Hazardous locations are areas where fire or explosion hazards may exist due to the presence of flammable gases/vapors, flammable liquids, combustible dust, or ignitable fibers. The type of hazard is defined as an “explosion hazard” in Class I and II locations, and a “fire hazard” in Class III locations.

Materials do not have to be in the gaseous state for an explosion. Combustible dusts, and atomized liquids are other examples.

Class I Locations:

Class I locations are areas in which flammable gases/vapors may be present in the air in quantities sufficient to produce explosive or ignitable mixtures:

• Group A: acetylene
• Group B: hydrogen, or gases/vapors of equivalent
• Group C: ethyl ether vapors, ethylene or cyclopropane
• Group D: gasoline, hexane, naptha, benzine, butane, benzol, lacquer solvent vapors, natural gas

Ignition temperatures:

The ignition temperature of a flammable material is critical in determining the acceptability of equipment/instrumentation that operates in hazardous locations as the external surface of the enclosure can act as an ignition source if the surface temperature exceeds the ignition temperature of the flammable material. High surface temperatures can result from normal operation of the equipment; abnormal operation (i.e. overheating); or because of a fault condition.

Equipment destined for installation in hazardous areas is marked with a code representing the maximum external surface temperature. Equipment cannot be used in hazardous areas when the maximum surface temperature is greater than the ignition temperature of the gases/vapors present.

Class II Locations

Class II deals specifically with combustible dusts. The dust may be suspended in the air, in a cloud or in a layer on electrical equipment. The ignition temperature of a dust layer is usually lower than for a dust cloud.

• Group E: metal dusts (aluminum, magnesium and their alloys) and other combustible dusts whose particle size, abrasiveness and conductivity present similar hazard
• Group F: carbonaceous dusts (carbon black, charcoal, coal or coke) that have more than 8% total entrapped volatiles, or dusts sensitized by other materials so they present an explosion hazard
• Group G: combustible dusts not included in groups E & G

Quick reference chart

Source: Monitor Technologies, LLC, 2005 "Hazardous Locations A Primer" PDF, Retrieved on Dec. 29th 2006

Sites of interest

UL: North American system overview: http://www.ul.com/hazloc/nasystem.html