In North America, hazardous locations have traditionally been defined by a combination of classes and divisions as follows:

Classes

Class I – A location made hazardous by the presence of flammable gases or vapors 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.

Divisions

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 appears 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.

Pressurization Systems

Type X
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 the power off immediately when the positive-pressure air system fails. You also must detect a failure of the system at the discharge end of the fan.

Type Z
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 types 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.

Non-Incendive System

Defined as systems having electrical/electronic circuitry that is incapable, under normal operating conditions, causing ignition of a specified flammable gas-air, vapor-air, or dust-air mixture due to arcing or thermal means.

See also: Strongarm Hazardous Area Systems