Area classification
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Where hazardous flammable atmospheres may occur, ATEX directive 1999/92/EC, implemented in the UK as the Dangerous Substances and Explosives Atmospheres Regulations 2002, applies. In these areas the employer is required to assess the risk, install suitable equipment to reduce the risk of ignition, and to implement procedures to ensure that safety is maintained throughout the life of the equipment.
Area classification is the name of the process carried out to identify the areas where hazardous flammable atmospheres may occur, and the likelihood that they may occur.
There are several steps to area classification: -
• Identifying the hazardous materials that may be present. Normal operation and abnormal operations, such as maintenance, should be considered.
• Identifying the sources of release of flammable vapours, dusts or mists. Typical sources of release are listed in European standards EN 60079-10 (vapours and gases), or EN 61241-10 (dusts), and may include: -
The interior of storage tanks containing flammable liquids.
Open vents from flammable liquid storage tanks.
Sack-tipping stations where potentially explosible dust clouds occur.
• Identifying the grade of release. There are 3 grades of release: -
Continuous - A release which is continuous or is expected to occur frequently or for long periods, (typically >1000 hours/year) Normally a Continuous release will result in classification of the area as zone 0,
Primary - A release which can be expected to occur periodically or occasionally during normal operation, (typically between 10 and 1000 hours/year). Normally a Primary release will result in classification of the area as zone1.
Secondary - A release which is not expected to occur during normal operation and, if it does occur, is likely to do so only infrequently and for short periods, (typically between 1 and 10 hours/year) Normally a Secondary release will result in classification of the area as zone 2.
However, if the degree or availability of ventilation is particularly good, or bad, local to the release, the normal zone type may be changed.
• Identifying the extent, or size, of the zone where the flammable atmosphere is likely to occur. The extent of the flammable zone depends on many factors. The Institute of Petroleum has developed codes of practice for use in the petroleum industry (published as Part 15 of the model code of safe practice in the petroleum industry). European standard EN 60079-10 provides additional information and describes how the zone extent is affected by ventilation in indoor situations. EN 61241-10 gives advice for zoning of dust releases. Area classification is best carried out by experts in the field; in the UK bodies known to carry out this work include SIRA, ABB Eutech, and Epsilon.
Electrical or mechanical equipment installed in the hazardous zones is constructed so as to minimise the likelihood of ignition.
See also
Electrical Equipment in Hazardous Areas
Further reading
Equipment and protective systems intended for use in_potentially explosive atmospheres
