Zones and equipment
Classification of areas
According to Directive 2014/34/EC, a potentially explosive area is a space in which the atmosphere could become explosive on account of the local and/or operational conditions. An explosive atmosphere is defined as a mixture with air, under atmospheric conditions, of flammable substances in the form of gases, vapours, mists or dusts in which, after ignition has occurred, combustion spreads to the entire unburned mixture.
The ATEX 137 Directive regulates the division of installations potentially containing an explosive atmosphere into zones with individual risks. The ATEX 95 Directive, on the other hand, is concerned with the classification of equipment in Groups and Categories. This combination is used to establish which Equipment Groups and Categories may be used in which zone. The classification of areas is based on the frequency and duration of the occurrence of an explosive atmosphere and includes gas, vapour and mist like the 3-zone concept for dust (see Table).
| Zone | Level of risk |
|---|---|
| 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 or frequently. |
| Zone 1 | A place in which an explosive atmosphere consisting of a mixture with air or 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 not likely to occur in normal operation but, if it does occur, will persist for a short period only. |
| Zone 20 | A place in which an explosive atmosphere in the form of a cloud of combustible dust in air is present continuously, or for long periods or frequently. |
| Zone 21 | A place in which an explosive atmosphere in the form of a cloud of combustible dust in air is likely to occur in normal operation occasionally. |
| Zone 22 | A place in which an explosive atmosphere in the form of a cloud of combustible dust in air is not likely to occur in normal operation but, if it does occur, will persist for a short period only. |
What equipment should be used in what environment?
Directive 2014/34/EC defines Equipment Groups and Equipment Categories for various risk areas. There are two Equipment Groups, I and II. Equipment Group I applies to equipment for use in underground parts of mines and their surface installations which might be endangered by firedamp and/or combustible dust. Equipment Group II applies to equipment for use in other areas which might be endangered by an explosive atmosphere. The Groups are also subdivided into Equipment Categories according to the frequency of occurrence of an explosive atmosphere. Group I is divided into Categories M1 and M2, whilst Group II is divided into Categories M1 to M3. M1 has a very high level of safety (constant risk of an explosive atmosphere forming), whilst M3 has a normal level of safety (rare risk of an explosive atmosphere forming). An EC-type examination must be performed and production must be subjected to quality assurance or individual testing, depending on the level of safety, or for Category 3 only evidence of internal in-process control is required. In each EU member country there are one or more so-called notified bodies (listed in Annex 4 of the ATEX guidelines) which can monitor the relevant requirements or perform the relevant procedures to provide evidence of compliance with Directive 2014/34/EC.
The following overview shows the classification of zones, which are subdivided into the hazard areas of "Gases, vapours and mist" on the one hand and "Dusts" on the other. Distinctions are also made between risk levels and hazard probability. The final step is to assign categories which define the level of equipment safety. The table indicates which category an item of equipment has to comply with before it can be operated within a certain zone.
| Hazard | Risk | Zone | Category | Equipment |
|---|---|---|---|---|
| Gases, vapours and mist | Constant, for lengthy periods or frequent | 0 | 1 G | Very high level of safety (safe despite two independent faults) |
| Gases, vapours and mist | Occasional | 1 | 1 G, 2 G | High level of safety (safe even if fault occurs which is normally to be expected) |
| Gases, vapours and mist | Rare, and then only for a brief period | 2 | 1 G, 2 G, 3 G | Normal level of safety (safe during normal operation) |
| Dusts | Constant, for lengthy periods or frequent | 20 | 1 D | Very high level of safety (safe despite two independent faults) |
| Dusts | Occasional | 21 | 1 D, 2 D | High level of safety (safe even if fault occurs which is normally to be expected) |
| Dusts | Rare, only for a brief period | 22 | 1 D, 2 D, 3 D | Normal level of safety (safe during normal operation) |
