Hazardous areas are classified into zones based upon the frequency of the occurrence and duration of an explosive gas atmosphere or combustible dust.
The gas area is classified into three zones: zone0、zone 1 and zone 2 .
The dust area is classified as three zones: zone 20、zone 21 and zone 22.
Zone 0: A place in which an explosive atmosphere consisting of a mixture with air of flammable substances in the form of gas, vapour or mist is present continuously or for long periods.
Zone 1: A place in which an explosive atmosphere consisting of a mixture with air of flammable 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 flammable substances in the form of gas, vapour or mist is likely to occur in normal operation but, if it does occur, will persist for a short period only. As a rule, “ for a short period only ” means no more than 2 hours.
Zone 20: Area in which combustible dust, as a cloud, is present continuously or frequently, during normal operation, in sufficient quantity to be capable of producing an explosive concentration of combustible dust in mixture with air and / or where layers of dust of uncontrollable and excessive thickness can be formed.
Zone 21: Area not classified as zone 20 in which combustible dust, as a cloud, is likely to occur during normal operation, in sufficient quantities to be capable of producing an explosive concentration of combustible dust in mixture with air. This zone can include, among others, areas in the immediate vicinity of powder filling or emptying points and areas where dust layers occur and are likely in normal operation to give rise to an explosive concentration of combustible dust in mixture with air.
Zone 22: Area not classified as zone 21 in which combustible dust clouds may occur infrequently and persist for only a short period, or in which accumulations or layers of combustible dust may be present under abnormal conditions and give rise to combustible mixtures of dust in air. Where, following an abnormal condition, the removal of dust accumulations or layers cannot be assured, then the area is to be classified zone 21.
