Atex introduced the concept of Categories and Equipment Protection Levels (EPL), partly as an attempt to simplify equipment selection.

Equipment, protective systems and components for use in explosive gas/air, vapour/air and mist/air atmospheres:

Category 3 ignition sources which can occur continuously or frequently (e.g. during normal operation of equipment, protective systems and components) shall be avoided.

Category 2 In addition to the avoidance of ignition sources specified for Category 3, ignition sources that can occur in rare situations (e.g. due to malfunctions of equipment, protective systems and components) shall also be avoided.

Category 1 In addition to the avoidance of ignition sources specified for Category 2, even ignition sources that can occur in very rare situations only (e.g. resulting from rare malfunctions of equipment, protective systems and components) shall be avoided.

Equipment, protective systems and components for use in explosive dust/air atmospheres:

Category 3 Ignition sources which can occur continuously or frequently (e.g. during normal operation of equipment, protective systems and components) shall be avoided. This applies to the ignition of a dust cloud as well as a dust layer. This includes also the limitation of surface temperatures to prevent the ignition of deposited dust during heat exposure for long periods.

Category 2 In addition to the avoidance of ignition sources as specified for Category 3, even ignition sources which can occur in rare situations only (e.g. due to malfunctions of equipment, protective systems and components) shall be avoided. This applies to the ignition of a dust cloud as well as a dust layer.

Category 1 In addition to the avoidance of ignition sources as specified for Category 2, even ignition sources which can occur in very rare situations only (e.g. due to rare malfunctions of equipment, protective systems and components) shall be avoided. This applies to the ignition of a dust cloud as well as a dust layer.

Resulting in 6 Atex Categories, designated '1', '2' and '3' representing levels of Risk 'High' 'Medium' and 'Low' respectively each with either 'G' and 'D' for Gas and Dust.

Atex Category EPL Equipment suitable for
1G Ga Zones 0, 1 & 2
1D Da Zones 20, 21 & 22
2G Gb Zones 1 & 2
2D Db Zones 21 & 22
3G Gc Zone 2
3D Dc Zone 22

Equipment is allocated a protection Level (EPL) of 'G' and 'D' then 'a', 'b' and 'c' representing the degree of protection dependent on a number of factors, particularly 'fault conditions. This concept1 has been around for a long time with intrinsic safety protection but now has been introduced into other protection techniques2 In simple terms to have a EPL of 'a' (for use in any zone) safety integrity must be maintained with 2 fault conditions, for EPL 'b' only 1 fault condition and EPL 'c' no fault conditions are considered.

Currently there is always a direct correlation based on each line of the table e.g. An area category 2G will be Zone 1 and equipment inside that area must have an EPL of Gb (or Ga which is a higher level of protection).

Whereas historically equipment was selected by Zone, the correct way is to specify the Category suitability or EPL required as this is more in line with risk assessment techniques.
E.g. for a low risk dust hazard specify an EPL Dc or suitable for Category 3D which can be used in Zone 22.

Where both Gas and Dust hazards exist at the same time (hybrid atmosphere) rules for equipment certification shall be based on the results of special investigation.
Currently it is believed there are no standard certified equipment which can be used in a hybrid atmosphere.

Please refer to labels for examples of how these concepts go together.

Notes

  1. The components in a Zener barrier are a simple way of demonstrating the concept of protection levels. ↩

  2. Protection levels now exist for protection method such as:
    Ex e now becomes Ex eb or Ex ec and Ex da, Ex db or Ex dc replacing Ex d. ↩

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