 2.160 < U ~110 2.5

110 < U ~175 3.2175 < U ~275 5.0275 < U ~420 8.420 < U ~550 10.0550 < U ~750 12.0

Table: Clearances

6.3.3 Requirements to be met by terminalsAs prescribed by Standard EN 50019, all terminals must be secured against working loose.The terminal must be designed so that the conductors cannot become detached from theterminal and sufficient contact pressure is ensured. The conductors must not be damaged inany way by clamping point of the terminal.

6.3.4 Admissible terminal connectionsFor <<increased safety e>> protection, the following types of internal connections arepermitted:

Threaded connections secured against working loose Crimping Soldering, provided the conductors are also held together mechanically. Brazing Welding

Where aluminum is used, special precautions have to be taken against electrolyte corrosion.

6.3.5 Junction boxesTo make sure the temperature limits in a plant are not exceeded, a maximum dissipationpower is set for each and every junction box. The admissible heating limits in these boxesdepend on two factors:

The number of terminals ad conductors inside the enclosure that create localheating inside it; and

The heating of individual terminals and conductors compared with the localtemperature around them

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To make things easier for the electrician, the junction bow fabricator provides a table for eachbox indicating the maximum number of terminals and conductors for various rated currentsand conductor cross-sections.

Cross-section in mm2Current[A] 1.5 2.5 4 6 103 426 42 4210 10 42 3616 18 36 3220 18 32 2625 24 26

Max. no. of terminals 21 21 17 16 13

6.3.6 IP protection classificationThe IP protection system uses two digits. The first digit indicates protection against accidentalcontact and foreign bodies, and the second protection against water. The degrees of relevantfor explosion proof apparatus are:

Degrees of protection against accidental contact and foreign bodies (first digit)

4 Protection against penetration of solid foreign bodies larger than 1mm (contact with tools,wires, etc., thicker than 1mm).

5 Total protection against accidental contact, protection against harmful dust deposits; dustpenetration is not totally prevented.

6 Total protection against accidental contact. Protection against penetration of dust.

Degrees of protection for water (second digit)

4 Protection against water splashed from any direction.

5 Protection against water jets from any direction.

6 Protection against temporary flooding, e.g. due to heavy seas.

7 Protection against water submersion t predetermined pressure for undetermined period.

8 Protection against water submersion at elevated pressure for undetermined period.

Example:IP 54 means protection against harmful dust deposits (5) and water splashed from anydirection (4).

6.3.6 Magnesium content of enclosure alloysBecause of possible spark formation due to impact, terminal boxes must not contain morethan 6% magnesium. Most enclosures on the market have an aluminum alloy of AlSi12. Lightalloy enclosures have been partially replaced by polyester enclosures, which easily meet thehigher standards in terms of resistance to impact and high temperatures.

6.4 Intrinsic safety <<i>> EN 50 020(EEx ia IIC, EEx ib IIC)

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Electrical apparatus is intrinsically safe if all circuits are intrinsically safe. A circuit isintrinsically safe if, as a result of limitation of current and voltage (a function of inductance,capacitance and resistance), no sparks or thermal effects can occur in it. The energy of suchcircuits is lower than the minimum ignition required igniting an explosive mixture.Subdivisions A, B and C, which are based on the experimentally determined minimumignition current (MIC), have been laid down for this degree of protection.

As far as intrinsic safety <<i>> is concerned, a distinction is made between the twocategories <<ia>> and <<ib>>. Category <<ia>> is subject to more stringent requirements onthe occurrence of faults and combinations of faults. Consequently, category <<ia>> isprescribed for use in Zone 0 and <<ib>> for use in Zone 1.

A further stipulation is that intrinsically safe connections must be marked as such (if colour isused, only light blue RAL 5012). In practice, this degree of protection permits the use ofnormal, non explosive proof thermostats, control equipment and monitoring devices, providedthat a tested, intrinsically safe arc suppression relay or suitable safety barriers are employed.Associated electrical apparatus with intrinsically safe circuits must be installed outside of thehazardous area. In the case of associated electrical apparatus, the type of protection isstated in brackets:[EEx ib] IIC T6.

6.4.1 Minimum ignition energyFor intrinsically safe electrical apparatus, gases and vapours are classified according to theratio between their minimum ignition current (MIC) and the minimum ignition current ofmethane. The CMI ratio must be determined using the method and test equipment laid downin the standard.

