Introduction to Electrical Area Classification

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Unformatted text preview: PDHengineer.com Course № E-1004 Introduction to Area Classification To receive credit for this course This document is the course text. You may review this material at your leisure either before or after you purchase the course. To purchase this course, click on the course overview page: http://www.pdhengineer.com/pages/E-1004.htm or type the link into your browser. Next, click on the Take Quiz button at the bottom of the course overview page. If you already have an account, log in to purchase the course. If you do not have a PDHengineer.com account, click the New User Sign Up link to create your account. After logging in and purchasing the course, you can take the online quiz immediately or you can wait until another day if you have not yet reviewed the course text. When you complete the online quiz, your score will automatically be calculated. If you receive a passing score, you may instantly download your certificate of completion. If you do not pass on your first try, you can retake the quiz as many times as needed by simply logging into your PDHengineer.com account and clicking on the link Courses Purchased But Not Completed. If you have any questions, please call us toll-free at 877 500-7145. PDHengineer.com 5870 Highway 6 North, Suite 310 Houston, TX 77084 Toll Free: 877 500-7145 administrator@PDHengineer.com Introduction to Electrical Area Classification Written by: Rebecca Johnson, PE Introduction Zones and Divisions for Area Classification There are several documents that can be used to aide in the decision to classify an area. How an area is classified determines the type of equipment that can be used in it. Most engineers are familiar with the terms hazardous, non-hazardous, classified, unclassified and Class I, Division 1, Class I, Division 2. Less familiar are the Class II, Divisions 1 and 2, Class III, Divisions 1 and 2, and the Class I, Zone 0, Class I, Zone 1, and Class I, Zone 2 designations. The designations are also further defined by groups which further breakdown the type of hazardous material. An area is classified based on the properties of the flammable vapors, liquids, gases, or combustible dust or fiber that may be present and the chance that a flammable or combustible concentration would be present. The class designations tell you what type of flammable or combustible material is present. Class designations use Roman numerals. The division designation tells you to what degree the hazard is present. Division designations and Zone designations use Arabic numbers. When flammable gases or combustible dusts are mixed in the proper proportion with air, a source of energy is all that is needed to create an explosion. This source could be from electricity - equipment that can produce arcs or sparks under normal operation when contacts are opened or closed (such as switches, circuit breakers, motor starters, or plugs and receptacles). Another source could be from heat. Lights (luminaries) and motors could have a surface temperature that may exceed the safe limits of a flammable atmosphere. Sometimes there could be a non-electrical source of ignition, such as a hammering on masonry or dropping a tool on a ferrous surface if the tool is not made of non-sparking material (such as aluminum). The Fire Triangle (also for explosions) The “fire triangle” refers to the three elements below that must be present for a fire (or explosion) to occur and sustain itself. 1. A flammable liquid or vapor or combustible dust must be present in sufficient quantity. 2. There must be the proper mixture of oxygen 3. There must be an ignition source. www.pdhengineer.com Page 1 of 10 Continuing Education for Engineers Each element above comprises one side of the triangle. If any side of the triangle is broken, then a fire cannot start or sustain itself. Article 500 of the NEC Article 500 of the NEC covers Class and Division designations. Class I Class I locations are those in which flammable gases or vapors exist or may be present in the air in enough of a quantity to produce explosive or ignitable mixture. Class I, Division 1 A Class I, Division 1 location is a location in which: 1. Ignitable concentrations of flammable gases or vapors can exist under normal operating conditions; 2. Ignitable concentrations of such gases or vapors may exist frequently because of repair or maintenance operations or because of leakage; 3. Breakdown of the equipment or a failure of the process might release ignitable concentrations of flammable gases or vapors and might also cause simultaneous failure of electrical equipment in such a way as to directly cause the electrical equipment to become a source of ignition. Examples include: • • • • Places where volatile flammable liquids or liquefied flammable gases are transferred from one container to another Locations containing open tanks or vats of volatile flammable liquids Interiors of spray booths and areas in the vicinity of spraying and painting operations where volatile flammable solvents are used Gas generator rooms and other portions of gas manufacturing plants where flammable gas may escape. In some Division 1 locations, ignitable concentrations of flammable gases or vapors may be present continuously or for long periods of time, such as in the inside of inadequately vented enclosures containing instruments normally venting flammable gases or vapors to the interior of the enclosure or the inside of vented tanks containing volatile flammable liquids. Class I, Division 2 www.pdhengineer.com Page 2 of 10 Continuing Education for Engineers A Class I Division 2 