In North America, hazardous locations have traditionally been defined by a combination of classes and divisions as follows:
Class I – A location made hazardous by the presence of flammable gases or vapors that may be present in the air in quantities sufficient to produce an explosive or ignitable mixture.
Class II – A location made hazardous by the presence of combustible or electrically conductive dust.
Class III – A location made hazardous by the presence of easily ignitable fibers or flyings in the air, but not likely to be in suspension in quantities sufficient to produce ignitable mixtures.
Division 1 – A location where a classified hazard exists or is likely to exist under normal conditions.
Division 2 – A location where a classified hazard does not normally exist but is possible to appear under abnormal conditions.
The Zone System
The U.S. and Canada have recently revised installation codes to recognize an international 3-Zone area classification system for equipment used in hazardous locations.
Zone 0 – An area in which an explosive gas atmosphere is continuously present for a long period of time.
Zone 1 – An area in which an explosive atmosphere is likely to occur in normal operation.
Zone 2 – An area in which an explosive gas atmosphere does not normally exist.
In Canada, all new installations must use the 3-Zone system. Existing installations may continue to use the 2-Division system or opt to re-classify using the 3-Zone system.
In the U.S., all installations (both new and existing) can either continue using the 2-Division system or re-classify their product using the 3-Zone system.
For Class I, Division 1 (or Zone 1) locations, you must use a Type X purging system. Type X purging systems reduce the hazards from Division I (or Zone 1) to unclassified. You must cut power off immediately when the positive-pressure air system fails. You also must detect failure of the system at the discharge end of the fan.
Type Z purging systems reduce the classification from Division 2 (Zone 2) to unclassified. If the control room location and/or equipment is suitable for these type of purges, then it’s not necessary to de-energize the power supply circuit to the control room equipment immediately upon a positive pressure air system failure. However, for safety’s sake, you should de-energize that equipment as soon as possible after you detect air failure, or that some means of monitoring the atmosphere within the room be started.
Defined as systems having electrical/electronic circuitry that is incapable, under normal operating conditions, of causing ignition of a specified flammable gas-air, vapor-air, or dust-air mixture due to arcing or thermal means.