Subdivision A: MIC ratio greater than 0.8Subdivision B: MIC ratio between 0.45 and 0.8Subdivision C: MIC ratio less than 0.45

6.4.2 Zener safety barriersZener safety barriers separate non-intrinsically safe circuits from the intrinsically safe circuit.Their function is to limit the voltage and the current in an intrinsically safe circuit to such anextent that the energy present is insufficient to ignite explosive mixtures. This limitation mustbe guaranteed both under normal conditions and in the event of faults. For this reason it isessential to observe the stipulations of the instruction. With Zener barriers, he specifiedcapacitances and inductances in the intrinsically safe circuit must not be exceeded. Fig. 12shows the schematically the circuit diagram of a safety Zener Barrier.

Zener safety barriers are installed either outside a potentially explosive area or in aflameproof enclosure.

6.4.3 Intrinsic safety protection relays(Switching amplifiers)

Intrinsic safety protection relays are used to transmit digital signals or to connect switchingdevices that are not explosion proof (terminals, pressure switches, micro switches, limitswitches, etc.). Intrinsically safe protection relays and switching amplifiers are available withrelay outputs and opt coupler outputs. Normally they have a broken-wire interlock and anoperation indicator. Intrinsically safe protection relays and switching amplifiers are locatedoutside the potentially explosive area in control equipment or in flameproof enclosures.

6.5 Intrinsically safe electrical system EN 50 039

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An intrinsically safe electrical system is totality of electrical apparatus connected together thatis documented with a system description. Those of its circuits that are used entirely or partlyin the explosion-hazard zone must be intrinsically safe circuits. It is not necessary for eachitem of electrical apparatus in an intrinsically safe electrical system to be certified individually,provided that each such item of electrical apparatus is positively identifiable.

6.6 Encapsulation <<m>> EN 50 028(EEx m II)

Encapsulation is a type of protection in which parts of a piece of a piece of electricalapparatus that are able to ignite an extremely explosive atmosphere by sparking orinadmissible heating are embedded in a casting compound. Thermosets, thermoplastics andelastomers are used for this purpose, with or without fillers (such as colorants).

6.7 Oil immersion <<o>> EN 50 015

A type of protection in which the electrical apparatus or parts of the electrical apparatus areimmersed in oil in such a way that an explosive atmosphere outside the oil cannot be ignitedby arcs, sparks or hot gases occurring under the oil.

6.8 Pressurized enclosure <<p>> EN 50 016(EEx p II)

Sparking apparatus (such as circuit breakers and contactors) and components with hotsurfaces are housed in an enclosure in such a way that they can be kept under pressure orpurged with air or inert gas. This makes it possible to expunge explosive mixtures that havepenetrated the enclosure prior to start-up and to keep such mixtures from entering duringoperation. Pressurized enclosure, like flameproof enclosure, is a protection type that must besubjected to type testing or acceptance testing. Besides continuous pressurization, theregulations call for monitoring of the air or inert gas supply. In the event of a malfunction, theelectrical apparatus housed in the enclosure must be de-energized immediately.

Preliminary purging is done with 5 times the enclosure volume. The purging time starts whenthe rated flow of the purging medium is reached (flow meter incorporated in the pressureswitch). Apparatus with simple compressed air connections without proper testingunfortunately occur frequently in practice, but in no way satisfy the relevant stipulations. Inview of the new product liability legislation and he low voltage regulations, the use of suchtechniques to avoid complying with the stipulations may have extremely unpleasant andcostly consequences.

6.9 Powder filling <<q>> EN 50 017A type of protection I which the parts at risk are surrounded with sand in an enclosure in sucha way that ignition of a potentially explosive atmosphere as a result of arcs or excessivetemperature is rendered impossible.

