IEC 60079-10-2: Classification of areas – Explosive dust atmospheres

The ATEX standard EN 60079-10-2 addresses the classification of areas where hazards may arise due to the presence of combustible dusts and therefore provides the basis for the correct selection and installation of equipment intended for use in hazardous locations.

It establishes the essential criteria against which ignition hazards can be assessed and provides guidance on design and control parameters that can be used to reduce such hazards. Both general and specific criteria are given for the process of identification and classification of ATEX areas.

The latest edition of the ATEX standard was published as EN on December 1, 2015. Among the aspects addressed in the standard is the treatment of hybrid mixtures (Annex C). A hybrid mixture is a combined mixture of a flammable gas or vapour with a combustible dust. Such a mixture can behave differently compared to the individual components. In particular, a hybrid mixture may form explosive atmospheres even outside the flammability limits of gases or vapours or the explosible concentration of dust when considered separately. It is recommended that a hybrid mixture be considered explosive when the concentration of gas or vapour exceeds 25% of its LEL.

The hazards presented by combustible dusts are mainly related to the formation of a dust cloud from any source of release, including layers or accumulations, which can generate an explosive atmosphere. In addition, the formation of dust layers, even if not likely to form a cloud, may lead to ignition due to self-heating or exposure to hot surfaces or thermal radiation, creating a fire hazard or overheating of equipment. An ignited layer can also act as an ignition source for an explosive atmosphere.

The classification of areas according to IEC 60079-10-2 has two main objectives: the determination of the type of hazardous zone and the definition of the extent of the hazardous area. The classification is developed through a structured process:

1. The first step consists in identifying whether the material is combustible and, for the purpose of assessing ignition sources, determining its main characteristics. The parameters to be considered include particle size, moisture content, minimum ignition temperature of both cloud and layer, and electrical resistivity. Based on these properties, the appropriate dust group is identified as:
   –Gruppo IIIA for combustible solid particles;
   –Gruppo IIIB for non-conductive dusts;
   –Gruppo IIIC for conductive dusts

1. The second step is the identification of the sources of dust release, including process equipment, loading and unloading points, possible leaks, and dust layers that can be disturbed and generate explosive clouds. This phase also includes the evaluation of the possible formation of dust deposits.
2. The third step consists of assessing the probability and duration of emissions from these sources, which determine the potential for explosive atmospheres to form. Continuous, primary, and secondary emissions are distinguished based on the frequency and duration of release.
3. The fourth step is the classification of the identified sources. Based on the frequency and duration of the presence of an explosive atmosphere, areas are classified as Zone 20, associated with continuous releases, Zone 21, associated with primary releases, and Zone 22, associated with secondary releases.

Finally, the extent of the ATEX zone is evaluated as the distance from the source of release beyond which the dust concentration is sufficiently below the minimum explosible concentration, such that the propagation of an explosion is no longer possible.