location is a location in which: a. Volatile flammable liquids or flammable gases are handled, processed or used but which will normally be kept within closed containers or systems from where they can escape only in the case of accidental rupture or breakdown of the containers (abnormal operation); b. Ignitable concentrations of gases or vapors are normally prevented by positive mechanical ventilation. The ignitable concentrations of the gases or vapors may become hazardous if the ventilation system fails; c. Any location that is adjacent to a Class I, Division 1 location where ignitable concentrations of gases or vapors might be communicated unless prevented by adequate positive-pressure ventilation from a clean source of air and safeguards against ventilation failure. Class I Groups Class I Group classifications are listed as Groups A, B, C, and D: Group A is acetylene. Group B is typically hydrogen, but is a flammable gas, flammable liquid produced vapor or combustible liquid produced vapor mixed with air that may burn or explode that has a maximum experimental safe gap (MESG) value less than or equal to .45 mm or a minimum igniting current ratio (MIC ratio) less than or equal to .40. Group C is typically ethylene, but is a flammable gas, flammable liquid produced vapor or combustible liquid produced vapor mixed with air that may burn or explode that has a maximum experimental safe gap (MESG) value greater than .45 mm and less than or equal to .75 mm or a minimum igniting current ratio (MIC ratio) greater than .40 and less than or equal to .80. Group D is typically propane, but is a flammable gas, flammable liquid produced vapor or combustible liquid produced vapor mixed with air that may burn or explode that has a maximum experimental safe gap (MESG) value greater than .75 or a minimum igniting current ratio (MIC ratio) greater than .80. MESG is the maximum experimental safe gap in millimeters. It is the maximum gap between mating surfaces of explosion proof enclosures that will prevent the flames or hot gases resulting from an explosion inside the enclosure from causing ignition of the surrounding external atmosphere. This can only be determined experimentally and is dependent on several factors. MIC is the minimum igniting current ratio. It is the ratio of the minimum current required from an inductive spark discharge to ignite the most easily ignitable mixture of a www.pdhengineer.com Page 3 of 10 Continuing Education for Engineers gas or vapor divided by the minimum current required from an inductive spark discharge to ignite methane under the same test conditions. Class II Class II locations are those in which there is the presence of combustible dust. There are three groups of combustible dusts based on the type of material – metallic, carbonaceous, or organic. Class II, Division 1 A Class II Division 1 location is a location: a. In which combustible dust is in the air under normal operating conditions in quantities sufficient to produce explosive or ignitable mixtures; b. Where mechanical failure or abnormal operation of machinery or equipment might cause such explosive or ignitable mixtures to be produced, and might also provide a source of ignition through simultaneous failure of electric equipment, through operation of protection devices, or from other causes; c. In which combustible dusts of an electrically conductive nature may be present in hazardous quantities. Combustible dusts that are electrically nonconductive include dusts produced in the handling and processing of grain, pulverized sugar, pulverized spices, dried hay and other organic materials that may produce combustible dusts when processed or handled. Only Group E dusts are considered to be electrically conductive for classification purposes. Dusts containing magnesium or aluminum are particularly hazardous and extreme precaution must be used with them. Class II, Division 2 A Class II, Division 2 location is a location where: a. Combustible dust is not normally in the air in enough quantity to produce an explosive or ignitable mixture and dust accumulations are normally not enough to interfere with the normal operation of electrical equipment. Combustible dust may be in suspension in the air as a result of infrequent malfunctioning of handling or processing equipment; b. Combustible dust accumulations around the vicinity of the electrical equipment may be enough to interfere with the dissipation of heat from electrical equipment or may be ignited by abnormal operation or failure of electrical equipment. www.pdhengineer.com Page 4 of 10 Continuing Education for Engineers The quantity of combustible dust that may be present and the adequacy of dust removal systems are factors in determining if the area should be classified or unclassified. Where products such as seed are handled such that there is a low quantity of dust, the amount of dust may be a factor in determining if the area should be classified. Class II Groups Class II Groups are listed as Groups E, F, and G: Group E is an atmosphere containing combustible metal dusts including aluminum, magnesium and their commercial alloys or other combustible dusts whose particle size, abrasiveness, and conductivity present similar hazards in the use of electrical equipment. Group F is an atmosphere containing combustible carbonaceous dust that have more than 8 percent total entrapped volatiles or that have been sensitized by other materials so that they present an explosion hazard. Coal, carbon black and charcoal are examples of carbonaceous dusts. Group G is an atmosphere that contains combustible dusts not included in Group E or F such as flour, grain, wood or plastic. Certain dusts may