7. Types of protection for Zone 2Apparatus approved for Zone 0 or Zone 1 may also be installed in Zone 2. In addition, the<<non-sparking apparatus n>> type of protection was created for Zone 2. The newCENELEC draft per EN 50 021 is based partially on IEC Report 79-15, which has alreadybeen published. The future harmonized CENELEC standard EN 50 021, which will apply allover Europe, will probably become effective in about 1996.The new protection type, which applies exclusively to electrical apparatus installed in Zone 2,permits low-cost solutions. Only normal operation is taken into consideration in this zone;there is no need to consider short-term malfunctions. Apparatus with <<n>> protection isclassified in five groups:

nV for non-sparking electrical apparatus (rotating machines, fuses, light fixtures,measuring instruments and low-energy apparatus)

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nW for apparatus producing operational arcs, sparks or hot surfaces (enclosed-breakdevices, non-incendive components, hermetically sealed equipment and sealeddevices)

nR for restricted breathing enclosurenP for simplified pressurized enclosurenL for apparatus and circuits with limited energy

7.1 Non-sparking electrical apparatus Ex nV II Zone 2In the case of this protection type, special precautions are taken to ensure an increaseddegree of safety and to prevent the occurrence of in admissibly high surface temperaturesand sparks or arcs inside or on external components of electrical apparatus in normaloperation. Special importance is attached to observance of the maximum surfacetemperature. Enclosures and junction boxes containing bare, live parts must comply with adegree of protection not less than IP 54. Those containing only insulated parts may bedesigned to a degree of protection not less than IP 44.

7.2 Apparatus producing operational internal arcs ad sparks7.2.1 Apparatus with operational sparking or hot surfaces

EE nW II Zone 2Apparatus and components with operational sparking must be encapsulated or sealed insuch a way that they will be capable of either withstanding an internal explosion or preventingthe external explosive atmosphere from penetrating. In the case of simplified flameproofenclosures, precautions must also be taken to ensure that a permissible internal explosioncannot be transmitted to the explosive atmosphere outside the enclosure (i.e., no sparkignition).

7.2.2 Restricted breathing enclosure Ex nR II Zone 2Apparatus with the internal sparking or arcing or inadmissible internal temperatures duringnormal operation may be used in Zone 2 if the enclosure complies with a degree of protectionof at least IP 54 and an internal gauge pressure of 4 mbar requires more than 80 seconds todecrease to one-half of the initial value (2 mbar). Enclosures and boxes meeting theserequirements qualify as <<restricted breathing>> enclosures. Unlike enclosures built to<<EEx e II>> and <<EEx d IIC>>, restricted breathing enclosures are not maintenance-free.The operator must ensure that the restricted breathing enclosures are inspected periodically.

7.2.3 Simplified pressurized enclosure Ex nP II Zone 2The simplified pressurized enclosure makes it possible to operate an enclosure underpressure. In the event of leakage or pressure loss, an alarm must be given but there is noneed to de-energize immediately. Simplified pressurized enclosures normally consist of theenclosure itself, an air or nitrogen inlet nozzle (sintered nozzle to reduce noise) and apressure monitor with intrinsically safe power supply.As things stood in June 1993, the possibility of tightening the requirements for simplifiedpressurized enclosures had been discussed at an international meeting. If adopted, thesewould require fabricators or suppliers to provide a pre-purging system and a flow monitor forpurging air or inert gas. That which is prescribed as mandatory for applications in Zone 1would be left to user’s discretion in Zone 2. The user would decide whether the apparatus inthe enclosure would be de-energized immediately or alarm would merely be tripped for theapplication in question.

7.3 Apparatus with limited energy Ex nL II Zone2A piece of electrical apparatus is intrinsically safe if its amperage and voltage are limited insuch a way (function of inductance, capacitance and resistance) that no sparks or thermaleffects can occur in it. The energy of such circuits is lower than the minimum ignition energyrequired to ignite an explosive mixture.

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8. Temperature classesFor the commercial use of explosion proof apparatus, the maximum surface temperatures aredetermined and temperature classes are established. Temperature class T1 has the highestpermissible surface temperature, temperature class T6 the lowest. Electrical apparatusconforming to higher temperature class (e.g. T5) may also be used for applications in which alower temperature class is required (T2 or T3 for example). A common error is to applytemperature classes only to electrical apparatus. Pipes (for steam and hot media) withsurface temperatures for a given temperatures class may be just as much a source of risk asthe hot surfaces of electrical apparatus.

8.1 Temperature class T1Mixtures with ignition temperature of t>450°C and a maximum surface temperature of 450°C.T1 includes the substances propane, carbon monoxide, ammonia, acetone, styrene, aceticacid, benzene, methane, toluene, hydrogen and town gas. T1 relates primarily to gasworksand the mining industry.

