Electrical equipment installation in atmosphere with flammable gases or vapors, flammable liquids, combustible dusts, ignitable fibers or flyings represents a risk for fire and explosion.
Areas with possible fire or explosion risks due to explosive atmospheres and/or mixtures - are called hazardous (or classified) locations or areas. These areas are in North America (United States and Canada) historically classified with the Class/Division system. In Europe and the rest of the world - but also more and more in North America - the Zone system is used.
The hazardous area classification system determines required protection techniques and methods for electrical installations in the location.
Class/Division System
The Class/Division/Group system is based on Article 500 of the National Electrical Code (NEC) where
- Classes - defines the general nature of the hazardous material in the surrounding atmosphere
- Divisions - defines the probability of hazardous material being present the surrounding atmosphere
- Groups - defines the type of the hazardous material in thesurrounding atmosphere
Class
Class defines the general nature (or properties) of the hazardous material in the surrounding atmosphere.
| Class | Nature of Hazardous Material |
|---|---|
| Class I | Hazardous because flammable gases or vapors are present (or may be present) in quantities sufficient to produce explosive or ignitable mixtures. |
| Class II | Hazardous because combustible or conductive dusts are present (or may be present) in quantities sufficient to produce explosive or ignitable mixtures. |
| Class III | Hazardous because ignitable fibers or flyings are present (or may be present) in quantities sufficient to produce explosive or ignitable mixtures. |
Division
Division defines the probability of the hazardous material being present in an ignitable concentration in the surrounding atmosphere.
| Division | Probability of Hazardous Material |
|---|---|
| Division 1 | The substance referred to by classhas a high probability of producing an explosive or ignitable mixture due to it being present continuously, intermittently, or periodically or from |
| Division 2 | The substance referred to by classhas a low probability of producing an explosive or ignitable mixture and is present only during abnormal conditions for a short period of time - such as a container failure or system breakdown |
Group
Group defines the type of the hazardous material in the surrounding atmosphere.
| Group | Type of Hazardous Material |
|---|---|
| Group A | Atmosphere containing acetylene. |
| Group B | Atmosphere containing a flammable gas, a flammable liquid produced vapor, or a combustible liquid produced vapor mixed with air that may burn or explode, having either a MESG (Maximum Experimental Safe Gap)1) value less than or equal to 0.45 mm or aMIC (Minimum Igniting Current)2) ratio less than or equal to 0.40 - such as hydrogen or fuel and combustible process gases containing more than 30% hydrogen by volume - or gases of equivalent hazard such as butadiene, ethyleneoxide, propylene oxide and acrolein. |
| Group C | Atmosphere containing a flammable gas, a flammable liquid produced vapor or a combustible liquid-produced vapor whose MESG is greater than 0.75 mm or MIC ratio is greater than 0.40 and less than 0.80 - such as carbon monoxide,ether, hydrogen sulfide, morphline, cyclopropane, ethyl, isoprene, acetaldhyde and ethylene or gases of equivalent hazard. |
| Group D | Atmosphere containing flammable gas, flammable liquid produced vapor, or combustible liquid produced vapor mixed with air that may burn or explode, having either a MESG value greater than 0.75 mm or a MIC ratio greater than 0.80 - such as gasoline, acetone, ammonia, benzene, butane, ethanol, hexane, methanol, methane, vinyl chloride, natural gas, naphtha, propane or gases of equivalent hazard. |
| Group E | Atmosphere containing combustible metal dusts, including aluminum, magnesium, bronze, chromium, titanium, zinc and their commercial alloys or other combustible dusts whose particle size, abrasiveness and conductivity present similar hazards in connection with electrical equipment. |
| Group F | Atmosphere containing carbonaceous dusts, carbon black, coal black, charcoal, coal or co*ke dusts that have more than 8% total entrapped volatiles or dusts that have been sesitized by other materials so they present an explosion hazard. |
| Group G | Atmosphere containing combustible dust not included in Group E & F - such as flour, grain, starch, sugar, wood, plastics and chemicals. |
1) MESG (Maximum Experimental Safe Gap) - The maximum clearance between two parallel metal surfaces that has been found under specified test conditions to prevent an explosion in a test chamber from being propagated to a secondary chamber containing the same gas or vapor at the same concentration.
