Step 1 — find the sources of release
Classification starts not from the plant but from the source of release. A flange, a valve gland, a pump seal, a filling point, a sampling point, a vent line, a hatch, a conveyor transfer point: each is a separate source, written down separately. Saying “this room is hazardous” is not classification.
Step 2 — grade of release
| Grade | Meaning | Zone that follows |
|---|---|---|
| Continuous | Release is continuous or for long periods | Zone 0 / 20 |
| Primary | Release occurs periodically in normal operation | Zone 1 / 21 |
| Secondary | Not expected in normal operation; if it happens, only briefly | Zone 2 / 22 |
Step 3 — ventilation
Ventilation enlarges or shrinks the zone that a given source produces. Three things are assessed: the degree of ventilation (high, medium, low), its availability (good, fair, poor) and the volume its effect reaches. Strong, reliable ventilation can reduce a zone; what happens on failure must be written down too.
Unclassified is also a decision
Calling an area “not a zone” is not standing outside the classification — it is a conclusion, and its reasoning belongs in the document. It is the question most often asked in an inspection: why was this area treated as unclassified?
Shown on the drawing
Classification is not conveyed by text alone. Zones are marked on the plan with their horizontal and vertical extents; only that drawing can say whether a motor or a socket box falls inside or outside a zone.