Hazardous Area Classifications

The purpose of a purge and pressurization system is to ensure compliance to specific safety standards for electrical equipment used in, around, or near an atmosphere with flammable gases or vapors, flammable liquids, combustible dusts, ignitable fibers or filings, where there is a risk of an explosion or fire.

Therefore, when selecting a purged and pressurization system, it’s important to have an understanding of the Class/Division System or Zone System used in defining these hazardous areas where the propensity for an explosion or fire exists.

Presently, two systems are used to classify hazardous areas: the Class/Division System used in the United States and Canada and the Zone System used in the rest of the world.

Class/Division/Group System Definitions

For purged and pressurization system(s) selection, hazardous location installations in the United States and Canada are identified and classified according to Class, Division, and Group. A review of these definitions is helpful assessing what type of purge system is required: X-purge, Y-purge, or Z-purge.

Class…Defines the nature and quantity of the hazardous material located in the surrounding environment.

Class I – Those locations where flammable gases/vapors may or may not exist in sufficient quantities to produce explosive or ignitable mixtures.

Class II – Those locations where combustible dusts may or may not be in sufficient quantities to produce explosive or ignitable mixtures.

Class III – Those locations where ignitable fibers may or may not be in sufficient quantities to produce explosive or ignitable mixtures.

Division…Defines the probability of the hazardous material being able to produce an explosive or ignitable mixture based on the presence of the hazardous material.

Division 1 – The hazardous material is present continuously, intermittently, or periodically under normal operating conditions resulting in a high probability of producing an explosive or ignitable mixture.

Division 2 – The hazardous material has a low probability of producing an explosive of ignitable mixture and is present only during abnormal conditions and for a short period of time.

Group…Defines the type of hazardous material in the surrounding atmosphere.

(Note: Groups A, B, C, and D are for gases and are Class I only; Groups E,F, and G are for dusts and flyings, and are Class II or III.

Group A – Atmospheres containing acetylene

Group B – Atmospheres containing a flammable gas, flammable liquid-produced vapor whose MESG is less than 0.45 mm or MIC ratio is less than 0.40. This would include hydrogen, acrolein, propylene oxide, butadiene, ethylene oxide.

Group C – Atmospheres containing a flammable gas, flammable liquid-produced vapor, or combustible liquid-produced vapor whose MESG is greater than 0.45 mm but less than or equal to 0.75 mm or MIC ratio is greater than 0.40 but less than or equal to 0.80. Gases in this class are ethylene, cyclopropane, acetaldehyde, and ethyl either.

Group D – Atmospheres containing a flammable gas, flammable liquid-produced vapor, or combustible liquid-produced vapor whose MESG is greater than 0.75 or MIC ratio is greater than 0.80. Gases in this class are ammonia, acetone, methane, ethanol, propane, gasoline, naphtha, benzene, butane, and natural gas.

Group E – Atmospheres containing combustible metal dusts. These metals include magnesium, aluminum, and many commercial alloys of these two metals.

Group F – Atmospheres containing combustible carbon-type dusts with 8% or more trapped volatiles such as coal, carbon black, and coke dust.

Group G – Atmospheres containing combustible dusts not included in Groups E and F. These dusts include flour, grain, wood, starch, plastic, and chemicals.

Zone System

For purged and pressurization system(s) selection, international (outside the US and Canada) hazardous locations are classified by the Zone system. There are two zones; gas or dust. Electrical equipment for gas atmospheres is divided into Groups and Subgroups.

The Zone identifies the probability of the gas or dust being present in enough quantity to produce explosive or ignitable mixtures. A review of these definitions is helpful assessing what type of purge systems is required: X-purge, Y-purge, or Z-purge.

Zone for GAS

Zone 0 – Ignitable mixtures of flammable gases or vapors are present continuously or for long periods of time

Zone 1 – Ignitable mixtures of flammable gases or vapors are likely to occur under normal operating conditions

Zone 2 – Ignitable mixtures of flammable gases or vapors are not likely to occur under normal operating conditions and if present are in the area for only a short period of time

Zone for DUST

Zone 20 – Combustible dusts or ignitable fibers or flyings are present continuously or for long periods of time

Zone 21 – Combustible dusts or ignitable fibers or flyings are likely to occur under normal operating conditions

Zone 22 – Combustible dusts or ignitable fibers or flyings are not likely to occur under normal operating conditions and if present are in the area for only a short period of time

Groups…The Zone system identifies three separate Groups for electrical equipment.

Group I – Equipment intended for use in mines susceptible to flammable mixtures of gases occurring naturally in the mine

Group II – Equipment intended for use in places with an explosive gas atmosphere other than mines where flammable mixtures occur naturally. Group II is further divided into three subgroups:

Group IIA – Atmospheres containing propane, or gases and vapors of equivalent hazard

Group IIB – Atmospheres containing ethylene, or gases and vapors of equivalent hazard

Group IIC – Atmospheres containing hydrogen or acetylene, or gases and vapors of equivalent hazard.

Group III – Equipment intended for use in places with an explosive dust atmosphere. Group III is further divided into three subgroups:

Group IIIA – Atmospheres containing combustible flyings

Group IIIB – Atmospheres containing non-conductive dust

Group IIIC – Atmospheres containing conductive dust