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Potentially Explosive Atmospheres exist where there is a risk of explosion due to mixtures of gas/air, vapour/air, dust/air or other flammable combinations. Process plants, refineries, oil and gas platforms etc are divided into Zones (European and IEC method) or Divisions (North American method) according to the likelihood of a potentially explosive atmosphere being present.
The following standards are used to define the classification:
- EN 1127-1:1997; Explosive atmosphere - Explosion prevention and protection. Part 1: Basic concepts and methodology
- EN 60079-10:2003; Electrical apparatus for explosive gas atmospheres. Part 10: Classification of potentially explosive atmospheres
- EN 50281-3:2002; Electrical apparatus for use in the presence of combustible dust. Part 3: Classification of areas where dusts are or may be present
- N.E.C. Chapter 5
It is worth noting that many -though by no means all - countries outside of Europe and North America use the IEC Standards as a basis for their own national standards.
Hazardous area zones are usually marked on a drawing of the plant, the drawing being referred to as the Hazardous Area drawing. these drawings also include temperature classification information and gas group information. By using these drawings the engineer can specify equipment suitable for the area in which it is to be located.
The following table provides a comparison between zones and divisions;
| European & IEC Classification | Definition of Zone or Division | North American Classification |
| Zone 0 (gases) | An area in which an explosive mixture is continuously present or present for long periods | Class I Division 1 (gases) |
| Zone 20 (dusts) | Class II Division 1 (dusts) | |
| Zone 1 (gases) | An area in which an explosive mixture is likely to occur in normal operation | Class I Division 1 (gases) |
| Zone 21 (dusts) | Class II Division 1 (dusts) | |
| Zone 2 (gases) | An area in which an explosive mixture is not likely to occur in normal operation and if it occurs it will exist only for a short time | Class I Division 2 (gases) |
| Zone 22 (dusts) | Class II Division 2 (dusts) | |
| Class III Division 1 (fibres) | ||
| Class III Division 2 (fibres) |
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