Purged and pressurized enclosures for

NFPA 496 Standard for Purged and Pressurized Enclosures for Electrical Equipment,... 

While some video display screens such as liquid crystal, gas plasma or vacuum fluorescent displays do not present the same charged screen hazards as CRTs, this does not imply that they are safe for use in hazardous locations. This requires special design and certification for use with a given flammable atmosphere. Non-certified equipment used in locations classified as hazardous under Article 500 of NFPA 70 National Electrical Code require a purged or pressurized enclosure to control ignition hazards as described in NFPA 496 Standard for Purged and Pressurized Enclosures for Electrical Equipment. The screen in this case is located behind a window in the enclosure. 

Standard for Purged and Pressurized Enclosures for Electrical Equipment in Hazardous (Classified) Locations Recommended Practice for Classification of Class I Hazardous Locations for Electrical Installations in Chemical Plants... 

Purged and Pressurized Enclosures for Electrical Equipment, NFPA 496, National Fire Protection Association, Quincy, MA, 1982 Approval Standard Intrinsically Safe Apparatus and Associated Apparatus for Use in Class I, II and III, Division 1, Hazardous Locations, Class No. 3610, Factory Mutual Research, Norwood, MA, Oct. 1979... 

National Fire Protection Association (NFPA), NFPA 496. Standard for Purged and Pressurized Enclosures for Equipment. NFPA Quincy, MA, 1993. 

The design and specification of intrinsically safe control equipment and systems is discussed by MacMillan (1998) and Cooper and Jones (1993). Purged and pressurized enclosures for electrical equipment are described in NFPA 496 (2003). 

NFPA 496 (2003) Standard for purged and pressurized enclosures for electrical equipment. (National Fire Protection Association). 

NFPA 496 Design and operation requirements of purged and pressurized enclosures for equipment used in hydrogen areas. 

Acceptable protection techniques for electrical and electronic valve accessories used in specific class and division locations include explosion-proof enclosures intrinsically safe circuits nonincendive circuits, equipment, and components dust-ignition-proof enclosures dusttight enclosures purged and pressurized enclosures oil immersion for current-interrupting contacts and hermetically sealed equipment. Details of these techniques can be found in the National Electrical Code Handbook, available from the National Fire Protection Association. 

For hazardous areas a purged and pressurized, or pressurized system and/or a NEMA 7 enclosure is appropriate. Purged and pressurized systems are evacuated and then refilled with an inert gas such as nitrogen. A pres-... 

Enclosures are typically designed for low pressures in the inches-of-water range. Other designs, such as those for nuclear reactor containment, are rated much higher, but cost much more. To function properly as an enclosure, a stmcture should be airtight and able to be purged and vented if a release occurs. 

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