1 Zona 20 ( ATEX category 1 D {DUST} ) A place in which an explosive atmosphere in the form of a cloud of combustible dust is present continuosly or for long period or frequently .
2 Zone 21 ( ATEX category 2D {Dust} ) . A place in which an explosive atmosphere in the form of a cloud of combustible dust in air likely to occur in normal operation occasionally .
3. Zone 22 ( ATEX category 3D {Dust} ) . A place in which an explosive atmosphere in the form of a cloud of combustible dust in air not likely to occur in normal operation but , if it does occur , will persist or a short period olny .
If there is no likelihood at all of flammable atmosphere the area is a safe one .
THe magnitude of the risk which determines which zone an area is in depends on such things as the process producing the flammable gas or vapour or dust cloud , the rate of production in relation to room size , the risk of leakage and the distance of the area from the source of the hazardous material . These factors are assessed by a safety specialist who designjates the zone classification of areas on a site and it is not usual for the electrical designer to hae to do this him/herself .
The type of risk depends on the properties of the gas , vapour , or dust concerned . For gases , dangerous , substances are , accordingly , classified into four groups , dependings on the minimum ignition energy of the gas and on the ability of a flame emerging from a narrow joint to ignite it :
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