8.2 Temperature class T2Mixtures with an ignition temperature of t>300°C and a maximum surface temperature of300°C. The main substance covered by T2 is isopentane, butyl acetate, ethyl alcohol andacetylene chemistry, which are used in industrially in acetylene chemistry.

8.3 Temperature class T3Mixtures with an ignition temperature of t>200°C and a maximum surface temperature of200°C. T3 covers benzene and the corresponding derivatives, which are mainly in thepetrochemical industry.

8.4 Temperature class T4Mixtures with an ignition temperature of t> 135°C and a maximum surface temperature of135°C. T4 includes mainly ethyl ether and acetaldehyde, which are used in the manufactureof plastics and solvents.

8.5 Temperature class T5Mixtures with an ignition temperature of t> 100°C and a maximum surface temperature of100°C. The practical importance of T5 is primarily in the manufacture of textile fibres.

8.6 Temperature class T6Mixtures with an ignition temperature of t>85°C and a maximum surface temperature of 85°C.This temperature class is of practical importance primarily in areas involving the use ofcarbon disulphide and ethyl nitrite.

8.7 General conditionsUsually electrical apparatus is designed for use in an ambient temperature range of -20°C to+40°C; in this case no additional making is necessary.

If the electrical apparatus is designed for use in a different temperature range, this isregarded as a special design. The ambient temperature range must be specified by themanufacturer and stated in the certificate. The marking must then contain either the specialambient temperature range or, if that is not possible, the character <<X>> (see section10.2.2.1)

Temperature Class Maximum SurfaceTemperature

Safety Clearance forPermanently hot surfaces

T1 450°C 10 Kelvin

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T2 300°C 10 KelvinT3 200°C 5 KelvinT4 135°C 5 KelvinT5 100°C 5 KelvinT6 85°C 5 Kelvin

9. Explosion proof apparatus9.1 CapacitorsCapacitors that remain connected with the circuits after they have been switched off musthave a discharging device regardless of their location (even if they are outside the potentiallyexplosive area). This discharging device must discharge within five seconds to a permanentenergy level of:

0.2mJ for electrical apparatus ofExplosion group II A

0.06mJ for electrical apparatus ofExplosion group II B

0.02mJ for electrical apparatusExplosion group II C

9.2 TransformersTransformers must be protected on the primary side against the effect of short-circuits and onthe primary and secondary sides against excessive heating as a consequence ofoverloading.

9.3 ConnectorsConnectors must be either mechanically or electrically locked to permit plugging orunplugging in the de-energized state only.

Deviations are permissible if connectors are assigned to only one item of apparatus and aresecured against unintentional disconnection. In these cases a label reading <<Do notdisconnect while energized>> is sufficient.

9.4 Cable entriesThe particular characteristics of the degree of protection of the apparatus must not beadversely affected by the cable entry. The sealing ring must be correct for the cable used. Adistinction is made between cable entries or direct entry into a flameproof enclosure andcable entries for entry into an <<increased safety e>> enclosure. Entries for leading cablesdirectly into flameproof enclosures and cable entries made of plastic must be certified.Metallic cable entries (normally protection lass IP 68) for <<EEx e>> enclosures do notrequire certification. Only metric threads may be used in explosion proof electrical apparatus.

9.5 Enclosures and distribution boxesFlameproof enclosures (EEx d IIC) and <<increased safety>> (EEx e II) junction boxes mustbe carefully sealed.No mechanical modifications may be made to flameproof enclosures (the drilling of holes isforbidden). The joints of flameproof enclosures may not be painted; only acid-freeanticorrosion agents may be applied.

Changes to the internal components (including the additional fitting of EX-e terminals) to EX-e enclosures are permissible only if the terms of the certificate allow them.

9.6 Terminals and connectionsOnly one wire may be connected to a terminal, unless the terminal is designed to receiveseveral conductors (example: mantle terminal). The ends of flexible conductors must be fittedwith cable lugs or ferrules.

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9.7 Condensation in Ex enclosuresRapid temperature changes may cause the temperature to fall below the dew point, possiblyleading to condensation in junction boxes. In the case of junction boxes for electric heaters,this can be prevented by leading in the heating cable directly. Junction boxes that containonly terminals may in special cases be fitted with an approved dewatering plug.

