1、 CLASSIFICATION OF LOCATIONS FOR ELECTRICAL INSTALLATIONS IN GAS UTILITY AREAS Revised by Distribution i.e., those areas where there may be some hazard from release of flammable gases, vapors or liquids. Gas utility electrical installations must be designed to avoid contributing to the probability o
2、f accidental ignition of flammable gases, vapors or liquids released to the atmosphere. Before any design can begin, the areas of potential hazard must be clearly defined as to Class, Group and Division or Zone. A properly designed installation would protect the area to a degree commensurate with th
3、e risk. It would provide special equipment wherever required for safety but would also include general purpose equipment wherever possible in order to save installation and maintenance costs. It is often possible to limit the installation of electrical facilities in areas where the atmosphere may be
4、 hazardous. Careful definition of the extent of the potentially hazardous area will often permit the use of general purpose circuit-breakers, transformers and starters in adjacent non-classified locations at a reasonable cost. Historically, in the United States, hazardous locations were classified a
5、s Division 1 or Division 2. A significant percentage of new electrical equipment for hazardous locations is designed to be marketed worldwide and thus must be labeled under the international Zone 0-2 classification system. The NEC has adopted the Zone classification system as an alternative to the D
6、ivision classification system. The NEC Article 505 should be consulted to determine conditions for installation of Zone equipment in Division locations and installation of Division equipment in Zone locations. The rest of this document will only refer to the division designations for clarity of conc
7、epts. 2 1.2 Scope The gas utility areas to which this publication applies are those in which flammable gases are produced, manufactured, stored, transmitted and distributed. It is not intended to include plants of a refinery nature, such as natural gas stripping plants or gasoline plants. In establi
8、shing the limits of classified locations, it is assumed that the gases of concern to this publication are materials classified under NFPA70-2005, as Group B or D in Article 500.6 or as Group IIA, IIB or IIC in Article 505.6 (see Appendix A). Factors such as corrosion, maintenance, equipment standard
9、ization, and interchangeability and possible process changes or expansion frequently dictate the use of special enclosures or installations for electrical systems. Such factors are outside the scope of this publication, which is entirely concerned with the proper application of electrical equipment
10、to avoid ignition of flammable vapors and gases. This publication further recognizes that certain environmental aspects of an installation, such as prevailing winds, site topography, proximity of other installations and climatic conditions, may exert sufficient influence to merit a classification ot
11、her than that which would have been made had these conditions not existed. All areas and distances indicated herein are guidance for a minimum level of safety and may be exceeded at the discretion of those responsible for the installation design. 3 2. The National Electrical Code (NEC) 2.1 National
12、Electrical Code Considerations The NEC (NFPA 70) is widely used as a set of minimum requirements for safe electrical installations and has been adopted as law at various federal, state and local levels. OSHA mandates its use in appropriate facilities. Many installations, even if not legally required
13、, conform to NEC as a matter of good practice. The NEC-90.1(C) very clearly states that the NFPA 70 is not a design specification. It is also not an operating or maintenance manual. Therefore, the guidance provided herein is not an attempt to rewrite or otherwise supersede the NEC or other applicabl
14、e codes or ordinances. It is intended, rather, to serve as a supplement. 2.2 Classes and Divisions Classified Areas. Article 500 of the NEC defines three classes of locations and two divisions within each class. Class I locations are those locations in which flammable gases or vapors are or may be p
15、resent in the air in quantities sufficient to produce explosive or ignitable mixtures. Class II (combustible dust) and Class III (easily ignitable fibers or filings) are not generally applicable to the natural gas industry. This publication, therefore, is limited to Class I locations. The intent of
