The hazardous area classification assessment is a probability analysis and a risk assessment evaluation of a manufacturing area or process that processes a potentially flammable atmosphere that focuses exclusively on minimizing or eliminating electrical energy as a source of potential ignition. The classification of hazardous areas is not intended to be a secondary line of defence against poor process design, poor installation and maintenance of equipment, the operation of faulty equipment or catastrophic vapour emissions.
Hazardous areas are divided into three different classes that are totally dependent on the type of material found in the process.
Class I Hazardous area:
These are places where flammable gases or vapors are or may be present in the air in sufficient quantities to produce an explosive or flammable mixture. In Class I areas that use the division concept methodology, two distinct divisions are predicted in the operational interpretation of versus normal versus abnormal and infrequent frequent.
In Class I hazardous areas that utilize the division concept methodology, four distinct groups are based solely on the liquid or gas ease of ignitability and its corresponding range of flammability or explosivity (ER, the range of explosivity in volume percent):
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Group A – Atmospheres that contain acetylene (2.5% ≤ ER ≤ 100%).
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Group B – Flammable gas (e.g. hydrogen) or vapour atmospheres having either a maximum experimental safe gap (MESG) less than or equal to 0.4 mm
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Group C – Flammable gas (e.g. ethylene) or vapour atmospheres having either a maximum experimental safe gap (MESG) greater than 0.45 mm
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Group D – Flammable gas (eg. propane) or vapour atmospheres having either a maximum experimental safe gap (MESG) greater than 0.75 mm .
Class II Hazardous area:
These are hazardous locations because combustible dust is present. Combustible dust is defined as any solid material 420 microns or less in diameter that present a fie or an explosion hazard when dispersed in air.
In Class II areas three distinct groups are based primarily on the physical characteristics of the dust:
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Group E – Atmospheres that contain combustible metal dust, including aluminium, magnesium, and their commercial alloys.
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Group F – Atmospheres containing combustible carbonaceous powders that have more than 8% volatiles trapped or that have been sensitized by other materials, so that they present a risk of explosion.
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Group G – Atmospheres containing other combustible dust, including flour, grain, wood flour, plastic and chemicals.
Class III Hazardous area:
These are hazardous locations because easily ignitable fibers and flyings are present. In Class III areas, there are no goupings as in Class I and Class II areas. There are, however, divisions that are based on how the material is processed.
- Division I is a location where easily ignitable fibers producing combustible flyings are handled, manufactured or used.
- Division II is a location where easily ignitable fibers are stored or handled other than in the manufacturing process