require additional precautions due to chemical phenomena that can result in the generation of ignitable gases. Class III Class III locations are those where there are easily ignitable fibers or where materials that produce combustible flyings are handled, manufactured or used. A Class III, Division 1 location is a location in which easily ignitable fibers or materials producing combustible flyings are handled, manufactured or used. For example, textile mills and fiber manufacturing and processing plants, cotton gins and cotton seed mills, clothing manufacturing plants. Easily ignitable fibers and flyings include rayon, cotton, and Spanish moss. A Class III, Division 2 location is a location in which easily ignitable fibers are stored or handled. Article 505 of the NEC Article 505 of the NEC covers Class and Zone designations. Class I Class I, Zone 0 www.pdhengineer.com Page 5 of 10 Continuing Education for Engineers A Class I, Zone 0 is a location where: a. b. Ignitable concentrations of flammable gases or vapors are present continuously; Ignitable concentrations of flammable gases or vapors are present for long periods of time. This includes inside vented tanks or vessels that contain volatile flammable liquids and inside inadequately vented spraying or coating enclosures where volatile flammable solvents are used. It is not good practice to install electrical equipment in Zone 0 locations except when the equipment is essential to the process or when other locations are not feasible. If it is necessary to install the equipment, it is good practice to use an intrinsically safe system. In Class I, Zone 0 locations only intrinsically safe wiring methods may be used. Class I, Zone 1 A Class I, Zone 1 location is a location where: a. b. c. Ignitable concentrations of flammable gases or vapors are likely to exist under normal operating conditions; Ignitable concentrations of flammable gases or vapors may exist frequently because of repair or maintenance operations or leakage; Equipment is operated or processes are carried on such that equipment breakdown or faulty operations could result in the release of ignitable concentrations of flammable gases or vapors and also cause simultaneous failure of electrical equipment in a mode to cause the electrical equipment to become a source of ignition; That is adjacent to a Class I, Zone 0 location from which ignitable concentrations of vapors could be communicated unless communication is prevented by adequate positive pressure ventilation from a source of clean air and effective safeguards against ventilation failure are provided. d. This includes locations where volatile flammable liquids or liquefied flammable gases are transferred from one container to another. Class I, Zone 2 A Class I, Zone 2 location is a location where: a. Ignitable concentration of flammable gases or vapors are not likely to occur in normal operation and if they do occur, will exist only for a short period; Page 6 of 10 Continuing Education for Engineers www.pdhengineer.com b. c. d. Volatile flammable liquids, flammable gases, or flammable vapors are handled, processed, or used but in which the liquids, gases or vapors normally are confined within closed containers of closed systems from which they can escape only as a result of accidental rupture or breakdown of the containers or system or as a result of abnormal operation of the equipment with which the liquids or gases are handled, processed or used; Ignitable concentrations of flammable gases or vapors normally are prevented by positive mechanical ventilation but which may become hazardous as a result of failure or abnormal operation of the ventilation equipment; That is adjacent to a Class I, Zone 1 location from where ignitable concentrations of flammable gases or vapors could be communicated, unless such communication is prevented by adequate positive-pressure ventilation from a source of clean air and effective safeguards against ventilation failure are provided. This includes locations where volatile flammable liquids or flammable gases or vapors are used but which would become hazardous only in case of an accident or unusual operating condition. Groups Groups are listed as Group I, IIC, IIB, and IIA. Group I is for atmospheres that contain a mixture of gases composed mostly of methane found underground. Group IIC is for atmospheres containing acetylene, hydrogen, or flammable gas or vapor mixed with air that may burn or explode, having either a maximum experimental safe gap (MESG) less than or equal to .5 mm or minimum igniting current ratio (MIC ratio) less than or equal to .45. Group IIC is equivalent to a combination of Class I, Group A and B. Group IIB is an atmosphere containing acetaldehyde, ethylene or flammable gas or vapor mixed with air that may burn or explode having either a maximum experimental safe gap (MESG) greater than .5 mm and less than or equal to .9 mm or minimum igniting current ratio (MIC ratio) greater than .45 and less than or equal to .80. Group IIB is equivalent to Class I, Group C. Group IIA is an atmosphere containing acetone, ammonia having either a maximum experimental safe gap (MESG) greater than .9 or minimum igniting current ratio (MIC ratio) greater than .80. Group IIA is equivalent to Class I, Group D. Unclassified Unclassified locations are locations that are not Class I, Division 1 or 2, Class II, Division 1 or 2, Class III, Division 2 or 2, Class I, Zone 0, 1 or 2. www.pdhengineer.com Page 7 of 10 Continuing Education for Engineers Dual Classification In cases of areas within the same facility classified separately, Class I, Zone 2 locations shall be permitted to abut, but not overlap Class I Div. 2 