16、Article 500 of the NEC is that electrical equipment and systems in classified locations should not provide a means of ignition for an explosive or ignitable mixture that may be present. Within each Class, Article 500 recognizes two degrees of hazard: Division 1 and Division 2. In Division 1 location
17、s, an ignitable mixture is likely to be present continuously or intermittently under normal conditions of operation, repair, maintenance or leakage. Normally expected conditions may include the release of gas from a pipeline blow-down vent or release of gas from an instrument vent. In Division 2 loc
18、ations, an ignitable mixture is normally handled and processed in closed piping systems or vessels. However, due to an abnormal condition, an ignitable mixture is likely to be present. Abnormal conditions may include gas leakage from a pipe flange or threaded pipe fitting. Unclassified locations are
19、 those that are not classified as Division 1, Division 2. These areas are sometimes referred to as “General Purpose” or Non-classified. 2.3 Division 1 Design Conditions Wiring methods, electrical equipment and materials installed in Division 1 or Division 2 locations must be as specified by NEC Arti
20、cle 501 Class I Locations. Electrical installations in Division 1 locations are designed so that normal operation or failure of any part of the electrical system will not release flames, sparks or hot gases and will not result in surface temperatures high enough to ignite the surrounding hazardous a
21、tmosphere. Electrical installations for Division 1 locations may be designed in a number of ways. No single method is best in all respects for all types of equipment used in gas utility areas. Types X and Y purged electrical equipment, intrinsically safe equipment and explosion-proof apparatus are u
22、sed primarily in Division 1 locations but can be used in Division 2 locations as well. Explosion-proof apparatus is enclosed in a case that is capable of withstanding an explosion of a specified gas or vapor that may occur within it. Explosion-proof apparatus will also prevent the ignition of a spec
23、ified gas or vapor surrounding the enclosure caused by sparks, flashes or explosion of the gas or vapor within it. The equipment must operate such that the external temperature of the enclosure will not ignite the surrounding flammable atmosphere. 4 In some cases, the requirement for hazardous-atmos
24、phere-rated electrical equipment within an enclosure can be reduced or eliminated by applying adequate positive pressure (above atmospheric) from a source of clean air in conjunction with effective safeguards against ventilation failure. The source of clean air is to be taken from a location outside
25、 the boundaries of the Division 1 or 2 area. Refer to NFPA 496, Standard for Purged and Pressurized Enclosures for Electrical Equipment, 2003, for design criteria. The NEC Article 504 also allows the use of intrinsically safe equipment and wiring in Division 1 locations. Intrinsically safe equipment
26、 and wiring are incapable of releasing sufficient electrical or thermal energy under normal or abnormal conditions to cause ignition of a specific hazardous atmospheric mixture in its most easily ignited concentration. Abnormal conditions will include accidental damage to any part of the equipment o
27、r wiring, insulation failure or other failure of electrical components, application of over-voltage and other similar conditions. The intrinsically safe wiring method allows for the use of general purpose enclosures and wiring methods normally used in non-classified areas. In areas along the Canadia
28、n border, it may be appropriate to refer to current Canadian Standards Association (CSA) standards for design criteria. Equipment and associated wiring listed by an organization, such as Factory Mutual, Underwriters Laboratories or Canadian Standards Association, which is acceptable to the authority
29、 having jurisdiction as intrinsically safe, may be installed in any location for which it is approved. 2.4 Division 2 Design Conditions Electrical installations for Division 2 locations are designed and arranged so that normal operation of the electrical system does not provide a source of ignition.
