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Explosion Proof: Is it Really?Case in PointDefinitions  
EXPLOSION PROOF: Is it Really?
Hazardous Locations: Understanding Class, Division and Group Definitions
 

While nothing can replace the published codes defining Class, Division and Group ratings for explosion proof applications, these guidelines provide a general overview of the terms and how they are used.

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Class: The term “Class” is used to categorize the nature of the hazard. For example, Class I comprises flammable gases and vapors; Class II comprises combustible dust; and Class III comprises ignitable fibers.

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_ Division: The term “Division” categorizes the area in which the equipment is to be installed. Division 1 covers all electrical devices including sensors and describes an environment that is usually or likely to be hazardous. Division 2 is applied alone to lighting to limit operating temperature and defines an environment that is usually safe but may become hazardous in the event of some kind of accident or failure.
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_ Groups: Within Class I and Class II, there are material groups representing the degree of hazard. For instance, Class I refers to gases and vapors; Group A belongs to Class I and is defined as atmospheres containing acetylene. (Acetylene is explosive when mixed
with air over a wide — 3 to 80% — range.)
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Groups A, B, C and D apply to Class I environments (containing combustible gases) as follows:

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Group A: acetylene

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Group B: hydrogen, fuel and combustible gases containing more than 30% hydrogen by volume or equivalent hazard such as butadiene, ethylene oxide, propylene oxide and acrolein.

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Group C: ethyl, either ethylene or gases or vapors of equivalent hazard

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Group D: acetone, ammonia, benzene, butane, cyclopropane, ethanol, gasoline, hexane, methanol, methane, natural gas, naphtha, propane or gases of equivalent hazard

Groups E, F and G apply to Class II environments (containing combustible dusts) as follows:

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Group E: metal dusts: aluminum, magnesium and their chemical alloys, or other combustible dusts whose particle size, abrasiveness and conductivity present similar hazards to the use of electrical equipment, characterized by resistivity < 100 ohm – centimeter.

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Group F: carbonaceous dusts: carbon black, charcoal, coal or coke dusts that have more than 8% total entrapped volatiles, or dusts that have been sensitized by other materials so that they may present an explosion hazard, characterized by resisitivity between 100 and 108 ohm – centimeter (> 105 ohm – centimeter in Division 2).

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Group G: combustible dusts not in Groups E or F, including flour, grain, wood, plastic and chemicals, characterized by resistivity > 108 ohm �centimeter.

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Published 2003, RdF Corp., expanded. Original publishing: PROCESS HEATING, July 1998



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