Zones are classified by the National Electric Code (NEC) and described in article 505.  One characteristic of Zone 2 areas is when ignitable concentrations of flammable gases or vapors are not normally present but could be present in certain cases that are clarified in the NEC standards.  Zones are also classified by ATEX 1999/92/EC Directive based on the frequency and duration of the occurrence of the explosive atmosphere.  Zone 2, according to ATEX 1992/92/EC Directive, is when an explosive atmosphere is likely to occur infrequently or for short periods of time.