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How Do We Define Hazardous Areas? (On photo: Flammable liquid warning)

http://electrical-engineering-portal.com/how-do-we-define-hazardous-areas April 12, 2013

How Do We Define Hazardous Areas?

Edvard

What is the hazardous area?

The first requirement is to know what a hazardous area is. The principal factors relevant t o t heclassifications of a hazardous area are the nature of the gases or dust present in the

pot entially explosive atmosphere and the likelihood of that at mosphere being present.

The concept o f zone classification has been developed to summarize these factors. Thenature of the atmosphere is characterized by the chemical composit ion of the gas or dust andits auto-ignit ion temperature.

The not ions of gas grouping and temperature classification have been developedto f ormalize this.

Before looking in more detail at these definit ions, it is instructive to consider how explosionsoccur.

A useful concept is that of the hazard triangle, Figure 1.
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The Hazard Triangle

Table 1 - IEC 79 classif ication of hazardous area zones

The three sides of the triangle represent fuel, oxygen and asource of ignition, all of which are required to create anexplosion. The fuel considered here is a flammable gas,vapour or liquid although dust may also be a potential fuel.

Oxygen is present in air at a concentration ofapproximately 21%. The ignition source could be a spark or a

high temperature.

If thepotentially flammable atmosphere is between theupper and lower flammable limits for the part icular materialand an ignition source is introduced then it will explode o rburn. Obviously if any side of the hazardous triangle can beremoved then a fire or explosion hazard cannot exist.

Given that a hazardous area may contain fuel and oxygen, the basis for preventing explosion isensuring that any ignit ion source is either eliminated or else does not come into contact withthe fuel-oxygen mixture.

If there is any possibility of oxygen enrichment, i.e. above 20% by volume, then specialconsideration is necessary to ensure safety.

Zone classification

Table 1 shows the IEC 79-10 zone classification used in Europe and most o ther parts of theworld. The British Standard BS 5345 Part 2will become obsolete and replaced byBS/EN/IEC 60079-10.

The table alsoindicates which typesof explosionprotectionare suitable fo r usewithin each zone.These explosionprotection conceptsare described later inthe chapter.

The Americansystem of hazardousarea classification isstructured in adif ferent way,according to theNational ElectricalCode NEC.

In brief,

hazardous locationsare classif ied aseither Class 1Division 1, where
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Table 2 - CENELEC/IEC gas grouping

ignitable concentrations o f f lammable gases or vapours may be present duringnormal operat ion, or Division 2, where f lammable gases or vapours occur in ignitableconcentrations only in the event of an accident or a failure of a ventilat ion system.

Class IIand Class III Divisions 1 and 2relate to combustible dust and fibres. The 1999edit ion of the National Electric Code (NEC) introduced for the first time in the USA the zoneclassif ication concept as an alternative to the class and division def init ions of hazardouslocations, e.g. Class 1 Zones 0, 1 and 2 for gases and vapour.

In the UK, the Factories Act states that where there is a risk of a f lammable dust cloud,explosion protection and measures to reduce the risk of ignition will be required.

The ATEX Directive legally requires dust hazards to be considered and classified aseither Zone 20, 21 or 22.

Gas grouping and temperature classification

Different gases require different amounts of energy (by hot surface or spark) to ignite themand the two concepts of gas grouping and temperature classif ication are used in Europe toclassify electrical apparatus according to its suitability f or use with explosive atmospheres ofpart icular gases.

Table 2 lists common industrial gases in their appropriate gas groups:

Gas group I is reserved forequipment suitable for use in coal mines.

Gas group II which contains gases found in other industrial applications is subdivided IIA, IIB, orIIC according to the relative f lammability of the most explosive mixture of the gas with air.

Table 3 defines eachtemperature classaccording to themaximum allowedapparatus surfacetemperature exposedto the surroundingatmosphere, andindicates common

gases for whichthese classif icationsare appropriate.

North Americanpractice defineshazardous materialsin classes.Flammable gases andvapours are Class 1

materials,combustible dustsare Class 2
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Table 3 - CENELEC/IEC temperature classif ication

(such as sawdust) areClass 3 materials.

Class 1 is subdividedinto four groupsdepending onflammability:

A (e.g. acetylene),

B (e.g. hydrogen),

C (e.g. ethylene)and

D (e.g. propane,methane)

Note that whencompared with the IEC

gas groupings, thesubgroup letters are inopposite order of flammability.

North American temperature classif icat ion is similar to IEC standards, but further subdivides theclasses to give more specif ic temperature data.

Resource: Newnes Electrical Pocket Book (Get it from Amazon)
http://www.amazon.co.uk/Newnes-Electrical-Pocket-Book-Books/dp/075062566X

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Electrical-Engineering-portal.com-How Do We Define Hazardous Areas