Three zones, one criterion: frequency
| Zone | Explosive atmosphere | Typical location |
|---|---|---|
| 0 | Continuously, for long periods or frequently | Inside a tank, the vapour space of closed vessels |
| 1 | Occasionally in normal operation | Around a filling point, at a sampling point, near a vent |
| 2 | Not expected in normal operation; brief if it occurs | The volume surrounding zone 1, well-ventilated enclosures |
Why it is frequency and not severity
An explosion in zone 2 can be as destructive as one in zone 0. What differs is how long the atmosphere exists. As frequency rises, the assurance that equipment will not ignite must rise with it — which is why classification feeds directly into equipment selection.
Zones nest inside one another
In real plants zones are nested: the inside of the tank may be zone 0, a metre around the filling point zone 1, and the volume beyond it zone 2. Classification is therefore not a list but a map; it has to say out to what distance each zone applies.
A common mistake
Classifying an area “as zone 1 to be safe” looks like caution but has two consequences: unnecessary equipment cost and — worse — the loss of the classification's authority. The zone that carries real risk disappears among the ones that were widened for comfort.