Ignition sources electrical area classification

The overall intent of electrical area classification is to provide for safety of personnel and equipment. This is achieved by the elimination of electrical ignition sources near combustible gases or vapors that could explode or bum. The specific reasons for classifying facilities into electrical hazardous areas typically are ... 

As discussed in this section, the intent of electrical area classification is to ensure that electrical equipment will not act as an ignition source. However, once the extent of a classified area is determined it is prudent to ensure that all potential ignition sources are eliminated or controlled. 

Area classification is employed to identify locations where electrical equipment could provide a source of ignition to flammable liquids and gases/vapours that may be present. It is also used to prevent dust explosions. Area classification is necessary for selecting and installing safer electrical and instrumentation equipment, and its review is very important for revamp projects as it may influence site selection and also project cost. In some cases it may require a change in electrical area classification for existing facilities and hence require replacement of some electrical and instrumentation equipment. 

An area or a zone defined as a three-dimensional space in which a flammable atmosphere is or may be expected to be present in such frequencies as to require special precautions and restrictions for the construction and use of electrical apparatus and hot surface exposures (e.g., lights) that act as an ignition source. Classified areas have specific restrictions based on the equipment involved, the gases encountered, and the probability of leakage. Several national and industry institutions have guidelines and codes for the classification of areas, e.g., defined in the United States by the National... 

Control of Ignition Sources—Exposed ignition sources (e.g., vehicles, smoking, etc.) should be spaced as far as practical from hydrocarbon-containing systems (maximize electrical area classification requirements). 

Tayout also has a significant role in minimizing the probability of ignition of a flammable release. Area electrical classification provides the basis for the control of electrical ignition sources. This classification is also used to determine the areas that require protection from vehicular access, etc. Frequently, highly hazardous processes that can result in overpressure (e.g., hydrogenation) are placed behind blast resistant structures/walls. 

Ignition of flammable Provide safe separation distances release resulting in fire. Develop appropriate area electrical or explosion. classification Provide ignition source control Place ignition sources in positive pressure enclosure and buildings Provide adequate ventilation API RP500 BS 5345 BS 5958 NFPA-70 NFPA-77... 

Electrical switches, relays, and other devices are described for safety reasons by several general categories. Since these devices are potential sources of ignition during normal operation (for example, arcing contacts) or due to malfunction, the area classification limits the types of devices which can be used. 

Where ventilation is installed to effect a reduction in the area electrical classification, the rate should be no less than 12 air changes per hour for an adequately ventilated area in accordance with API RP 500. Areas containing ignition sources, such as control and switchgear buildings, gas turbine acoustical enclosures, and power generators, should be pressurized in accordance with NFPA 496, if located in an electrically classified area. 

Some sources of ignition can be discounted immediately this example, for instance, does not involve pyrophoric materials. As there are no vent lines leading to atmosphere, there is no danger from sources of ignition outside the plant. In practice, the presence of naked flames can also usually be discounted as they will normally be excluded from areas where flammable materials are handled. All electrical equipment should be checked to ensure that it is suitable for the area classification in which it is sited. 

RP 14J stresses the importance of keeping ignition sources away from areas from which hydrocarbons may be released. Further guidance is provided in API RP 500—Classification of Locations for Electrical Installations at Petroleum Facilities. This standard also provides guidance to do with ventilation and area classification. 

Many of the decisions to do with safe operations and maintenance depend on the area classification systems that is being used. Area classification is primarily to do with the design of electrical systems. However, the guidance provided can be used whenever flammable materials are present, regardless of the potential sources of ignition. Therefore, an overview of the topic is provided here. 

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