ATEX Area Classifications/Zoning Definitions
ATEX Area Classifications/Zoning Definitions
It is of upmost importance that any possible hazardous or potentially explosive atmosphere is identified correctly. In great Britain, the requirements of the "Use" directive, 99/92/EC were put into effect through the dangerous Substances And Explosive Atmospheres regulations (dSEAr).
The requirements in DSEAR apply to most workplaces where potentially explosive atmospheres may occur. Some industry sectors are excluded as they are generally covered by other pieces of legislation.
DSEAR requires that employers eliminate or control the risks from dangerous substances and this is usually done by carrying out detailed and comprehensive risk assessments of all aspects of the working area, processes, materials, methods etc.
When a potentially explosive atmosphere is assessed the hazard could be comprised of flammable gasses/vapours, dusts or a combination of both. With reference to the descriptions shown below it can be seen that the definition of any particular zone is based on the frequency of exposure to the hazardous material.
Hazardous areas can usually be classified into three groups, I, II and III.
Group I - is typically reserved for underground applications such as mines.
Group II - is the most common group and deals with most surface applications.
Group III - is related to electrical equipment intended for used in places with an explosive dust atmosphere other than mines.
Zone 0 - A place in which an explosive atmosphere consisting of a mixture with air of dangerous 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 of dangerous 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 dangerous 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.
For hazardous areas involving dusts then the above classifications are prefixed with a "2", so zone 1 for dusts becomes zone 21, as below:-
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 21 - A place in which an explosive atmosphere in the form or 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.
ATEX categories are categories which are used to define equipment to be used in the various zones as detailed on the previous page.
|ATEX CATEGORY||TYPICAL ZONE COMPATIBILITY|
|1G||EQUIPMENT SUITABLE FOR ZONE 0|
|ID||EQUIPMENT SUITABLE FOR ZONE 20|
|2G||EQUIPMENT SUITABLE FOR ZONE 1|
|2D||EQUIPMENT SUITABLE FOR ZONE 21|
|3G||EQUIPMENT SUITABLE FOR ZONE 2|
|3D||EQUIPMENT SUITABLE FOR ZONE 22|
From this we can see that if an oven is to be used in a zone 1 application then equipment suitable for use in category 2 will need to be used. Another important aspect of correct classification is the determination of a correct "T Rating". The "T Rating" determines the maximum allowable surface temperature of any item within the potentially explosive area.
The determination of the "T Rating" is related to the flammable or potentially explosive material being processed and is often related to the auto ignition temperature of the material in question. For Example, if the "T Rating" of the system as a whole is determined to be T3, this means that no item in the potential explosive atmosphere can ever be hotter than 200°C and the system and its controls will need to be designed to ensure this compliance.
|T CLASS||MAX SURFACE TEMP IN °C|
From the table above, it can be seen that potentially explosive materials are grouped together and for example, most paint drying/solvent evaporation applications would be classified as Gas Group IIB. In order to make sense of all the information detailed above, we must now briefly describe the various protection concepts commonly used to ensure that the equipment to be used in the potentially explosive is compliant with the ATEX classification determined by the employer relative to ATEX137 or 99/92/EC. The protection concepts are as follows:
|Ex i - Intrinsic safety||Ex p - Pressurised|
|Ex d - Flameproof||Ex m - Encapsulation|
|Ex e - Increased safety||Ex o - Oil immersion|
|Ex n - Non sparking||Ex q - Powder fill|
Airflow has vast experience of supplying bespoke systems for use within hazardous and potentially explosive atmospheres. The protection concepts employed are usually a combination of Ex d equipment such as flameproof fan motors etc and intrinsically safe equipment.
The Ex i concept includes for devices mounted in the main control panel in the safe area. These devices called barriers are used to limit the electrical energy supplied to devices in the hazardous area such as pressure switches, thermocouples, etc. Using all the information above, we can now make sense of a typical ATEX classification as shown here.
This classification is for an EC (ATEX) compliant system for surface equipment that has an equipment category of 2 (Zone 1) and is for gasses as opposed to dusts. The protection concept is "d" which is flameproof, the gas group is IIC and the maximum allowable temperature is 200°C.