Hazardous locations classification

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... 

The classification of hazardous locations involves the determination of four factors ... 

National Fire Protection Association (NFPA), NFPA 497A. Classification of Class I Hazardous Locations for Electrical Installation in Chemical Process Areas. NFPA, Quincy, MA, 1992. 

NFPA 497 Classification of flammable gases and hazardous locations for electrical installation. 

Conducting an electrical area classification An electrical area classification involves identifying the extent (if any) of location(s) in a facility where combustible materials could be present during normal and/or abnormal conditions. According to Article 500 of the NEC, major categories of hazardous locations are ... 

NFPA 497. Recommended Practice for the Classifications of Liquids, Gases or Vapors and of Hazardous Locations for Electrical Installations National Fire Protection Association Quicy, MA, USA, 1997 Edition. 

Design for Hazardous Areas. This type of area classification tends to simplify the electrical design by directive means, but poses alternate choice problems, dictated by economics. The cost of an explosion-proof installation is as much as twice that using standard equipment. As an example of more costly electrical equipment, rigid metal conduit with threaded explosion-proof boxes and fittings are required for all Class I, Division 1, hazardous locations either rigid metal conduit or electrical metal tubing is approved for both Class I and II, Division 2, locations. 

Most laboratories do not represent hazardous locations in the context of requiring special electrical wiring and fixtures, although there may be individual equipment items which may need to be treated as such. The classification of a facility as hazardous in a regulatory context, depends upon the type of materials employed in the facility and whether flammable fumes or gases, electrically conducting materials, or explosive dusts are present in the air within these facilities in the normal course of routine activities or only sporadically due to some special circumstance. Explosion proof wiring and fixtures are substantially more expensive than ordinary equivalents and should be used only if there are no acceptable alternatives. Where the need does exist, however, they definitely should be used. 

For the purpose of complying with 1910.309, the classification of hazardous locations as described in article 500 of the National Electrical Code for DBCP shall... 

Zone systems are followed in area classification by ATEX and lEC, and even NFPA has included zone systems of classification in their standard. The zone classification system probably has the edge over the class/division system because many consider the zone classification system to be more flexible and safer in hazardous locations. However, zone systems do not take care of the consequence of release. In case the consequences are important, as per the advice of the HSE.UK, it is better to go for a more conservative equipment selection. Although not officially accepted yet, various sources have tried to place time limits on these zones. The most commonly used values are as shown in Table X/3.3.2-l based on probability and duration of explosive atmosphere. 

The discussion on hazardous area standards is concluded with the recommendation that the applicable standard part must be read thoroughly before starting any equipment selection. Since area classification is mainly carried out by mechanical and safety engineers, good coordination is helpful. Gas detection systems for hazardous locations is another area of concern for instmmentation and control engineers so brief discussions on combustible gas detection systems will be dealt with in the next clause. 

Employers in control of hazardous areas need to determine the locations and extents of these zones in each of the hazardous areas, an exercise known as hazardous area classification. At present, in 2001, there is no specific legal duty to carry out hazardous area classification, apart from the general duties to carry out risk assessments and, in the context of mines, in Regulation 19 of the Electricity at Work Regulations. However, the Protection of Workers Potentially at Risk from Explosive Atmospheres Regulations expected to be enacted soon, will contain specific provisions relating to area classification and the need to record the area classification information in an explosion protection document. 

NFPA 497 Classification of Flammable Liquids, Gases, or Vapors and of Hazardous (Classified) Locations for Electrical Installations in Chemical Process Areas, 1997 edition. National Fire Protection Association, Quincy, MA. 

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