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The term hazardous location once meant a place within a petrochemical or some other equally volatile plant. Today, hazardous manufacturing processes include coatings, adhesives, and flammable liquids in parts-cleaning tanks and dry-cleaning plants. Even food processing and other agricultural-related industries are becoming familiar with hazardous designations by the nature of the processes involved, such as grain elevators or flour and feed mills. Textile mills and any plant that creates sawdust also fall into the hazardous category.
Selecting equipment that may be used in such locations requires an understanding of how hazards are classified. The NEMA, Underwriters' Laboratories, and the National Fire Protection Association have categorized hazardous environments by the following classes with divisions similar to European zones.
The division defines the probability of an explosive mixture being present:
Classification of hazardous areas varies slightly between the United States and other countries. The European classification of hazardous areas generally follows that of the International Electrotechnical Commission (IEC) and is affiliated with the International Organization for Standardization (ISO), as its electrotechnical division. The IEC classifications are as follows. A hazardous area is divided into zones:
The following table shows the zone and division cross reference used in IEC countries and North America.
The responsibility for the area classification of a plant rests jointly with the engineering, process, safety, and any other departments involved in its design and operation. Factors to be considered include:
Product Classification
Two other safety aspects are commonly included in the classification of devices for use in hazardous areas-gas groupings and temperature classification. The gas grouping takes account of the maximum amount of energy which can be released under operating or fault conditions, whereas the temperature classification is concerned with the maximum temperature which can be attained by the external surface of the device.
Temperature classification
Gas-air mixtures can be ignited by contact with a hot surface, and consequently all electrical equipment used in hazardous atmospheres must be classified according to its maximum surface temperature. Table 11.2 shows the ignition temperature to be taken into consideration in Europe (IEC) and the United States (NEC) for the gases and vapors stipulated there. All temperature classifications, unless otherwise specified, are assessed with reference to a maximum ambient temperature of 400C (1040F). If the equipment is used in a temperature higher than this, then its temperature classification should be reassessed.
Gas or Apparatus Grouping The same gas groupings are used for flameproof and intrinsically safe equipment, and tables are available showing the equipment classification which can be used with particular groups of gases. Table 11.3 gives some examples with the IEC and North American classifications, Gas groups E, F, and 0 are concerned with hazards associated with the presence of dust. The overall term used in Europe covering all methods of protection is explosion-proof, and the symbol Ex is used. In the United States engineers tend to use the term explosion-proof as being synonymous with the European term flameproof and this often causes confusion. Methods for Safe Control Understanding design methods for operating equipment in hazardous areas starts with the combustion triangle. Fuel, oxygen, and a source of ignition (spark or temperature) must be present at the same time (and in the necessary proportions) for combustion to occur. One approach to creating a safe environment for hazardous areas is confinement; isolating the area reduces the possibility of accidents. Isolation involves removing or confining any possible element which could create a spark and ignite an explosion. There are three common methods of providing safety within a hazardous location, categorized by the power technology used: (1) pneumatic, (2) explosion-proof; and (3) intrinsically safe systems. Pneumatic Systems Pneumatic systems are, by their nature, a safe means of control be- cause they are powered by air. Pneumatic systems are generally clean and easy to service, but the number of control operations performed by pneumatic sequencers is limited. This can be augmented by an electropneumatic interface which enables pneumatics to be controlled by a Programmable Logic Controller. The drawbacks to pneumatic systems are in distance and reaction times. Where installations are spread over a wide area, the slow reaction time of pneumatic systems increases control reaction time. The length of control circuits in a total pneumatic system must be limited for the control cycle time to remain short. Explosion-proof housings Explosion-proof housings provide a simple means to adapt electric and electromechanical and electropneumatic controls to hazardous locations. Explosion-proof housings are designed to withstand the explosion of a mixture inside the enclosure and to prevent the spread of the flame to the outside. These enclosures are effective, especially for interrupting high currents to motors using limit switches. However, this method lacks flexibility in the use of sensing techniques because of the size of the devices. In addition to the space required for explosion-proof devices, material and labor costs for installation and service may be high. NEMA classifications. The NEMA classifies electrical equipment and enclosures for industrial applications as follows:
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