Rank: Forum user 
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I am after a bit of advice, I am looking into the DSEAR regs and need to zone off some of the areas in the factory. I know that the areas to be zoned off will be class 1 but what I am struggling with is the size that the area needs to be. If we have an explosive gas atmosphere in one area of the room have I got to classify the whole room as zone 1 or just the area which the process is taking place? If it is just that area what size has the zoned area got to be?
Many thanks in advance
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Rank: Super forum user 
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The size of the zone is determined by the extent of the area that will contain the flammable atmosphere.
ie Regulation 7 of the Dangerous Substances and Explosive Atmospheres Regulations 2002
Zone 0: A place in which an explosive atmosphere – consisting of a mixture with air of dangerous substances in the form of gas, vapour or mist – is present continuously, for long periods, or frequently
Zone 1: A place in which an explosive atmosphere – consisting of a mixture with air of dangerous substances in the 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 dangerous substances in the form of gas, vapour or mist – is not likely to occur in normal operation but, if it does occur, will persist for a short period only
Follow this link for lots of advice http://www.hse.gov.uk/el...ity/atex/definitions.htm
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Rank: Super forum user
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Just telling you the Zone classification definitions, doesn't answer the question what you are asking.
Two documents are normally quoted to determine the extent/size of your hazardous areas and what zone classification they should be given.
These are BSEn 60079-10-1 Explosive Atmospheres - Classification of Areas
or
Energy Institute (used to be called Insitute of Petroleum) IP Pt 15 3rd Edition Area classificaion code for flammable liquids.
Another alternative would be to model with software using the DNV Phast consequence modelling software.
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Rank: Super forum user 
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you beat me to it JJ but here also is the HSE link:
http://www.hse.gov.uk/co...ech/techmeasareaclas.htm
Hazardous Area Classification for Flammable Gases and Vapours
Area classification may be carried out by direct analogy with typical installations described in established codes, or by more quantitative methods that require a more detailed knowledge of the plant. The starting point is to identify sources of release of flammable gas or vapour. These may arise from constant activities; from time to time in normal operation; or as the result of some unplanned event..
The most commonly used standard in the UK for determining area extent and classification is BS EN 60079 part 101, which has broad applicability. The current version makes clear the direct link between the amounts of flammable vapour that may be released, the ventilation at that location, and the zone number. It contains a simplistic calculation relating the size of zone to a rate of release of gas or vapour, but it is not helpful for liquid releases, where the rate of vaporisation controls the size of the hazardous area.
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Rank: Super forum user
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Hi Khan
Pikeman has provided a concise response that answers your question. All you have to do is use one or more of the accepted methods to dertmine where the limits of your various zones will be.
As identified by JJ, essentially, your definition of a Zone will be marginally beyond the foresseable limits of flammability/explosion of the substance[s] under consideration.
This will mean that if you have more than one relevant substance [ie vapour + dust] from ther same plant/process, you must ensure that your Zone limit accomodates both [all] relevant substances.
These will range from using existing data gathered as part of an existing atmosphere monitoring process through to deciding which of the available modelling processes is most appropriate to your situation - there as some very good BSs that help here - 2 quoted by JJ.
You will also require the refer to SDSs and CLP for known substances/mixtures; but where the flam/explosive atmosphere is created as a consequence of your production process, you may need additional advice on the characteristics of the gases/vapours/etc being produced/emitted.
Frank Hallett
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