30、 Protection against ignition during electrical breakdown is provided. However, electrical breakdowns are sufficiently rare that the chances of one occurring simultaneously with accidental release of an ignitable mixture is extremely remote. Arcing and sparking devices are permitted only if suitably
31、enclosed or if the sparks are of insufficient energy to ignite the mixture. As previously mentioned, Division 1 design techniques may be used in Division 2 locations. In addition, non-incendive type design techniques can be used in a Division 2 location. Non-incendive circuits and components are tho
32、se circuits and components that under normal operating conditions do not release sufficient electrical or thermal energy to ignite a specific hazardous atmospheric mixture in its most easily ignited concentration (this is in contrast to intrinsically safe devices that must be low energy under normal
33、/abnormal conditions). The principles involved with non-incendive circuits and components are based on the low probability of the presence of an ignitable gas-air mixture occurring for a substantial period of time in a Division 2 area. This mixture must be coincident with an abnormal condition in th
34、e electrical equipment that is capable of igniting the gas mixture. For a complete description and design criteria for non-incendive circuits and components, refer to ANSI/ISA Standard ISA-S12.12.01-2000 or the current revision. Type Z purging techniques can be used to reduce or eliminate the requir
35、ement for hazardous-atmosphere-rated electrical equipment within an enclosure in a Division 2 classified area. Refer to the NFPA Standard 496 for design criteria. 5 3. Conditions Necessary for Ignition 3.1 Basic Conditions Three basic conditions must be satisfied for the occurrence of a fire or expl
36、osion of the type under consideration. These are: a) A flammable gas, vapor or liquid must be present. b) This must be mixed with air or oxygen in the proportions required to produce an explosive or ignitable mixture. Furthermore, within the context of this Publication, there must be a sufficient am
37、ount of this mixture to provide an ignitable atmosphere surrounding the electrical installation. c) There must be an ignition source. This can only occur when sufficient incendiary energy is applied to a significant amount of an ignitable mixture. Within the context of this publication, the potentia
38、l source of ignition is understood to be the electrical installation operating at energy levels sufficient to release incendiary energy. 3.2 Dispersion In applying these principles to any potential hazard, the quantity of the substance that might be liberated, its physical characteristics and the na
39、tural tendency of vapors to disperse in the atmosphere must be recognized. There is a definite range of concentrations of any flammable vapor in air that is necessary before the vapor can be ignited. The upper and lower limits are listed for various flammable liquids, gases and volatile solids in NF
40、PA Fire Protection Guide to Hazardous Material. Whenever the concentration of a flammable substance in air is above or below the flammable range the vapors cannot be ignited. (See Appendix A and/or NFPA 497.) Lighter-than-air gases diffuse into the atmosphere so readily that, except in enclosed spac
41、es, they seldom produce ignitable mixtures in the areas close to grade where most electrical installations are located. Heavier-than-air gases will not diffuse readily into the atmosphere unless the diffusion is assisted by air currents. They are more likely, therefore, to produce ignitable mixtures
42、 at elevations close to or below grade level. Refer to definitions in NFPA 497-2004 4.2.1 and 4.2.2. Vapors from volatile flammable liquids, even when they evolve rapidly, have a natural tendency to disperse into the atmosphere and thus rapidly become diluted to concentrations below the lower flamma
43、ble limit. This tendency is greatly accelerated by air movement. Many years of experience have established that outdoor areas likely to be classified are only a small fraction of those classified areas that might theoretically result from any given rate of release of flammable liquid. Liquids of low
44、 vapor pressure seldom evolve sufficient quantities of vapor to require that any significant area be classified. The prevention of the continued existence of a potentially ignitable atmosphere; e.g., by detection, alarm and corrective action, is one of the best ways to ensure safety. 6 4. Determinat
45、ion of the Existence of a Classified Location 4.1 General The determination of whether a location should be classified requires careful consideration of the following factors: a) The flammability or combustibility of the product. b) The likelihood of product release within a given area. c) The size
46、of the release. d) Flammable gases and vapors. The first step in classifying locations for electrical installations is to determine if the product being handled is flammable. The flammable substances normally found in utility gas plants include natural gas, liquefied natural gas (LNG), propane or li
47、quefied petroleum gas (LPG) and the vapors of flammable liquids. Natural gases commonly encountered include mixtures of methane with small quantities of other low-molecular-weight hydrocarbons, with the mixtures generally being lighter than air. LNG is produced by reducing the temperature of natural
48、 gas sufficiently to cause condensation. Since natural gas is mostly methane, the condensation occurs at -258 F under atmospheric pressure of 14.73 psia. One cubic foot of LNG is equivalent to approximately 600 cubic feet of natural gas in the gaseous phase, measured under standard conditions of 14.
49、73 psia and 60 F. Synthetic natural gas includes mixtures of 96-98 percent methane with small quantities of other low-molecular-weight hydrocarbons and hydrogen. The mixture is generally lighter than air. LPGs include propane, propylene, butane and mixtures thereof, which have vapor densities of 1.3 to approximately 2.0 times that of air. Vapor pressures exceed 40 psia at 100 F (37.8 C). 4.2 Combustible and Flammable Liquids Combustible and flammable liquids vary in volatility. They are defined in NFPA Standard No. 30 or 497. Combustible liquids have a flash point at or above 100 F (37.
