Zone Classification for Hazardous Areas
Zone classification is the process of dividing a facility into areas based on the probability and duration of an explosive atmosphere being present. It determines what type of Ex-protected equipment is required in each location.
Gas and Vapour Zones (IEC 60079-10-1)
| Zone | Definition | Typical Duration | Required EPL |
|---|---|---|---|
| Zone 0 | Explosive atmosphere present continuously or for long periods | >1,000 hours/year | Ga |
| Zone 1 | Explosive atmosphere likely during normal operation | 10–1,000 hours/year | Gb (or Ga) |
| Zone 2 | Explosive atmosphere not likely during normal operation; if it occurs, only briefly | <10 hours/year | Gc (or Gb/Ga) |
Zone 0 Examples
- Inside storage tanks containing flammable liquids above their flash point
- Inside process vessels during normal production
- Inside pipelines carrying flammable gases
Zone 1 Examples
- Area surrounding Zone 0 (e.g., around tank vents)
- Areas near equipment that frequently releases flammable substances
- Pump and compressor seal areas
- Sample points and drainage systems
- Loading/unloading areas for flammable liquids
Zone 2 Examples
- Areas surrounding Zone 1
- Areas where release only occurs during maintenance, breakdown, or abnormal conditions
- Flange connections in good condition
- Valve stems under normal conditions
- Areas near Zone 1 with adequate ventilation
Dust Zones (IEC 60079-10-2)
| Zone | Definition | Required EPL |
|---|---|---|
| Zone 20 | Explosive dust cloud present continuously or frequently | Da |
| Zone 21 | Explosive dust cloud likely during normal operation | Db (or Da) |
| Zone 22 | Explosive dust cloud not likely; only briefly if it occurs | Dc (or Db/Da) |
Zone 20 Examples
- Inside silos, hoppers, and cyclone filters
- Inside pneumatic conveying lines
- Inside mixers and blenders processing combustible dusts
Zone 21 Examples
- Areas around dust filling/emptying points
- Near conveyor transfer points with frequent spillage
- Bag filter rooms during normal operation
Zone 22 Examples
- Areas surrounding Zone 21 with adequate housekeeping
- Near equipment that rarely releases dust
- Warehouse areas storing bags of combustible powder
Important: Dust layers on surfaces can form Zone 22 (or higher) even without airborne dust. A layer of 5mm of combustible dust, if disturbed, can create an explosive cloud.
North American Classification (NEC/CEC)
The US (NEC Article 500/505) and Canada (CEC Section 18) use a parallel system:
| Zone System | NEC/CEC Equivalent | Hazard Level |
|---|---|---|
| Zone 0 | Class I, Division 1 | Continuous/frequent |
| Zone 1 | Class I, Division 1 | Normal operation |
| Zone 2 | Class I, Division 2 | Abnormal only |
| Zone 20 | Class II, Division 1 | Continuous/frequent dust |
| Zone 21 | Class II, Division 1 | Normal operation dust |
| Zone 22 | Class II, Division 2 | Abnormal only dust |
NEC Article 505 also recognizes the Zone system directly. The Division system is less granular — Division 1 covers both Zone 0 and Zone 1.
Class III covers fibers and flyings (textile mills, woodworking) — no direct Zone equivalent.
Zone Classification Methodology
Classification is typically performed by a team including:
- Process engineers (source and release characterization)
- Safety/HSE engineers
- Electrical engineers
- Operations personnel
Key Factors
- Source of release — where flammable material can escape (flanges, valves, seals, vents)
- Grade of release
- Continuous → forms Zone 0/20
- Primary (expected during normal operation) → forms Zone 1/21
- Secondary (not expected during normal operation) → forms Zone 2/22
- Ventilation — affects zone extent and possibly zone type
- Good ventilation reduces zone size and may downgrade zone type
- Poor ventilation extends zone size and may upgrade zone type
- Negligible ventilation (enclosed spaces) — zone extends to fill the entire space
- Relative density of gas — lighter gases (hydrogen) rise; heavier gases (propane) pool at ground level
Zone Extent
The physical size of a zone depends on:
- Release rate and velocity
- Ventilation rate and effectiveness
- Relative density of the flammable substance
- Flash point and vapour pressure
- LEL of the substance
Standards provide calculation methods: IEC 60079-10-1 Annex B (gases), IEC 60079-10-2 Annex B (dusts).
Zone Documentation
The output of a zone classification study is:
- Hazardous Area Classification drawings — plans and sections showing zone boundaries
- Schedule of release sources — tabulated list of all potential sources
- Data sheets — substance properties, release characteristics
- Zone classification report — methodology, assumptions, results
These documents form the basis for selecting equipment with the correct EPL and protection method.
Common Pitfalls
- Forgetting that lighter-than-air gases (hydrogen, methane) accumulate at ceiling level
- Ignoring dust layers — a 5mm layer disturbed becomes a dust cloud
- Not reassessing when processes change
- Confusing "Zone 2" with "non-hazardous" — it still requires Ex equipment
- Under-estimating ventilation failures (what happens when the fan stops?)
Related Files
- Perusteet — explosion triangle, basic concepts
- Gas Groups — how different fuels are classified
- EPL — equipment protection levels required per zone
- Protection Methods — types of protection suitable per zone
- Standardit — IEC 60079-10-1 and 60079-10-2