Hazardous Area Classification
Mohamed Tamer
Expert Instrumentation and Control Engineer at DG Impianti Industriali S.p.A.
Hazardous areas are defined in DSEAR as "any place in which an explosive atmosphere may occur in quantities such as to require special precautions to protect the safety
Area classification is a method of analyzing and classifying the environment where explosive gas atmospheres may occur. The main purpose is to facilitate the proper selection and installation of apparatus to be used safely in that environment, taking into account the properties of the flammable materials that will be present. DSEAR specifically extends the original scope of this analysis, to take into account non-electrical sources of ignition, and mobile equipment that creates an ignition risk.
Over the past 100 years, systems have evolved for classifying hazardous locations. Agencies throughout the world developed and implemented standards for classifying hazardous locations and the products to be used in those locations. Certifications for manufacturing and installing apparatus in hazardous locations were established and codes and safety practices for operating in hazardous locations were developed.
1- Classifying Hazardous Locations
In the 1920s, standards for determining where a hazardous location existed were developed by the National Fire" Protection Association (NFPA). These standards became part of the National Electrical Code (NEC) for the United States. The division system was established to differentiate between hazardous or explosive areas, and safe areas where no explosive atmosphere is present. After World War II, the system was modified to provide for two divisions in order to designate where an explosive atmosphere existed under normal conditions and where it existed under abnormal conditions (the result of equipment breakdown or fault). The two-division systems have been used worldwide for classifying hazardous locations for almost fifty years.
In 1996, the NFPA introduced NEC Article 505, which is an lEC-style zone system of hazardous location classification applies to applications in the United States. The new NEC article provides a zone classification system in addition to the traditional class system. The new system encompasses modifications as needed to account for the uniqueness of the different national electrical codes and practices used in the United States.
In 1998, the Standards Council of Canada (SCC) adapted the IEC standards with provisions for the Canadian Electrical Code (CEC). The new CEC standards have been approved by the SCA as CAN/CSAE79-95.
2- Why Classify Hazardous Locations?
It would be ideal if all electrical apparatus could be installed in safe areas separated from any explosive atmosphere, or that no danger from electrical ignition was possible. Isolating all apparatus is unrealistic in an industrial setting. Therefore, it is necessary to design apparatus or enclosures for the specific operating conditions of the location. An essential step in the design process is a systematic approach to classifying hazardous locations.
Classifying locations with regard to their fire or explosion hazard is a complex subject. The principles used to classify hazardous locations are universal, However the systems and terminology used in countries around the world differ. The classification factors are discussed below, and the systems used in North America and Europe are described in the sections that follow.
2.1 - North American Division System
The division system consists of three components: Class, Division, and Group. Class describes the physical form of flammable material in the atmosphere. Division describes the likelihood or degree of a hazardous atmosphere. Group describes the type of flammable material in terms of chemical composition and ignition energy
Classes
In North America, hazardous locations are categorized into three classes, depending on the form of flammable material present.
Natural gases and petroleum gases/vapors/liquids are examples of Class I hazards. Grain or coal dust suspended in high concentrations in the air is example of a Class II hazard. Suspended fibers, such as those that may be present in textile and paper manufacturing are examples of Class III hazards.
Divisions
The division system was first used in the 1920s in the petroleum refining industry to designate an area where flammable gases, vapors, or liquids were present under normal operating conditions (Division 1). In 1947, in an effort to contain installation costs, Division 2 was added to indicate an area adjacent to Division 1 and where a hazardous condition would exist only periodically or as a result of a fault or breakdown.
Until the 1960s, the division system was used worldwide. In the 1970s, Europe adopted the zone system for classification of hazardous locations. Although the division system is still used in North America, a modified zone concept was adopted in the United States in 1996 and in Canada in 1998.
2.2- North American Modified Zone System
Currently, the North American classification system is in transition, from a division to a modified zone system. For more than fifty years, the division system was used to classify hazardous locations in terms of classes, divisions, and groups. So, why change the system? Because companies have become global and industrial end-users and manufacturers want to harmonize with international standards so that a plant built in one country can use the same equipment and installation standards as one built in another country.
2.3- European Category/Zone System
The ATEX Directive updates the Euro norm (EN) standards to the new generation E and adds new standards and requirements for equipment used in hazardous locations. Key elements in the Directive are the separation of Zone 0 and Zone 1, the definitions of Categories 1, 2, and 3, and the new requirements for production quality and the contents of installation instructions. The classifications are not fundamentally changed by the ATEX Directive. The only change is that all approved equipment is divided into categories, which determines in which Ideation they can be used. Before the ATEX Directive, prescribing which equipment can be used in which location\was based on individual national standards. Now the prescription is harmonized for the complete European Community.
Division versus Category (Zone)
The division system and category (zone) system have many similarities and a few significant differences. The differences in the requirements can impact the ability of manufacturers to sell products and the ability of users to install these same products in hazardous locations around the world.
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Under both the division and category (zone) systems, electrical equipment for use in hazardous locations are identified based on the:
The following table shows the differences between the North American and European classifications for hazardous locations.
North American Division 1 includes the corresponding European Categories 1 and 2. Currently, there is no direct equivalent in the North American Division system to the European Category 2. However, the new North American standards (1996 NEC Article 505 and 1998 CEC) have adopted a modified zone system that incorporates a corresponding Category 1 as modified to meet country codes
3- Cross References of Classifications
Most countries are using standards based on IEC zone system for classifying hazardous location. Europe has adopted ATEX category system. The table below shows across-reference of classifications
4- Ignition Temperature and ignition Energy
Ignition temperature, also called auto-ignition temperature, is the minimum temperature required to ignite or cause self-sustained combustion of a solid, liquid, or gas independently of the ignition source or heating element.
Ignition energy is the minimum electrical or thermal energy in the form of an electric spark or heat source that is necessary to cause ignition of the most easily ignited concentration of a flammable gas or combustible dust.
Group classifications categorize the type of flammable material present in terms of chemical composition and ignition energy. Class I hazardous locations are divided into four groups based on the type of gas or vapor. Class II locations are divided into three groups based on the type of dust or powder present. Class III, locations where combustible fiber or flyings are present, is not divided into groups.
Class I
Class II
Class III
Gas / Dust / Fiber Groups and Ignition Energy – North America and Europe
5- Surface Temperature
Electrical apparatus produce heat during their operation and potentially a larger amount of heat under a fault condition. The surface temperature of the apparatus is a measure of the amount of heat produced. In order for an apparatus to be safely installed in a hazardous location, it is important to know the maximum surface temperature that the apparatus generates during normal operation and under a fault condition. An explosion can occur if the surface temperature of the apparatus is greater than the ignition temperature of the gas in the surrounding atmosphere.
The maximum surface temperature of an apparatus is the basis for the classifications. Each heat producing apparatus is assigned a T-number based on the highest temperature it produces under the most severe operating conditions. The maximum surface temperature is measured in relationship to a reference ambient temperature. Usually the ambient temperature of 40°C is used as the starting point, unless otherwise stated on the apparatus labeling.
Surface temperature classifications are divided into six classes—T1 through T6. North American and European standards for surface temperature classifications
6- References: