Hazardous area is classified into 3 zones
Zone0
A zone in which an explosive gas/air mixture is continuously present or present for long periods (estimated to be more than 1000Hr).
Locations: - inside closed tanks, closed process vessels or containers which contain flammable gases, vapors and/or liquids.
Zone1
A zone in which an explosive gas/air mixture is likely to occur in normal operation. (Estimated to be in between 10 to 1000Hr). This classification is applied to areas in which a hazardous atmosphere is likely to occur at any time and therefore requires application of measures to prevent the occurrence of an hazardous electrical condition at any time and at any circumstances.
Locations: - Above storage tanks, relief valves, vents or devices which are likely to release flammable gases and vapors under normal condition.
Also in a Zone2 area where any situations which allows a hazardous atmosphere to collect such as a pit or trench although it may be in open air shall be classified as Zone1 area. Especially since the heavy vapors travel along distances if air currents do not assist diffusion.
Zone 2
A zone in which an explosive gas/air mixture is not likely to occur in normal operation and if it occurs it will exist only for a short time. (Estimated < 10 hr/year). ‘i.e. A hazardous atmosphere is likely to occur only under abnormal operating conditions. This is applicable only where a fire as an explosion hazard is unlikely and may be caused only by the simultaneous and improbable occurrence of an arc or spark resulting from an electrical failure and a hazardous atmosphere arising through the failure of the conditions of control.
Locations: - Pumps, valves, compressors, around tanks and vessels etc.
Unclassified area: The area which has no hazardous (safe area)