the equipment itself under normal operating conditions.
The substance referred to by class has 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 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 a MIC (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 coke 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.
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).
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.
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.|
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 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.|
A room with a propane gas installation will typically be classified with the
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