Many are aware that electrical equipment arcing and sparking -can cause chemical atmospheres (gases, vapors and dusts) to ignite. However, we often forget that thermal ignition of these chemical atmospheres also can occur if the chemicals are exposed to high temperatures. A careful study of the installation and equipment requirements in the National Electrical Code (NEC) is necessary, but a thorough understanding of the overall concepts of chemical characteristics also is critical in providing a safe electrical installation in a hazardous (classified) area.
For more information on chemical--laden atmospheres, Section 500.4(B) references NFPA 497—2004, the Recommended Practice for the Classification of Flammable Liquids, Gases, or Vapors and of Hazardous (Classified) Locations for Electrical Installations in Chemical Process Areas and NFPA 499—2004, Recommended Practice for the Classification of Combustible Dusts and of Hazardous (Classified) Locations for Electrical Installations in Chemical Process Areas. These two documents are not codes or standards but are recommended practices that provide information on how to classify these areas based on specific chemicals. A “recommended practice” is defined in Section 3.2.1 in NFPA 497 as “a document that is similar to a code or standard” but only contains non-mandatory provisions, using “should” rather than “shall.” Anyone associated with the design, installation, inspection or maintenance of hazardous classified areas must become familiar with these two documents.
The definition in 3.3.2 of NFPA 497 for “Autoignition Temperature (AIT)” is the minimum temperature required to initiate or cause self-sustained combustion of a solid, liquid or gas that is entirely independent of any heating or heating element. In other words, the chemical can ignite based on the actual temperature of the solid, liquid or gas. Table 4.4.2 in NFPA 497 provides an alphabetical listing of selected gases and vapors along with their relevant physical properties, including the AIT. For example, gasoline has an AIT of 280°C and can ignite when the temperature of the gasoline vapor exceeds that temperature. Electrical equipment used in a gasoline atmosphere therefore, must not exceed 280°C.
Section 500.8(C)(4) and the accompanying Table 500.8(C) in the NEC provide marking and classification of the electrical equipment used near the gasoline, so the designer, installing electrician and maintainer of the equipment can ensure the AIT of 280°C is not exceeded. The marking on the equipment must specify the temperature class (commonly called a “T-Code”) based on an operating temperature at a 40°C ambient; the equipment must never exceed the ignition temperature of the gas, vapor or dust.
For electrical equipment located in a gasoline atmosphere, a T-Code of T2A through a T6 would be chosen, so the AIT of 280°C of the gasoline would not be exceeded. Remember, T-Codes are available in the NEC, but the chemical characteristics, such as the AIT, are in NFPA 497 for liquids, vapors or gases or NFPA 499 for combustible dusts.
Section 500.8(D)(2) provides similar temperature-marking requirements for electrical equipment based on the AIT available in NFPA 499 for combustible dust. Combustible dust can be ignited as a dust cloud or as a dust layer. As dust accumulates on electrical equipment, the layer of dust increases the operating temperature of the equipment and the temperature of the dust. As moisture in the dust is driven off by the heat (dehydrates), the dust becomes more dense with a further increase in equipment temperature and a resultant increase in dust temperature. Where dusts of organic materials, such as grain, plastics or pharmaceuticals, accumulate on electrical equipment, heating of the dusts often results in degradation or carbonization of the dust layer. This carbonized dust layer often has a significantly lower ignition temperature. In both cases, dust can reach its AIT and ignite. Where organic dust may dehydrate or carbonize, the temperature marking of the electrical equipment must not exceed the ignition temperature of the dust or 165°C.
Familiarity with the NEC is necessary, but there is critical information in other NFPA documents that must be accessed before safe electrical installations in hazardous (classified) locations can be assured.
ODE is a staff engineering associate at Underwriters Laboratories Inc., in Research Triangle Park, N.C. He can be reached at 919.549.1726 or at email@example.com.