Gas zones classify areas based on the likelihood and duration of explosive gas, vapour, or mist atmospheres. This classification directly determines which equipment categories may be installed and informs all other protective measures under the ATEX Workplace Directive.

Zone 0 - Continuous Hazard

Zone 0 is defined as an area where an explosive atmosphere consisting of a mixture of air and flammable substances in the form of gas, vapour, or mist is present continuously, for long periods, or frequently.

• Typical locations: Inside storage tanks containing flammable liquids, interior of process vessels during normal operations, inside pipework and manifolds, vapour spaces above volatile liquids
• Duration guideline: More than 1000 hours per year (approximately 10% of time)
• Equipment requirement: Category 1 equipment with EPL Ga (very high protection)
• Typical extent: Usually confined to the interior of containment—does not normally extend outside vessels

Zone 0 represents approximately 1-5% of classified areas in typical industrial facilities. Equipment for Zone 0 must provide protection with two independent means of protection or remain safe with two simultaneous faults.

Zone 1 - Likely Hazard

Zone 1 is defined as an area where an explosive atmosphere is likely to occur occasionally during normal operations.

• Typical locations: Areas surrounding Zone 0 (tank vent outlets, manhole covers), proximity to process equipment with regular releases (pump seals, compressor seals), sample points and drain connections, areas around loading/unloading operations
• Duration guideline: Between 10 and 1000 hours per year (0.1% to 10% of time)
• Equipment requirement: Category 1 or Category 2 equipment with EPL Ga or Gb
• Typical extent: 1-3 metres from release source, depending on ventilation and release rate

Zone 1 typically comprises 10-30% of classified areas in process plants. Most common protection types for Zone 1 include flameproof (Ex d), increased safety (Ex e), and intrinsic safety (Ex ia/ib).

Zone 2 - Unlikely Hazard

Zone 2 is defined as an area where an explosive atmosphere is not likely to occur in normal operations, but if it does occur, will persist for a short period only.

• Typical locations: Areas surrounding Zone 1, locations where releases occur only from equipment failure (flange gaskets, valve packing), ventilated rooms containing flammable liquid storage, outdoor areas downwind from potential release points
• Duration guideline: Less than 10 hours per year (under 0.1% of time)
• Equipment requirement: Category 1, 2, or 3 equipment with EPL Ga, Gb, or Gc
• Typical extent: 3-15 metres from Zone 1 boundary, extending to effective dilution

Zone 2 represents the largest classified area in most facilities, typically 60-80% of total classified space. Category 3 equipment (self-certified under Module A) is sufficient, offering significant cost advantages over Zone 1 equipment.

Zone Extent Determination

Zone extent depends on multiple factors. Release rate determines the volume of flammable material entering the atmosphere—a typical pump seal leak releases 0.1-1 kg/hour. Substance properties including LEL, vapour density, and diffusion coefficient affect dispersion behaviour. Ventilation effectiveness based on air velocity and air changes per hour influences dilution. Obstructions and enclosures can trap vapours and extend zones significantly.

EN 60079-10-1 provides calculation methods, but pragmatic approaches using empirical data from similar installations are common. Standard configurations with published zone extents reduce assessment effort.

Transition Between Zones

Zone boundaries should be positioned at logical physical features where practical—walls, floors, equipment boundaries. Gradual transitions are acceptable; sharp boundaries at arbitrary distances may be impractical. Some installations use conservative "worst case" zone classification to simplify equipment selection and reduce the number of different equipment types required.