Article 500 of the National Electrical Code (NEC 500) defines the classification of hazardous locations based on the type and severity of the explosion hazard.
Potentially Explosive Areas According to NEC 500
The NEC 500 is the fundamental reference for the installation of electrical equipment in areas at risk of explosion ( known in the U.S. as “Hazardous Locations” or HazLoc ). It divides these areas into three classes based on the severity of the explosion hazard and the type of hazard source. Classes I and II are further subdivided into a total of six explosion groups, which are determined by the ignition temperature of the substances. There is no further subdivision for Class III.
Class I of the NEC 500
This class includes all flammable or explosive gases and vapors, such as hydrogen, acetylene, propane, or methane. This class is subdivided into Groups A, B, C, and D. Explosive gases can pose a significant hazard, which is why the safety regulations for HazLoc areas where they may be present are the most stringent. Class II of the NEC Code
Class II includes all combustible dusts, which are further subdivided into subcategories E (metal dusts), coal dusts (F), and grain dusts (G). According to the NEC, any particles with a diameter of no more than 420/500 microns are generally considered dusts.
Class III – Fibers and Lint
All particles larger than 420/500 microns are considered fibers or lint and fall into Class III; these include, for example, paper and cotton fibers, as well as larger wood particles.
NEC Code – It is not always possible to clearly distinguish between classes
While an explosive gas generally cannot exist simultaneously as solid particles, it is possible for many solids to have individual particles ranging in size from over 420/500 microns to below that threshold. Consequently, particles of Classes II and III are often present simultaneously; in such cases, the area is classified into the higher hazard class, i.e., Class II.
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Division I and II – Explosive atmospheres classified according to the NEC Code are divided into two categories
The NEC 500 further subdivides hazardous locations into two divisions; however, these divisions are not based on the properties of the flammable substances, but rather on the likelihood of an explosive atmosphere occurring. All areas in which there is a risk of explosion under normal conditions —whether continuously or only occasionally—belong to Division I. If, on the other hand, the risk of explosion occurs only under unusual conditions (such as technical malfunctions or operator errors), the area is classified as Division II.
Division or Ex-Zone: What’s the Difference?
In addition to the classification of explosive atmospheres just discussed, the NEC also provides for a further subdivision into various explosion zones (see NEC 505). Explosion zones focus on the frequency of occurrence of a potentially explosive atmosphere, whereas the exact definition of the combustible substance takes a back seat. A distinction is made only between gases and mists on the one hand and dusts on the other.
Division I therefore corresponds to Ex Zones 0 and 20, as well as 1 and 21, as defined in Articles 505 and 506 of the NEC, respectively, while Division II corresponds to Zone 2 and 22, respectively.
What does the NEC 500 mean for the operation of electrical systems in North America?
At first glance, NEC 500 provides only a definition of hazardous locations. From this perspective, it only becomes relevant to explosion protection in conjunction with articles of the National Electrical Code, such as NEC 504, which deals specifically with the installation of intrinsically safe systems. It also relates to NFPA 79 and other standards that define protection classes, specific requirements for equipment and their enclosures, and much more.
Nevertheless, the few requirements set forth in NEC 500 are of the utmost importance for German equipment manufacturers who export to the United States or Canada, as they ensure that their products can be put into service there.
The NEC 500 and Its Significance for Export Business to the U.S.
The requirements that NEC 500 imposes on equipment are very general. Section 500.8 merely requires that the equipment be suitable for use in the respective class, group, and temperature class and be marked accordingly. Above all, two mistakes must be avoided:
- The temperature class should not be confused with the explosion group; they are two different things.
- The ATEX marking used in Europe in accordance with DIN EN ISO 80079-36 /-37 is not recognized in the United States.
In Canada, the Canadian Electrical Code (CEC) is a codified system that is fully consistent with that of the NEC 500; consequently, CSA markings are recognized in the United States.
Zusammenfassung
Article 500 of the National Electrical Code (NEC 500) is the legal standard in effect in the United States for defining hazardous locations. It competes with the Ex-zone classification specified in NEC 505. Both articles only become relevant in practice in conjunction with more detailed provisions of the NEC or NFPA 79, as well as other regulations, some of which are influenced by the in-house standards of UL or CSA.
