Zone classification is the systematic assessment and documentation of areas where explosive atmospheres may occur. It forms the foundation of ATEX workplace compliance under Directive 1999/92/EC, determining which equipment categories may be installed in each location.

Purpose of Zone Classification

Zone classification serves multiple essential functions. It identifies the likelihood and duration of explosive atmospheres, guides appropriate equipment selection for each area, informs other protective measures such as ventilation and procedures, forms a core component of the Explosion Protection Document (EPD), and provides a basis for permit-to-work systems and maintenance planning.

Without proper zone classification, employers cannot demonstrate compliance with ATEX workplace requirements, and inappropriate equipment selection could create ignition risks or result in unnecessarily expensive overspecification.

Zone Definitions for Gases, Vapours and Mists

Directive 1999/92/EC defines three zones for Group II (non-mining) gas hazards:

• Zone 0: Explosive atmosphere present continuously, for long periods, or frequently. Typically the interior of vessels, tanks, and pipework containing flammable liquids or gases. Guideline: more than 1000 hours per year
• Zone 1: Explosive atmosphere likely to occur occasionally during normal operations. Areas around Zone 0, near potential release points such as pump seals, sample points, and relief valves. Guideline: 10-1000 hours per year
• Zone 2: Explosive atmosphere not likely during normal operations; if it occurs, will be infrequent and of short duration. Areas where releases only occur during abnormal conditions such as flange leaks or equipment failure. Guideline: less than 10 hours per year

Zone Definitions for Combustible Dusts

Parallel classifications exist for dust hazards. Zone 20 is where explosive dust cloud is present continuously, frequently, or for long periods—inside silos, hoppers, and dust collection systems. Zone 21 is where explosive dust cloud is likely occasionally during normal operations—areas near powder transfer points, bag filling stations, and dust-generating processes. Zone 22 is where explosive dust cloud is not likely normally but may occur briefly—general areas where dust may accumulate and become disturbed.

Dust layer accumulation must also be considered. A 5mm layer disturbed into a 5-metre-high cloud creates a concentration of approximately 1000 g/m³—well above most dust explosion limits (typically 20-60 g/m³ minimum).

Classification Methodology

Zone classification follows a structured approach based on EN 60079-10-1 (gases) and EN 60079-10-2 (dusts). The process involves source identification, determining grade of release, assessing ventilation, and establishing zone type and extent. Release grades are categorised as continuous (constant release or long period), primary (periodic release during normal operations), or secondary (infrequent release, short duration, abnormal conditions only).

Continuous releases typically create Zone 0/20, primary releases create Zone 1/21, and secondary releases create Zone 2/22, though ventilation can modify these outcomes.

Ventilation Effects

Ventilation significantly influences zone classification. Effective ventilation can reduce zone extent by diluting releases below LEL at shorter distances, reduce zone type by preventing atmosphere formation, and eliminate zones entirely in some cases with adequate air changes. EN 60079-10-1 provides methods for calculating ventilation effectiveness, considering air change rates (typically 2-12 air changes per hour for adequate natural ventilation) and availability (continuous, normally available, or poor).

Documentation Requirements

Zone classification must be documented with clear zone boundary drawings showing plan and elevation views, source of release schedules listing all potential release points, calculation records demonstrating zone extent derivation, and assumptions about ventilation, maintenance, and operating conditions. This documentation forms part of the Explosion Protection Document and must be reviewed when process conditions change.