locations. Class I, Zone 0 or Zone 1 locations shall not abut Class I, Division 1 or 2 locations. Reclassification A Class I, Division 1 or Division 2 location shall be permitted to be reclassified as a Class I, Zone 0, Zone 1 or Zone 2 location, provided all of the space that is classified because of a single flammable gas or vapor is reclassified under the proper requirements. Equipment Equipment that is listed for a Zone 0 location can be used in a Zone 1 or Zone 2 location of the same gas or vapor. Equipment that is listed for a Zone 1 location can be used in a Zone 2 location of the same gas or vapor. The equipment may be listed for a specific gas or vapor or a combination of gases or vapors, such as IIB +H2 (hydrogen – hydrogen being more common than acetylene). Equipment listed for Classes and divisions shall be marked to show the class, group and operating temperature or temperature class referenced to a 40 degree C ambient temperature. Equipment identified for Class I, Division 1 or 2 shall be marked accordingly, and may also be marked for Class I, Zone 1 or 2 as applicable, or with the gas classification group or with temperature classification from the Zone section. Equipment that is approved for Division 1 may be used in Zone 1 or Zone 2 areas. Division 2 equipment may be used in Zone 2 areas. However, the reverse is not necessarily permitted. Zone 0, Zone 1 and Zone 2 equipment may be used in a Division 2 location for the same gas and a suitable temperature rating. No Zone rated equipment can be used in a Division 1 location because a Division 1 location includes areas that could be considered Zone 0. Protection Techniques When an area is classified in accordance with zones, electrical and electronic equipment may be protected by the following techniques. “d” “p” “i” Flameproof, for Class I, Zone 1 or 2. Similar to explosion proof. Purged and Pressurized, for Class I, Zone 1 or 2 for which it is listed Intrinisic Safety, for Class I, Zone 0, 1 or 2 ia = Zone 0 www.pdhengineer.com Page 8 of 10 Continuing Education for Engineers ib = Zone 1 “n” for Class I, Zone 2 locations which under normal operation are not capable of igniting a surrounding explosive gas atmosphere and not likely to create a fault that is capable of causing ignition. nA non sparking equipment nC Sparking equipment where the contacts are suitably protected other than by restricted breathing enclosure nR Restricted breathing enclosure “o” “e” Oil Immersion, for Class I, Zone 1 or 2, immersion in a protective liquid Increased Safety, for Class I, Zone 1 or 2, commonly used for motors, generators, and fluorescent luminaries Encapsulation, for Class I, Zone 1 or 2, arcing parts are completely surrounded in a compound in such a way that an explosive gas or vapor cannot be ignited in a Class I, Zone 1 area Powder filling, arcing parts are surrounded by glass or quartz powder to prevent ignition of an external explosive atmosphere in a Class I, Zone 1 area “m” “q” Electrical equipment of types of protection “e,” “m,” “p,” or “q” shall be marked Group II. Electrical equipment of types of protection “d,” “ia,” or “ib” shall be marked Group IIA, IIB, or IIC, or for a specific gas or vapor. Electrical equipment of types of protection “n” shall be marked Group II unless it contains nonincendiary components in which case it shall be marked Group IIA, IIB, or IIC, or for a specific gas or vapor. Electrical equipment of other types of protection shall be marked Group II unless the type of protection utilized by the equipment requires that it be marked Group IIA, IIB, or IIC, or for a specific gas or vapor. Temperature Class Temperature class is referenced to a 40 degree C ambient temperature. The temperature class, if provided, shall be indicated using the temperature class codes, also known as the T Codes. There are tables to refer to for these numbers. The lower the T Code, the higher the temperature rating. Class I temperature shall not exceed the ignition temperature of the specific gas or vapor to be encountered. The ignition temperature is the minimum temperature required to initiate or cause self-sustained combustion independent of the heating or heated element. www.pdhengineer.com Page 9 of 10 Continuing Education for Engineers Class II temperature shall be less than the ignition temperature of the specific dust to be encountered. For organic dusts that may dehydrate or carbonize, the temperature marking shall not exceed the lower of either the ignition temperature or 165 degrees C. There are exceptions for temperature marking, both for Division rated equipment and Zone rated equipment. Refer to the appropriate guiding document for further information. Equipment Marking Example: Class I, Zone 1, Aex, ia, IIC, T6 Class I: Zone 1: Aex: ia: IIC: T6: Class Zone American Standard (European Common Markets are Eex) Protection Technique Group Temperature Class References National Electric Code Articles 500 – 504 (Divisions) National Electric Code Article 505 (Zones) API/RP 500, 501, 505 Recommended Practice for the Classification of Locations for Electrical Installations at Petroleum Facilities Classified as ClassI, Division 1 and Division 2 (or Class I, Zone 0, Zone 1 or Zone 2) NFPA 497 Recommended Practice for the Classification of Flammable Liquids, Gases, or Vapors and of Hazardous (Classified) locations for Electrical Installations in Chemical Process Areas NFPA 499 Recommended Practice for the Classification of Combustible Dusts and of Hazardous (Classified) Locations for Electrical Installations in Chemical Process Areas. www.pdhengineer.com Page 10 of 10 Continuing Education for Engineers ...
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