In Europe, hazardous areas are classified into three zones which are directly related to the predicted occurrence of when and the duration that an explosive atmosphere may be present in the area. These zones are:
· Zone 0: Where an explosive atmosphere is continuously present, or present for long periods of time.
· Zone 1: Where an explosive atmosphere is likely to occur in normal operation.
· Zone 2: Where an explosive atmosphere is not likely to occur in normal operation and if it does occur, it will exist only for a short time.
Furthermore, European standards require that apparatus be subdivided into two groups: I and II. Group I is for apparatus to be used in mines where the danger is represented by methane gas and coal dust. Group II is for apparatus to be used in surface industries where the danger is represented by gas and vapor that has been subdivided into three groups: A, B and C.