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in the Chemical labomtory Edifed by N O R M A N V. STEERE, 140 Melbourne Ave., S.E. Minneapolis, Minn., 55414
XL. Explosion-Proof Electrical Equipment* T. E. EHRENKRANZ, los Alomos Scientific Loborotory, los Alamos, New Mexico Article 500 of the National Electrical Code (NEC) divides "hazardous locations" into Classes, Groups, and Divisions and specifies required electrical equipment in each category. The principal agency which tests and rates electrical equipment for such locations is the Underwriters' Laboratories, Inc. (UL). Electrical equip2 by the National ment csn ~ 1 be~ specified Electrical Manufacturers Association (NEMA) Type numbers. There are Class I, Class 11, and Class I11 haamdous locations according to the NEC. In e a ~ hClass, the NEC also recognizes a Division 1 location where hazardous atmospheric concentrations can exist under normal operating conditions, and Division 2 where, in general, sucb hazardous conditions can exist only infrequently through an accident or an abnormal situation. Division 2 may include areas adjacent to Division 1locations. The National Electrical Code defines Class I, Divisions 1 and 2 as follows: Class I, Division 1. Locations (1) in which hazardous concentration of flsmmable gases or vapors exist continoously, intermittently, or periodically under normal operating conditions, (2) in which hazardous concentrations of such gases or vapars may exist frequently because of repair or maintenance operations or because of leakage, or (3) itr which breakdown or faulty operation of equipment or processes which might release hszardous concentrations of flammable gases or vapors, might also cause simultaneous failure of electrical equipment. This classification usually includes locations where vohtile flammable liquids or liquefied flammable gases are transferred from one container to mother, interiors of spray booths and areas in the vicinity of spraying and painting opemtions where volrttile flammable solvents are used; locations containing open tanks or vats of volatile flammable liquids; drying rooms or compartments for the evaporation of flsmmahle solvents; locations containing fat and oil extraction apparatus using volatile flammable solvents; portions of cleaning and dyeing plants where hazardous liquids are used; gas generator rooms and other portions of gas manufacturing plants where flammable gas may escape;
* Appeared in Health and Safety Newsletter of Los Alrtmos Scientific Laboratory and is reprinted with permission.
inadequately ventilated pump rooms far flammable gas or for volatile flammable liquids; the interiors of refrigerators and freeaers in which volatile, flemmable materials are stored in open, lightly stoppered, or easily n~pturedcontainers, and all other locations where hazardous concentrations of flammable vapors or gases are likely to occur in the course of normal operations. Chss 1, Division 2. Locations (1) in which volatile flammable liquids or flammable gases are handled, processed, or used, but in which the hazardous liquids, vapors, or gases will normally be confined within closed containers or closed systems from which they can escape only in case of accidental rupture or breakdown of sucb containers or systems, or in oase of abnormal operation of equipment, (2) in which hazardous concentrations of gases or vapors are normally prevented hy positive mechanical ventilstion, hot which might become hszardous through failure or abnormal operation of the ventilating equipment, or (3) which are adiacent to Class I. Division 1 h a -
tion is prevented by adequate pasitivepressure ventilation from a source of clean sir, and effective safeguards against ventilation failure are provided. This classification nsuslly includes locations where volatile flammable liquids or Bammilble gases or vapors are used, but which, in the judgment of the Code enforcing authority, would beoome hasardous only in case of an accident or of some unusual operating condition. The quaotity of hazardous material that might escape in case of accident, the adequacy of ventilating equipment, the total area. involved, and the record of the industry or business with respect to explosions or fires are all factors that. should receive consideration in determining the classification and extent of each haaardous ares. A Class I location is one in which flammable gases or vapors may be present in flammable concentrations. This Class includes Groups A, B, C, and D. Group A is for atmospheres containing acetylene, Group B for atmospheres containing hydrogen, or gases or vepors of equivalent hesard such as manufactured gas, Group C for atmospheres containing ethyl ether vapors, ethylene, or cyclopropane, and Group D is for atmospheres containing
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gasoline, hexane, naphtha, benzine, butane, propane, sleohol, acetone, benzol, lacquer solvent vspors, or natural gas. The approximate flammable limits in sir for Group A (acetylene) is 2 5 8 1 % by volume; for Group B (hydrogen) it is 4-7570; for the Group C grtses the spread is from 2 to 30%. -. and for G r o u ~D it is only 1-17%. In general, electrical equipment for Class I, Division 1locations does not have gas-tight enclosures. Enclosures are built so that they can resist an internal explosion if gas seeps inside and is ignited by s spark, and that the resulting flame is quenched so that a gas mixture outside the enclosure will not be ignited. For example, a Class I, Group C and D oest iron junction box is hydrostatically tested to four times the internal peak pressure that is attainable with a stoichiametric mixture. Its bolted metal-to-metal joints, if, for example, 18 mm or 3 / r in. wide, have a maximum clearance of 0.05 mm or 0.002 in., or, if 25 mm or 1 in. wide, a maximum of 0.09 mm or 0.0035 in. so that only cooled combustion products can issue. If an item is not to be hydrostatically tested by UL, then a calculated factor of safety of 5 must be demonstrated, unless the material is rolled steel for which UL socepts lower factors of safety. It is important to note that for Groups A and B ground joints as described above are not acceptable; only threaded joints are allowed. With Class I fixtures surface-temperature l i t a t i o n s are usually easily met, but in the case of lighting fixtures surface temperature is a controlling factor. Class