Siemens SIPART PS2 Operating Instructions Manual page 324

Glossary
Zone 20
Zone 20 is an area in which a potentially explosive atmosphere in the form of a cloud of
combustible dust in air is present continuously, over a long period, or frequently.
Zone 21
Zone 21 is an area in which a potentially explosive atmosphere in the form of a cloud of
combustible dust in air can be occasionally produced during normal operation.
Zone 22
Zone 22 is an area in which a potentially explosive gaseous atmosphere in the form of a cloud
of combustible dust in air never develops or develops only for a short while during normal
operation.
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SIPART PS2 with PROFIBUS PA
Operating Instructions, 02/2016, A5E00127926-AB