9.8 SealsSince explosion proof apparatus is normally exposed to environmental influences, particularattention must be given the sealing materials. Studies have shown that, for example, nitriderubbers are not suitable for all installations. High temperatures and corrosive ambient airrequire modern materials such as Vinton seals. These seals can be obtained from the cablegland manufacturers. Their longer life and increased safety justify the extra cost.

10. Installations10.1 GeneralThe general rules and regulations that apply to normal apparatus apply as a matter ofprinciple. Normally the regulations laid down for explosion proof apparatus must be satisfiedin addition. It is a matter of <<and>>, not <<either-or>>.

10.2 Use of explosion proof electrical equipment10.2.1 Inscriptions on nameplatesThe inscription must bear the following inscriptions:

1. The name of the manufacturer or his registered trade mark2. Ex classification (the symbol EEx, the sign for each type of protection used, the

symbol of the group of the electrical apparatus, temperature class and if required alsothe zone [0 or 2])

3. The indication of the testing station and the certificate reference4. The mark <<X>> or <<U>> (if required)5. Power rating6. Vo l tage7. Amperage where appropriate (important for fuse selection)8. Instruction number (if required)9. Extended or restricted ambient temperature range10. Factory serial number

Plus the following additional information for motors:11. Ratio of starting current to rated current IA/IN12. Tripping time tE for <<increased safety>> motors

Or

13. Response time of the temperature monitoring unit tA for thermistor-type protectivesystem (TMS).

The nameplate f apparatus approved for Zone 0 or only for Zone 2 must carry reference tothe respective zone after EX classification (Ex nR II T5 Zone 2). If the nameplate does notrefer to a specific zone, the piece of apparatus may be used in Zones 1 and 2 (EEx de IICT3).

10.2.2 Instructions10.2.2.1 Additional designation <<X>>If special conditions must be maintained when a piece of apparatus is installed, an <<X>> isadded after the certification reference. For the installer, this means that the manufacturer’sinstruction sheet must be read carefully and its directions relative to safe operation of theapparatus must be followed to the letter.

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10.2.2.2 Additional design <<U>>The supplement <<U>> tells the user that the apparatus is incomplete and cannot be used onits own. This additional marking is added in the case of components such as enclosures,terminals, microswitches etc, since these only achieve the status of a complete apparatuswhen installed or assembled.

10.3 CablesAt present there are no definitive regulations for installations where there is a potentiallyexplosive atmosphere. But it is advisable to use an appropriate industrial quality that isadditionally flame-retardant or even self-extinguishing. Particular attention must be paid to theouter sheaths, so that the cables can withstand the expected mechanical and thermal effectsand do not give rise to safety risks in service. Where different plastics are used, it is importantto ensure that they are mutually compatible.

10.3.1 Shielded cables

Cables and conductors with a shield or metal braiding must also have an outer sheath ofplastic.

10.3.2 Cables for movable apparatusFor movable apparatus with a rated voltage of up to 750V reinforced rubber cables orequivalent PUR cables must be used as connecting cables.

10.4 Fire partitioning of cablesPenetrations fire cables and wires into areas that do not contain a potential explosiveatmosphere must be sufficiently tightly sealed, for example by sand seals, mortar seals orspecial compounds. Long runs of cables or wires must be suitably subdivided in explosion-hazard zones into individually protected fire sections.

10. Installation of cables and wiresAt locations that are particularly at risk due to thermal, mechanical or chemical influences, thecables and wires must be protected. This can be done by installing them in conduits, plastichoses or metal hoses with edge protection (plastic ferrules or guards).Unused openings and cable entries on electrical apparatus must be sealed with approvedplugs. In Zone 2, non-certified plugs may be used. Inside control cabinets, and in the interiorof switchgear and distribution boards, special measures must be taken where there is a riskof confusion of the wiring of intrinsically safe circuits with that of ones that are not intrinsicallysafe- for example where there is a neutral wire coloured blue.

10.6 Minimum cross-sectionsFor mechanical reasons, the following minimum cross-sections must be observed:

10.6.1 Minimum cross-section for single-core wires1 mm

2for stranded conductors

1.5 mm2

fr single-wire conductors

10.6.2 Minimum cross-section for multi-core cables with up to 5 cores (3L + N + PE)0.75 mm

2for stranded conductors

1 mm2

for single-wire conductors

10.6.3 Minimum cross-section for multi-core cables with more than 5 cores0.5 mm

2for stranded conductors

1 mm2

for single-wire conductors

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10.7 Fuse protection of apparatusWhere apparatus is protected by fuses, the rated current of the loads when stationary andtheir tripping characteristic must be taken into account in relation to the starting current. Indoubtful cases the relevant rated current must be measured.Solenoid valves in Zone 1 must be protected by a separate fuse as marked (appliance fusefor max. 1.5 x rated current).

