Explosion Proof, Is it Really? Hazardous Locations

Explosion Proof: Is it Really?

Case in Point

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.

	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.

	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.

	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.)

Groups A, B, C and D apply to Class I environments (containing combustible gases) as follows:
	Group A: acetylene

	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.

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

	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:
	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.

	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).

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

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

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