Hazardous Area Classess, Division and Group System
Whilst in North American installations are classified by Classes, Divisions and Groups to ascertain the level of safety required.
Hazardous Area Class – The Class defines the general nature (or properties) of the hazardous material in the surrounding atmosphere which may or may not be in sufficient quantities.
- Class I—Locations in which flammable gases or vapors may or may not be in sufficient quantities to produce explosive or ignitable mixtures.
- Class II—Locations in which combustible dusts (either in suspension, intermittently, or periodically) may or may not be in sufficient quantities to produce explosive or ignitable mixtures.
- Class III—Locations in which ignitable fibers may or may not be in sufficient quantities to produce explosive or ignitable mixtures.
Hazardous Area Division – The Division defines the probability of the hazardous material being able to produce an explosive or ignitable mixture based upon its presence.
- Division 1 indicates that the hazardous material has a high probability of producing an explosive or ignitable mixture due to it being present continuously, intermittently, or periodically or from the equipment itself under normal operating conditions.
- Division 2 indicates that the hazardous material has a low probability of producing an explosive or ignitable mixture and is present only during abnormal conditions for a short period of time.
Gas & Dust Group – The Group defines the type of hazardous material in the surrounding atmosphere. Groups A, B, C, and D are for gases (Class I only) while groups E, F, and G are for dusts and flyings (Class II or III).
- Group A— Explosive atmospheres tmospheres containing acetylene.
- Group B— Explosive atmospheres containing a flammable gas, flammable liquid-produced vapor, or combustible liquid-produced vapor whose MESG is less than 0.45 mm or MIC ratio is less than 0.40. Typical gases include hydrogen, butadiene, ethylene oxide, propylene oxide, and acrolein.
- Group C— Explosive 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. Typical gases include ethyl either, ethylene, acetaldehyde, and cyclopropane.
- Group D— Explosive atmospheres containing a flammable gas, flammable liquid-produced vapor, or combustible liquid-produced vapor whose MESG is greater than 0.75 mm or MIC ration is greater than 0.80. Typical gases include acetone, ammonia, benzene, butane, ethanol, gasoline, methane, natural gas, naphtha, and propane.
- Group E— Explosive atmospheres containing combustible metal dusts such as aluminum, magnesium, and their commercial alloys.
- Group F— Explosive atmospheres containing combustible carbonaceous dusts with 8% or more trapped volatiles such as carbon black, coal, or coke dust.
- Group G— Explosive atmospheres containing combustible dusts not included in Group E or Group F. Typical dusts include flour, starch, grain, wood, plastic, and chemicals.
Explosion Protection Techniques and Methods
Various explosion protection techniques and methods have been developed and employed, thus reducing or minimizing the potential risks of explosion or fire from electrical equipment located in hazardous locations.
Not all methods are listed.
Class/Division system
- Explosion-proof—A type of protection that utilizes an enclosure that is capable of withstanding an explosive gas or vapor within it and or preventing the ignition of an explosive gas or vapor that may surround it and that operates at such an external temperature that a surrounding explosive gas or vapor will not be ignited thereby.
- Intrinsically Safe—A type of protection in which the electrical equipment under normal or abnormal conditions is incapable of releasing sufficient electrical or thermal energy to cause ignition of a specific hazardous atmospheric mixture in its most easily ignitable concentration.
- Dust Ignition-proof—A type of protection that excludes ignitable amounts of dust or amounts that might affect performance or rating and that, when installed and protected in accordance with the original design intent, will not allow arcs, sparks or heat otherwise generated or liberated inside the enclosure to cause ignition of exterior accumulations or atmospheric suspensions of a specified dust.
- Non-incendive—A type of protection in which the equipment is incapable, under normal conditions, of causing ignition of a specified flammable gas or vapor-in-air mixture due to arcing or thermal effect.
Zone System
The below concepts are high-level protection concepts.
There are also sub-levels of protection that may or not be applicable to each type. Also, some equipment may combine multiple types of protection.
