NEC 505

With the NEC 505, the internationally accepted classification of potentially explosive atmospheres into three zones each for gases/liquids and dusts was introduced in the United States.

NEC 505: Explosion Protection in the United States

Potentially explosive areas—known as Ex zones, hazardous areas, or classified locations —are classified according to their level of hazard in both the United States and Europe. However, for a long time, only a two-class or two-division classification system was used in the United States. It was not until 1996 that Article 505 of the National Electrical Code introduced the three-zone classification, as applied, for example, in the EU under the ATEX Directive 1999/92/EC. Since then, the following zones have been distinguished.

Ex Zones According to NEC 505

Article 505 classifies potentially explosive areas into three zones—0, 1, and 2 (Ex zones)—based on the frequency or duration of the explosion hazard. In addition, a distinction is made based on whether the air contains flammable liquids (mists, vapors), gases, or solids. If solids are present, the zone number is prefixed with a 2 (i.e., zones 20, 21, and 22). Thus, there are a total of six Ex zones.

Rare: Zones 2 and 22

If an explosive atmosphere occurs only rarely and for short periods, the affected area is classified as Zone 2 or 22. “Rarely and for short periods” refers to a period during which the risk of explosion occurs in less than 50 percent of operating time and lasts for less than 30 minutes per year. Conversely, this means that even if an explosive atmosphere is expected for only 5 minutes per year, the affected area is already classified as an Ex zone.

Occasionally: Zones 1 and 21

In Zones 1 or 21 as well, an explosive atmosphere is expected for less than 50 percent of the operating time, but for a total of more than 30 minutes.

Common: Zones 0 and 20

According to NEC 505, all areas in which an explosive atmosphere frequently occurs during normal operation are classified as Zone 0 or Zone 20. According to the standard definition, “frequently” means the presence of an explosive atmosphere for more than half of the operating time. In practice, these zones primarily include vessels and piping filled with explosive substances.

Don’t Confuse Them: NEC 505 and NEC 500

Also, the Article 500 of the National Electrical Code provides a definition of hazardous locations; however, it uses a different classification system, focusing on the type of hazard source (gas, liquid, dust, etc.) and the severity of the explosion hazard . Furthermore, the various areas are not referred to as zones, but rather asclasses, which are further subdivided intogroups.

With regard to the frequency of occurrence of a potentially explosive atmosphere, Article 500 recognizes only two categories, known as “divisions,” distinguishing only between the risk of explosion during normal operation (Division I) and during malfunctions or defects (Division II).

Which NEC article is relevant?

The answer to this question is simple: both. Article 500 provides the definition commonly used in the U.S. for Class I and II hazardous locations. With NEC 505, the internationally accepted three-zone classification was introduced in the United States. Both standards coexist, allowing system integrators to choose whichever of the two systems is more cost-effective for them or their customers.

Practical Relevance for Explosion Protection

Article 505 merely provides definitions related to explosion hazards. It only becomes relevant in practice for manufacturers and users of control panels and other electrical systems when considered in conjunction with other sections of the National Electrical Code and other regulations and standards. In this context, the UL 1203 , which can be considered the standard for the use of equipment in hazardous locations.

However, the specific protective measures to be taken depend not only on legal considerations but also on economic factors and operational requirements. For example, explosion-proof enclosures can be used, or components that represent a potential ignition source can be isolated from the ambient atmosphere by an inert gas. Of course, protection must still be ensured even when a door of the control cabinet is opened. Which type of ignition protection is best suited to prevent an explosion must therefore always be determined on a case-by-case basis.

Zusammenfassung

Article 505 of the National Electrical Code incorporates the internationally established classification of potentially explosive atmospheres—divided into three zones each for liquids/gases and dusts—into U.S. explosion protection regulations. This classification thus corresponds to our ATEX Operating Directive 1999/92/EC. The article only becomes relevant in practice in conjunction with other regulations, particularly UL 1203.