2) MIC (Minimum Igniting Current) Ratio - The ratio of the minimum current required from an inductive spark discharge to ignite the most easily ignitable mixture of a gas or vapor, divided by the minimum current required from an inductive spark discharge to ignite methane under the same test conditions.
Groups A, B, C, and D are for gases (Class I only). Groups E, F, and G are for dusts and flyings (Class II or III).
The specific hazardous materials within each group and their automatic ignition temperatures can be found in Article 500 of the National Electrical Code and in NFPA 497.
Zone System
The Zone system is based on Article 505/506 of the National Electrical Code (NEC) and follows the international method of area classification as developed by the International Electrotechnical Commission (IEC).
- Zones - defines the general nature (or properties) of the hazardous material - if its gas or dust, and the probability of the hazardous material in the surrounding atmosphere
- Groups - defines the type of the hazardous material and (partly) the location of the surrounding atmosphere
Zone
Zone defines the general nature - if it is a gas or dust - and the probability of hazardous material being present in an ignitable concentration in the surrounding atmosphere. The Zone system has three levels of hazard for gas or dust where the Division system has two.
Gases, Vapors and Mists
Article 505 National Electrical Code (NEC)
| Zone | Nature and Probability of Hazard Material |
|---|---|
| Zone 0 | Ignitable concentrations of flammable gases or vapors which are present continuously or for long periods of time. |
| Zone 1 | Ignitable concentrations of flammable gases or vapors which are likely to occur under normal operating conditions. |
| Zone 2 | Ignitable concentrations of flammable gases or vapors which are not likely to occur under normal operating conditions and do so only for a short period of time. |
Dusts
Article 506 National Electrical Code (NEC)
| Zone | Nature and Probability of Hazard Material |
|---|---|
| Zone 20 | An area where combustible dusts or ignitable fibers and flyings are present continuously or for long periods of time. |
| Zone 21 | An area where combustible dusts or ignitable fibers and flyings are likely to occur under normal operating conditions. |
| Zone 22 | An area where combustible dusts or ignitable fibers and flyings are not likely to occur under normal operating conditions and do so only for a short period of time. |
Zones compares to classes and divisions in the Class/Division system.
Group
Group define the type of hazardous material and (partly) the location of the surrounding atmosphere. Group is divided in three groups where Group I is reserved for mining locations. Group II is for explosive gases (Zone 0, 1 and 2) and Group III is for explosive dusts (Zone 20, 21 and 22).
| Group | Type of Hazardous Material and Location of Atmosphere | |
|---|---|---|
| Group I | Mines susceptible to firedamp (flammable mixture of gases naturally occurring in a mine). | |
| Group II | Explosive gas atmosphere other than mines susceptible to firedamp. Group II equipment is subdivided into three subgroups. | |
| A | Atmospheres containing propane, acetone, benzene, butane, methane, petrol, hexane, paint solvents or gases and vapors of equivalent hazard. | |
| B | Atmospheres containing ethylene, propylene oxide, ethylene oxide, butadiene, cyclopropane, ethyl ether, or gases and vapors of equivalent hazard. | |
| C | Atmospheres containing acetylene, hydrogen, carbon disulphide or gases and vapors of equivalent hazard. | |
| Group III | Explosive dust atmosphere. Group III equipment is subdivided into three subgroups. | |
| A | Atmospheres containing combustible flyings. | |
| B | Atmospheres containing non-conductive dust. | |
| C | Atmospheres containing conductive dust. | |
Example - Hazardous Area Classification
A room with a propane gas installation will typically be classified with the
- Class/Division system as: Class I, Division 2, Group D
- Zone system as: Zone 2, Group IIA
- Protection Techniques for Hazardous Areas - North America
Recommended reading for this topic:
- National Electrical Code, NFPA 70, Chapter 5, Article 500
- 29 CFR 1910 Subpart S, Electrical 1910.307
- NFPA 497, "Classification of Gases, Vapors, and Dusts for Electrical Equipment in Hazardous Classified Locations"
- NFPA Handbook, "Electrical Installations in Hazardous Locations, " by P. J. Schram and M. W. Earley
- NFPA 70E, Chapter 5, "Hazardous (Classified) Locations"
- NFPA (Fire) HAZ-10, "Fire Protection Guide to Hazardous Materials"
- ANSI/UL 913, "Intrinsically Safe Apparatus"
- NFPA 496, "Purged and Pressurized Enclosure for Electrical Equipment in Hazardous Locations."
