Provides a classification of areas according to the likely existence of an explosive gas/air mixture, including areas in which the mixture is continuously present or present for long periods (Zone 0), areas in which the mixture is likely to occur in normal operation (Zone 1), and areas in which the mixture is not likely to occur or will exist for only a short time (Zone 2). Basic principles involved in classification are given and the factors to be considered in determining the type and extent of zones are described. Similar to IEC 79-10 in terminology, definitions and general treatment of the subject.