In many industries, there are areas where there is a risk of explosion. These so-called Ex zones occur where flammable gases, vapors, or dusts accumulate in the air, and they impose special requirements on the planning, design, and selection of equipment. A key foundation for this is the zone classification in explosion protection. It defines where and how frequently an explosive atmosphere may occur—and what technical measures are required.
This article provides an overview of Ex zones according to European and international regulations and explains what matters in practice.
Ex-Zones: Definition
An “Ex” zone refers to an area where an explosive atmosphere may form due to gases, vapors, mists, or dust. The classification is based on the frequency and duration of the occurrence of this atmosphere.
What is an “ex-zone”?
“Ex zones” are therefore specifically designated areas where there is a risk of an explosion or dust explosion, since flammable concentrations of combustible gases, vapors, mists, or dusts may occur there. Depending on the hazard situation, a distinction is made between different Ex zones.
Zone Classification in Explosion Protection
The zoning system in explosion protection is used to systematically assess areas with a potential risk of explosion. The key factor is how frequently and for how long an explosive atmosphere consisting of gases, vapors, mists, or dusts may occur.
International Classification: ATEX, NEC, and IECEx
There are no uniform international regulations governing explosion protection. In Europe, the ATEX Directives and the IEC/EN 60079 series of standards form the basis for the design and operation of explosion-protected systems. In the U.S., the National Electrical Code (NEC) and supplementary UL standards define the requirements for electrical equipment.IECEx complements these systems as an international certification system based on IEC standards and enables the worldwide recognition of tests and certifications for explosion-protected equipment and services.
Classification of Explosive Atmospheres in Europe (ATEX)
In Europe, potentially explosive areas are classified into three different zones based on the frequency and duration of explosive atmospheres, with this classification further distinguishing between gases, vapors, and mists on the one hand, and dusts on the other. This results in a total of six Ex zones.
Ex Zones 0 and 1
Zone 0 designates areas where an explosive gas atmosphere is present continuously or for long periods of time, such as inside containers or pipelines. In Ex Zone 1, an explosive gas atmosphere may occasionally occur during normal operation, for example at filling or emptying points.
What is the Ex-Zone 2?
Zone 2 is particularly relevant in practical applications. In this zone, an explosive gas atmosphere is expected to occur only rarely and for short periods, for example as a result of malfunctions or leaks. Many industrial applications—especially those involving control cabinets and electrical components—fall into this category.
The legal regulations do not specify what “rarely” means. In practice, however, the prevailing view is that even a single occurrence of an explosive atmosphere in a year requires an area to be classified as an Ex Zone 2. If the risk of explosion lasts for less than 30 minutes per year, this is generally referred to as a “short period.”
Ex-Zones 20, 21, and 22
Zones 20, 21, and 22 apply to combustible dusts. The basic principle is the same as that for gas zones, but it is adapted to the specific behavior of dust atmospheres. The key factor here is that dust deposits, dust clouds, and ignition sources behave differently than gases, which is why separate criteria apply for frequency and risk.
An Overview of the Ex-Zones
The division into Ex zones is not merely a theoretical classification. It defines specific requirements for the design, selection, and use of electrical equipment and forms the basis for planning in accordance with standards.
The zone classification shown below complies with the European ATEX system and the U.S. zone model according to NEC 505/506:
Explosion-Protected Zones: Guidelines, Laws, and Standards in the United States
An explosion at an industrial facility can have dangerous consequences. For this reason, the relevant regulatory authorities have enacted comprehensive regulations on explosion protection in general (zone classification, protective measures, etc.) and on the design and operation of electrical equipment in hazardous areas in particular. In the U.S., Articles 500 through 505 of the National Electrical Code (NEC) specify the requirements for equipment and installation intended for use in hazardous locations. These areas are referred to in North America as “Hazardous Locations,” or “HazLoc” for short.
Zoning in the United States
The zoning system in the United States differs fundamentally from the European ATEX model. Traditionally, the Class/Division system based on NEC 500 is used there, which distinguishes more clearly between types of substances and their properties.
Ex Zones According to NEC 505: The Three-Zone Model
With the introduction of NEC 505, a zone model was also introduced in the U.S. that is based on the internationally standard three-zone classification (Zones 0, 1, and 2). As a result, both systems now exist in parallel.
Although the zone numbering is similar, the requirements, testing standards, and certifications are not identical to those of ATEX. Therefore, an Ex Zone 2 as defined by European standards is not automatically equivalent to a Zone 2 under NEC 505.
Zone Classification in Explosion Protection: Practical Criteria
The classification of zones for explosion protection is based on a risk assessment. Key criteria include:
- Substance Properties (e.g., ignition temperature, explosion limits)
- Degree and Frequency of Release
- Ventilation Conditions
- Operating Conditions and Malfunctions
- Cleaning and Maintenance Processes
The result is a documented assignment of the areas to defined Ex zones.
Which systems and control cabinets are permitted for use in hazardous areas?
In all explosion-protected zones, only equipment and control cabinets that are explosion-proof or dust-explosion-proof may be used. Compliance with these requirements is best demonstrated through certification in accordance with UL 1203. Appropriate labeling certifies the cabinet to local regulatory authorities as suitable for use in hazardous areas, thereby significantly simplifying the commissioning process.
As a UL-certified control cabinet manufacturer, we at Krayer manufacture control cabinets that meet the strict requirements of UL 1203.
Conclusion: Understanding Ex-Zones Correctly and Implementing Them Technically
Ex zones form the foundation of explosion protection. A clear zoning system in explosion protection is a prerequisite for:
- Safe system designs
- electrical systems that comply with standards
- solutions that can be used internationally
For control cabinet construction, this means that only those who correctly classify Ex zones can develop electrical solutions that are technically safe, compliant with standards, and suitable for international use.
