Groups, zones, categories and devices

Internationally, IEC 60079-0 and in Europe EN 60079-0 define groups for potentially explosive atmospheres. There are three groups. Group I covers underground mines or mines susceptible to firedamp, Group II surface environments with Gas, and Group III surface environments with Dust. In Product Directive 2014/34/EU there are only two groups defined, Group I for underground mines or mines endangered by firedamp and/or combustible dust, and Group II for all surface installations.

The group designation is essentially based on where equipment can be used (equipment classification).

Explosive gases, vapors and dusts have different chemical properties that affect the likelihood and severity of an explosion. Such properties include flame temperature, minimum ignition energy, upper and lower explosive limits, and molecular weight. Based on the nature of the explosive gas/dust, gases are grouped into IIA, IIB and IIC and dusts into IIIA, IIIB and IIIC.

Temperature classes are defined for equipment based on its maximum surface temperature. When selecting a piece of equipment for a potentially explosive atmosphere, the equipment’s maximum surface temperature must be lower than the ignition temperature of the possible potential gas or dust mixture.

Within industries, all potentially explosive atmospheres are required to have an area classification called Zones.

The zone system is used all over the world and nowadays also accepted as an alternative system in North America.

Authorities normally determine the area, but that can also be performed by a third party; a notified body or other experts. It is the owner’s responsibility to ensure that the classification of their site is performed before suitable products can be selected and installed at the location.

Globally, a Zone system is used to classify potentially explosive areas. The Worker Protection Directive 1999/EC and the international standards IEC 60079-10-x, EN 60079-10-x define these zones. In all cases, Zone classification is the responsibility of the owner of the site where the potentially explosive atmosphere exists. 

There are 6 zones:

• Zones 0 (for gas) and 20 (for dust), where there is a continuous presence of explosive atmosphere.

• Zones 1 (for gas) and 21 (for dust), where there is an occasional occurrence of potentially explosive atmosphere.

• Zones 2 (for gas) and 22 (for dust), where potentially explosive atmospheres can occur by accident, not during normal operation. 

Equipment categories are used in ATEX. The category indicates which safety level of product must be used in each zone. In Zone 0/20, category 1 devices must be used; in Zone 1/21, category 2 devices; and in zone 2/22, category 3 devices. 

Classification into categories is of particular importance, because all the inspection, maintenance and repair duties of the end user will depend on the category of the product/equipment and not on the zone where it is installed.

The latest revisions of the IEC and EN standards introduce the concept of “equipment protection levels”, which identify products according to the ignition risk they might cause. EPL also takes the potential consequences of possible explosion into consideration. For zone 0/20, the equipment protection level required would be “a”; for zone 1/21, the level would be “b”; and for zone 2/22, the level would be “c”.