10.8 Safety shutdownIt must be possible to switch off electrical apparatus immediately (safety shutdown) from a

location outside the hazardous area if continued operation of the apparatus in the event offaults would increase the hazard – for example by the spread of fires. This safety shutdownfacility need not be provided in Zone 2. Apparatus that must continue to operate in the eventof malfunctions in order to prevent a spread of the hazard should not be included in theshutdown circuit; instead it should be included an independently switchable circuit.

10.9 Connection of protection and monitoring devicesProtection and monitoring devices such as over current trips, safety temperature, safetytemperature limiters and pressure switches must, on tripping, disconnect the relevant part ofthe installation in all phases and may not automatically reconnect it. On switching it on againor unlocking, the operability of the protective device must be checked. Disconnection mustnot lead to lead to increased risk. Installations must be designed to switch to a safe conditionwhen shut down.

10.10 Equipotential bonding

Equipotential bonding is essential within explosion-hazard zones where protective measuresare employed with an earth continuity conductor for protection against indirect contact.Conducting structural components such as supports, pipes and containers must beconnected together and to the earth continuity conductor.

11. Maintenance

11.1 DisconnectionBefore any enclosure is opened in an explosion-hazard zone, the relevant part of theinstallation must be switched to de-energized state. A suitable auxiliary device must beprovided to ensure that accidental or unintended energizing of the circuit is not possible.Normally the consent of the operator is needed for maintenance work on installations of thisnature.

11.2 Safety precautionsOn switching off for the purpose of carrying out maintenance work, it is essential to ensurethat unintentional switching on is rendered absolutely impossible. The recommended methodis to provide safety switches which can be locked by the maintenance personnel withpad locks.

11.3 Ex measuring instrumentsWhen carrying out measurements in areas with potentially explosive atmospheres, it isimportant to ensure that the measuring instruments are explosion proof. For the use ofnormal measurement instruments, a <<work permit>> must be obtained from the operator.Special measurements, for example those involving high-voltage instruments and insulationtesters, may, on connection and disconnection of the measuring voltage, give rise to sparksthat possess sufficient energy to ignite explosive mixtures. The same conditions apply toelectronic calculators as to measuring instruments, if they are fitted with large enoughbatteries. In certain cases electronic calculators are permitted for Zone 2.

11.4 Maintenance and inspection work

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There are three levels of maintenance and inspection work: Visual inspection of the closed apparatus Visual inspection of open apparatus Detailed check of the open apparatus

Experience has shown that periodic inspections are highly important. A visual inspection, forexample, covers the junction box and its cable entry, the pushbuttons and the indicatorwindows. Small defects or open junction boxes can often jeopardize the degree of protectionof the installation. In addition to the <<official>> inspections, further inspections may becarried out by the operator or the manufacturer. All checks must be logged. Thermostats andprotective devices such as a level switches, flow switches and the like must undergofunctional test at regular intervals.When carrying out insulation testing by means of hand-driven generators (megers), high-voltage instruments or insulation meters, it is essential to obtain a permit to carry out the workand a fire permit. Pulse reflecto meters maybe used to locate faults (earth faults or opencircuits) in heating cables. The location of the fault can be determined to within a meter onthe basis of the characteristic pulse propagation velocity of the cable.

12. Repairs and modifications to explosion proof electrical apparatusAs required by EN 50014, repair work on explosion proof electrical apparatus should becarried out by the manufacturer only. This rule can be waived in cases where the repairs arecarried out by instructed and specially trained, skilled personnel and only genuine spares areused. In no case is it permissible for modifications and rework to be carried out thatjeopardize the characteristics essential to explosion protection (type of protection andtemperature class). Rewiring of installations is permissible if carried out by trained skilledpersonnel. The rewiring work done must be properly logged. It is forbidden to rework the gapto apply paint or other coatings to the respective joints. Such modifications could impair theenclosure’s flameproof ness and cause an uncontrolled explosion.

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Tel: +31 653370472Fax: +31 108920718E-Mail: info@hqc.info