The classification of hazardous areas

TGL Technische Gute- und Lieferbedingungen, Technical Regulations for Quality and Delivery) have defined the classification of hazardous areas (TGL 30042) and the requirements for construction and testing of explosion protected electrical equipment (mainly TGL 19491). 

For the classification of hazardous areas the notable standards that are most frequently used are IEC60079 Part 10 for UK and Europe, BS5345 Part 2 for UK only, API 500 A for USA. Although the territories of origin are given it is found in practice that an oil company operating in a particular country may adopt any one or a combination of standards to comply with the governing rules of the country e.g. European standards are adopted in Abu Dhabi and Qatar, whereas the standards of the USA are preferred in Saudi Arabia. A similar situation exists in the Far East for example. 

A description of the hazards inherent in the process and plant operation, and the safety features and precautions to be taken to overcome them. This may include the classification of hazardous areas. 

Classification of hazard areas according to the probability of a flammable concentration of vapour occurring (to BS 5345 Part 1)... 

Classification of hazard areas according to the probability of a flammable concentration of vapour occurring (to BS 5345 Part 1) Zone 0 Area in which an explosive gas-air mixture is continuously present, or present for long periods Zone 1 Area in which an explosive gas-air mixture is likely to occur in normai operation Zone 2 Area in which an explosive gas-air mixture is not likely to occur in normal operation, and if it occurs will exist only for a short time Safe area By implication, an area that is not classified Zone 0, 1 or 2 is deemed to be a non-hazardous or safe area with respect to BS 5345 ... 

Hazardous places are classified in terms of zones (divisions) on the basis of the frequency and duration of the occurrence of an explosive atmosphere (IEC 60079-10 200X, Classification of Hazardous Areas European Standard EN 50281-3 200X, Classification of Areas Where Combustible Dusts Are or May Be Present ). 

In the United States the discrimination between categories ia and ib is necessary only if the IEC-based classification of hazardous areas has been adopted. According to the differentiation into Divisions 1 and 2 (still predominant), one single type of intrinsic safety (IS) has been defined, requiring a two-fault assessment. 

Some of the earliest work on the subject of hazardous area classification was documented by the API, IP and BS institutions, and the chemical manufacturing company ICI. Their particular documents are given in Table 10.1. 

The concepts of hazardous area classification and the legislation that supports the subject are in a continuous state of revision and so the engineer should keep abreast of such developments. 

The relevant standards have been affected by the standard harmonisation process within the EU through the European Committee for Electrotechnical Standardisation (CENELEC) and through the International Electrotechnical Commission (lEC). Individual parts of BS EN 60079 deal with different aspects concerning electric apparatus for potentially explosive gas atmospheres, namely classification of hazardous areas, electrical installations in hazardous areas (other than mines) and inspection and maintenance of electrical installations in hazardous areas (other than mines). BS 50014 covers general requirements. 

Classification of hazardous areas into zones, observance of the corresponding minimum requirements for the respective zone and marking of potentially explosive areas. 

Where there are flammable materials that constitute an explosion risk, the locations in which the explosion risk exists are called hazardous areas. These areas are classified into zones according to the extent of the risk, using guidance published in BS EN 60079-10 1996 Electrical apparatus for explosive gas atmospheres. Part 10 Classification of hazardous areas. The zones are also defined in the European Directive on minimum requirements for improving the safety and health protection of workers potentially at risk from explosive atmospheres. The zones are ... 

Electiical classification. Article 500 of the National Electric-Code provides for the classification of the hazardous nature of the process area in which the measurement device will be installed. If the measurement device is not inherently compatible with this classification, suitable enclosures must be purchased and included in the installation costs. 

NFPA 497. 1997. Recommended Practice for the Classification of Flammable Liquids, Gases, or Vapors and of Hazardous (Classified) Locations for Electrical Installations in Chemical Process Areas. National Fire Protection Association, Quincy, MA. 

Figure 2.1 Classification of an area hazardous due to combustible gases/liquids into zones 0, 1 and 2 [50], The value R is tabulated in Table 2.4 as a function of the filling rate into the tank (liquid with a Hash point <35°C). Dimensions given in metres.

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