I1 locations contain electrically conducting or combustible dusts. This Class includes: Group E for atmospheres containing metaldust, includingaluminum, magnesium, and their commercial alloys, and other metals of similarly he~ardous characteristics, Group F for a.tmospheres containing carbon black, cod, or coke dust, and Group G for atmospheres containing flour, starch, or grsin dusts. Fixtures approved for this Class must be so constructed that they are "dust-ignitionproof." This means that enclosures exclude ignitible amounts of dust; or amounts which would affect equipment performance and permit an interior ignition to be commnnicated to the exterior of the enclosure; and that surface temperstures are below the ignition temperatures of the combustible dusts, about 166-199T (330-390°F). Class I11 designates areas which contain ignitible fibers. I t has no groups but is classified into two divisions. Equipment approved for such locations may have a maximum surface temperature of 1 6 9 T (32g°F), or 120°C (248°F) if subject to (Continued on page A603)
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overloading. Covers must minimize entrance of fibers and escape of sparks. Wiring far Classes I, 11, and I11 in general is in rigid conduit or M I (mineral insulated) cable with approved end fittings. There are also approved types of flexible conduit. The NEC prescribes the use of special conduit seals to prevent the psssage of combustible gases from one fixture to another or from a hazardous mee. into a nonhszardous area. These seals and associated breather and drsin fittings must be installed according to the manufacturer's recommendations. Some typical NEMA Types m e the following: Type I-general purpose Type &watertight (excludes water from a.bossstream) Type 7, with letter (corresponds to Class I with same group letter; airbreak equipment) Type 8, with letter (corresponds t o Class I with same group letter; oilimmersed equipment) The Type 4 is especially useful in areas where the main service requirement is for equipment t o withstand periodic washingdown. The NEC also recognizes the principle of 6'. Intrinsic safety!' Equipment approved
as "intrinsiertlly safe" for use in specific hazardous atmospheres is defined as incapable of releasing sufficient electricd energy to cause ignition under normal or abnormal conditions. Such equipment does not need the expensive, specialized enclosures =,hi& chhraeterize "explosionproof" equipment. This approach to the explosion hazard is not yet widely utilized in the USA, but in Great Britain, for example, there is a formidable list of lamps, instruments, communications equipment, and test apparatus certsed "intrinsically safe" for atmospheres which correspond to our Class I Groups C and D. Other ways to avoid the high cost of "explosion-proof" installations are to locate as much of the electrical equipment as possible outside the haettrdaus area, or to locate conventional equipment in an enclosure which is under a constant purge of clean air, or preferably, nitrogen. Around hydrogen facilities one is faced with one of two problems: the unavailability, or the high cost of Class I GroupB equipment. When the above schemes are not practical, there are two schools of thought. One is to use Group C or D equipment; the other is to use "enclosed and gasketed" equipment. (The latter used to be called "vapor-proof" and more recently "vapor-tight" but these t e r n s are now obsolete and not recognized by the NEC or UL.) I n spite of the fact that "explosion-proof" equipment is not gas tight, and "enclosed and gasketed" equip(Continued on page A604)
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ment "breathes" upon heating and cooling, the above alternatives are often acceptable and the following reasoning is offered: The ignition energy and the quenching distance for hydrogen, as for gases in general, are lowest a t stoichiometrio proportions, then increase rapidly. Likewise, explosion pressures are highest in these regions. Since equipment for each Group is designed for safety at stoichiometrio mixtures, a Group C or D fixture may be entirely safe up to 10-15% hydrogen. As for "enclosed and gssketed" fixtures, even in a high concentration of hydrogen they are not likely to "breathe" enough of the hazardous atmomhere for the internal conperiod.
(a) Turn offthe lock-out power before opening housings. This includes lighting fixture servicing. ( b ) When ground joints need cleaning use s t 3 bristle brush (not wire brush) and a high flash point solvent such as kerosene. Wipe dry and apply light oil or special lubrict~ntsfor corrosive locations as recommended by manufacturer. Be sure that there are no nioks or grit in the groundjoint surfaces. (c) Uae original bolts when replacing covers, and tighten them all evenly. There must be no missing bolts. ( d ) Perform ( b ) every time cover is removed. (e) Do not paint over nameplates. (f) Avoid painting e.p. fixtures containing heat-sensitive devices as dark paint will alter their functioning. (g) Repairs to e.p. motors technically negate the UL label, and should preferably be undertaken by the manufacturer, especially extensive work such as rewinding. (h) Some drains in sealed conduit work rtutomatieally, but those which do not must be periodically drained manually. (i) Some people in laboratories specify a watertight joint as one made with a sealant, and made up to be "wrenchtight."
Bibliography 1. National Electrical Code, 1966. 2. Standards Nos. 886 and 844, Underwriters' Laboratories. 3. Electrical Accidents and Their Causes, H.M.S. Stationery Office, 1960. 4. K. A. Woodard, "The Application of Commercial Electrical Equipment.. . ," Steams-Roger Manufacturing Co., 1954. 5. J. R. Petree, "NEC, UL and Practical Factors Applying to Class I Groups A.
(Cmtinued on page A607)
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comrnuaic:nt,iou, Cn,llseHinds Co. 7. v o ~ELBEAND SCOW, ASII-TI)T-
6. Personal
62-3027. 4. LEWISand
VON E I . ~ : "Cu~lihwtion, , Flames and Explosions of Gases," Academic Press, 1951. 9. W. F. RICKES, "Intrit~sidly Safe Electronic 1nstrument;ttion." IRA Joz~mal,8, No. 7, pp. 42-66 (1961). 10. C. F. KISSELSTEIN,"Keeping the 'Proof' in Explosion-Prod," IS.1 .JmrnLrnal, June pp. 60-62 (1961). 11. L. G. M\TTFIEWS,Union C d > i d e Cnrp., personal communirittion.
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