- Flame-proof—A type of protection in which an enclosure can withstand the pressure developed during an internal explosion of an explosive mixture and that prevents the transmission of the explosion
to the explosive atmosphere surrounding the enclosure and that operates at such an external temperature that a surrounding explosive gas or vapor will not be ignited there. This type of protection is referred to as “Ex d”.
- Intrinsically Safe—A type of protection in which the electrical equipment under normal or abnormal conditions is incapable of releasing sufficient electrical or thermal energy to cause ignition of a specific hazardous atmospheric mixture in its most easily ignitable concentrations. This type of protection is referred to as “Ex i”.
- Increased Safety—A type of protection in which various measures are applied to reduce the probability of excessive temperatures and the occurrence of arcs or sparks in the interior and on the external parts of electrical apparatus that do not produce them in normal service. Increased safety may be used with flame-proof type of protection. This type of protection is referred to as “Ex e”.
- Type n—A type of protection applied to electrical equipment such that in normal operation it is not capable of igniting a surrounding explosive atmosphere. This type of protection is referred to as “Ex n”.
- Type t—A type of protection in which the electrical equipment is equipped with an enclosure providing dust ingress protection and a means to limit surface temperatures. This type of protection is referred to as “Ex t”.
Equipment Protection Level (EPL) Markings
The EPL marking indicates the level of protection that is given to equipment based on the likelihood of its becoming a source of ignition and distinguishing the difference between explosive gas atmospheres, explosive dust atmospheres, and the explosive atmospheres in mines susceptible to firedamp.
Temperature Code (T Code)
A mixture of hazardous gases and air may be ignited by coming into contact with a hot surface.
The conditions under which a hot surface will ignite a gas depends on surface area, temperature, and the concentration of the gas. The same can be said about combustible dusts. The T Class or Temperature Class Rating of a product denotes the maximum surface temperature that a given product will not exceed under a specified ambient temperature. For example, a product with a T code of T3 means that its maximum surface temperature will not exceed 200°C provided it is operated in a ambient temperature defined by the manufacturer.
Class/Division System
Approved hazardous area equipment is marked according to which Class (I, II, or III), Division (1 or 2), Group (A, B, C, D, E, F, or G), and temperature code (T1 through T6) that it is rated for.
For intrinsically safe equipment the words “Intrinsically Safe” or “IS” will precede the actual approval marking to indicate it as being intrinsically safe.
Examples are listed below: Class I Division 1 Group B,C,D T5 CL I Div 2 GP ABCD T5 IS CL I,II,III Div 1 GP ABCDEFG CL II,III Div 1,2 GP EFG T4 Zone system Approved equipment is marked according to the protection concept for which it has been designed (Ex i, Ex d, Ex n, and etc.), the group (I, IIA, IIB, IIC, IIIA, IIIB, or IIIC), and temperature code (T1 through T6) that it is rated for.
For the United States it will be preceded by which Class and Zone it is approved for.
Examples are listed below:
Ex ia IIC T5
Ex d IIB+H2 T6
Ex nA IIC T6
Class I Zone 2 AEx nC IIC T5
ADDITIONAL HAZARDOUS AREA & EXPLOSIVE ATMOSPHERE TERMINOLOGY
Although the following terminology is not permitted for markings it is commonly used to describe the various types of approvals or when speaking of them.
XP—Flameproof approval for Hazardous Area Class I Division 1
EXP—Flameproof approval for Hazardous Area Class I Division 1
NI—Non-incendive approval for Hazardous Area Class I Division 2
DIP—Dust Ignition Proof approval for Hazardous Area Class II Division 1
S—“Suitable For” for Hazardous Area Class II Division 2
IS—Intrinsically Safe
International Approvals
Countries participating in the IECEx Scheme (International Electrotechnical Commission on explosion protected equipment, known as “Ex”) can issue either an international certification or a national certification of explosion protected equipment. Each country within the IECEx scheme establishes an ExCB (Ex Certification Body) which can approve products. ExCB’s can issue the national certification for their country based upon the IECEx standards (including any national deviations) and the international certification. Currently, Australia is the only country accepting international certifications for use in their country.