In last month’s column, I introduced the traditional approach to hazardous locations. This month, I discuss hazardous materials grouping with some specific occupancy requirements, and I compare the traditional way of classifying locations with the International Electrotechnical Commission (IEC) classifications in Articles 505 and 506.
Traditionally, the National Electrical Code (NEC) has classified hazardous locations as Class I, II and III. It subdivides those classifications into Division 1 and Division 2, depending on the likelihood of the hazard existing under normal operating conditions.
The hazardous materials are further subdivided into four groups of gases and vapors (A, B, C and D) and three groups for dusts (E, F and G). Combustible fibers and flyings do not have individual groups. They typically fall into the equivalent of Group G.
The criteria for the groups differ between Class I and Class II. The Class I groups are based on the hazardous material’s degree of hazard based on standardized tests. A detailed list of the group classifications for flammable and combustible gases and liquids can be found in NFPA 497 Recommended Practice for the Classification of Flammable Liquids, Gases, or Vapors and of Hazardous (Classified) Locations for Electrical Installations in Chemical Process Areas.
Dusts fall into three groups: Group E dusts are combustible metal dusts, Group F dusts are carbonaceous dusts, and Group G dusts are the remaining combustible dusts (e.g., flour, grain, wood, plastics and chemical dusts). The wood dusts are typically wood flour, which is finely ground wood particles. Information on the dust groups can be found in NFPA 499 Recommended Practice for the Classification of Combustible Dusts and of Hazardous (Classified) Locations for Electrical Installations in Chemical Process Areas. NFPA 499 also provides information that is helpful in classifying Division 1 and Division 2 locations. There are several other National Fire Protection Association (NFPA) codes and standards that address combustible dusts, including NFPA 61, NFPA 652 and NFPA 654.
Articles 511 through 516 provide specific requirements for some commonly encountered occupancies that have hazardous locations. As a quick historical aside, Article 517 on healthcare facilities is located in this series of NEC articles because, when it was introduced into the Code, the primary focus was on mitigating the hazards associated with flammable anesthetics. These are no longer in use in the United States.
The classifications in Articles 511 through 516 did not originate in the NEC. These NEC requirements have been extracted from other standards. Many of these classifications are contingent on compliance with the other requirements in the standards from which the requirements have been extracted. A number of NFPA occupancy standards contain requirements for area classification. Some other industry standards provide recommendations or requirements for area classification.
Class III locations are not subdivided into groups. Standards such as NFPA 664 address facilities where there are combustible fibers and flyings.
The IEC classification system
The IEC classification system is found in Articles 505 and 506. Article 505 is an alternative to Articles 500 and 501. Article 506 is an alternative to Articles 500, 502 and 503. One of this system’s notable features is that underground mining is a group of its own, Group I. For locations that are hazardous based on flammable gases, flammable-liquid-produced vapors or combustible-liquid-produced vapors, there are only three groups: IIC, IIB and IIA. The more hazardous materials are in Group IIC, whereas in the traditional system, the more hazardous materials are in Groups A and B. Group IIC includes the materials that are in Group A in the traditional system. Group IIB includes the materials in Group C, while Group IIA includes the materials that are in Group D.
Article 506 is the IEC equivalent of both Articles 502 and 503, along with some of the general requirements of Article 500 that apply to Class II and III locations.
The traditional system uses two divisions. Articles 505 and 506 use three zones.
Gases, flammable and combustible liquid vapors
The most significant difference between the two systems is that, in Article 505, the equivalent of a Division 1 location can be two zones, Zone 0 and Zone 1. The Code covers these in detail.
Zone 0 is the most hazardous because the ignitible concentrations are present continuously or for long periods of time. These areas tend to be very limited, and the wiring is limited to equipment protected by intrinsic safety. Zone 1 is also very hazardous because ignitible concentrations are likely to exist under normal operating conditions, or they could exist frequently, due to maintenance or leakage. The duration of an ignitible concentration is limited in a Zone 1 location. A Zone 1 location can also exist where a breakdown could cause simultaneous failure in which an ignition source could exist at the time where the material is released. A Zone 1 location will also exist adjacent to a Zone 0 location.
In comparing the Class/Division approach with the Class/Zone approach, the significant difference is the presence of the Zone 0 classification. Using the Class/Division approach Class I, Zone 0 or Zone 1 locations are both treated as Class I, Division 1 locations.
Zone 2 locations are the equivalent of a Division 2 location. The definitions are slightly different, but they are equivalent. In a Zone 2 location, the hazardous material is normally confined in a closed system, or the hazard is kept under control by positive mechanical ventilation.
The 2005 NEC introduced the zone classification system for combustible dusts and ignitible fibers. The dust and ignitible fibers and flyings zones are Zones 20, 21 and 22. These classifications are the dust equivalent to Zones 0, 1 and 2, and the definitions feature parallels. The zones are defined in 506.3. The following is a brief summary is:
- Zone 20 locations are areas in which ignitible concentrations of combustible dust or flyings are present continuously or for long periods of time.
- Zone 21 locations are areas in which ignitible concentrations of combustible dust or flyings are present in normal operating conditions.
- Zone 22 locations are areas in which ignitible concentrations of combustible dust or flyings are not likely to be present in the air under normal operating conditions. Often the material is confined to closed systems. They may also be areas adjacent to Zone 21 areas.
The material groups differ from the groups in the traditional system. The combustible metals are in their own group, while the agricultural and carbonaceous dusts are in the same group. The combustible fibers and flyings are assigned to a group. The groups are as follows:
- Group IIIC is the equivalent to Group E, which are the combustible metal dusts.
- Group IIIB is the equivalent to Groups F and G.
- Group IIIA is the equivalent to the combustible fibers and flyings in Class III.
Many of the NFPA codes and standards that have requirements for hazardous locations address both classification schemes. For example, Articles 511 through 516 use both classification schemes. NFPA 497 and 499 also provide classification information based on both systems. The materials and their associated hazards are the same regardless of the area classification approach; it is simply two systems and approaches to mitigating the same hazards.
Equipment must be approved for the class, division and group (traditional system), or zone and material group (IEC system) to be encountered. It is important to note that Class I equipment isn’t necessarily suitable for a Class II or Class III location unless specifically approved for the additional locations. The hazards are different. It is possible that a location may have Class I and Class II or III hazards. If that is the case, the equipment must be able to withstand simultaneous exposure to both hazards.
The classification of an area is most often not the electrical professional’s responsibility. It is often the responsibility of someone with a fire protection or industrial engineering background. There are a number of resources that can be used to classify the areas as well as to ensure the installation remains safe once the area has been classified. To ensure a safe installation, it is helpful for the electrical professional to understand the basis for the classification. The hard part of this is getting the area properly classified. Once that is completed, it is simply a matter of applying the relevant installation requirements contained in the NEC hazardous (classified) location articles.
About The Author
EARLEY, P.E., is an electrical engineer. Retired from the National Fire Protection Association, he was secretary of the National Electrical Code Committee for 30 years and is president of Alumni Code Consulting Group.