Why areas are classified
Where flammable gas, vapour, dust or fibres can be present, ordinary electrical equipment can ignite them. A spark from a switch or a hot surface on a luminaire is enough. Hazardous area classification identifies these places and how likely the hazard is, so that equipment can be chosen to suit.
Typical examples include refineries, fuel depots, chemical and pharmaceutical plants, grain handling, sawmills, wastewater treatment, paint booths, mines and offshore platforms. Usually only part of a site is classified, often in zones around a specific source of release.
The Class and Division system
North America has long used the Class and Division system, set out in the NEC in the United States and in the Canadian Electrical Code. The class names the kind of material, and the division names how likely it is to be present.
- Class I: flammable gases and vapours
- Class II: combustible dusts
- Class III: ignitable fibres and flyings, as in textile mills and woodworking
- Division 1: the hazard is present, or likely, in normal operation
- Division 2: the hazard is present only under abnormal conditions, such as a leak or an equipment failure
The Zone system
Most of the world uses Zones, defined in the IEC 60079 series of standards. Europe applies them through ATEX, and the IECEx scheme provides international certification. Zones divide the likelihood of a hazard into three levels instead of two.
Zones 0, 1 and 2 cover gas and vapour; Zones 20, 21 and 22 cover dust. Zones are also recognized in North America. The NEC permits them under Articles 505 and 506, and the Canadian Electrical Code uses Zones in Section 18, keeping Class and Division rules for existing facilities in an annex. Broadly, Division 1 covers Zones 0 and 1, and Division 2 corresponds to Zone 2.
- Zone 0 or 20: present continuously or for long periods
- Zone 1 or 21: likely to occur in normal operation
- Zone 2 or 22: not likely in normal operation, and only for short periods if it does occur
Groups and temperature codes
Materials are also grouped by how easily they ignite. In the Division system, gases fall into Groups A to D (acetylene, hydrogen, ethylene and propane types) and dusts into Groups E to G (metal, carbon and other dusts such as grain). In the Zone system, gases are Group IIA, IIB or IIC, with IIC the most demanding, and dusts are IIIA, IIIB or IIIC.
The temperature code, or T code, gives the highest surface temperature the equipment can reach, from T1 (450 °C) to T6 (85 °C). It must be below the ignition temperature of the gas, vapour or dust present. Luminaires produce heat by nature, so their T code deserves a careful check.
How equipment is protected
Equipment for hazardous locations uses one or more protection concepts, each with a code that appears in its marking.
The marking also shows the equipment protection level, such as Gb for Zone 1 gas areas or Dc for Zone 22 dust areas. In North America, luminaires for hazardous locations are evaluated to standards such as UL 844.
- Explosion-proof or flameproof enclosure (Ex d): contains an internal explosion and cools the escaping gases
- Increased safety (Ex e): extra measures that prevent sparks and hot spots in the first place
- Non-sparking equipment (Ex nA, now Ex ec): for Zone 2
- Dust protection by enclosure (Ex t): keeps dust out and limits surface temperature
- Encapsulation (Ex m) and intrinsic safety (Ex i): common for components, sensors and control circuits
Practical advice
A luminaire is only suitable if every part of its marking matches the area where it will be installed.
- The area classification is set by the owner, usually with a qualified engineer, and accepted by the authority having jurisdiction, not by the equipment supplier
- Read the full marking on each luminaire: class or zone, division or protection level, gas or dust group, and T code
- Check the certificate behind the marking, including any special conditions of use
- Make sure the certification is accepted in the country where the luminaire is installed
- Install, seal and maintain the equipment as its instructions and the electrical code require
- Keep the records: inspectors will ask for them