Zone Classification Principles
Zone classification isn't arbitrary—it follows defined principles based on how, where, and how often explosive atmospheres are likely to form. Understanding these principles enables consistent, defensible classification decisions.
The Source of Release Concept
Zone classification starts with identifying "sources of release"—points from which flammable substances may be released into the atmosphere. Each source is characterised by its grade:
- Continuous grade: Release is continuous or expected to occur frequently or for long periods. Examples: open liquid surfaces, continuous vents, liquid surface inside an open tank.
- Primary grade: Release is likely to occur periodically or occasionally during normal operation. Examples: pump seals, sample points, relief valves that operate during normal conditions.
- Secondary grade: Release is not expected during normal operation and, if it occurs, is likely to be infrequent and for short duration. Examples: flanged connections, valve stems, equipment seals that should not leak.
Matching Sources to Zones
The relationship between release grade and zone is straightforward in principle:
| Release Grade | Gas/Vapour Zone | Dust Zone |
|---|---|---|
| Continuous | Zone 0 | Zone 20 |
| Primary | Zone 1 | Zone 21 |
| Secondary | Zone 2 | Zone 22 |
However, this is modified by ventilation and dispersion characteristics that affect whether released material actually forms an explosive atmosphere.
The Role of Ventilation
Ventilation can reduce zone extent or even eliminate the need for classification. EN/IEC 60079-10-1 (for gases) and EN/IEC 60079-10-2 (for dusts) provide detailed guidance on assessing ventilation effectiveness.
Three ventilation levels are recognised:
- High: Can reduce zone extent significantly and may reduce zone type
- Medium: Can control concentration while release persists but cannot prevent some explosive atmosphere formation
- Low: Cannot control concentration and zone type is determined primarily by release grade
Ventilation availability matters too—is it present continuously, or could it fail? A system relying on mechanical ventilation needs consideration of what happens when ventilation stops.
Determining Zone Extent
Zone extent depends on release rate, release velocity, LEL of the substance, ventilation rate, and physical barriers. For simple releases, calculation methods in the standards can estimate extent. For complex scenarios, computational fluid dynamics (CFD) modelling may be used.
General principles for extent:
- Heavier-than-air vapours spread along floors and into pits
- Lighter-than-air gases rise and accumulate at high points
- High release velocities cause greater dispersion but wider initial spread
- Outdoor releases may have minimal extent due to natural ventilation
- Indoor releases in confined spaces may fill the entire room
"Normal Operation" Definition
The directive's definitions refer to "normal operation"—which means "the situation when installations are used within their design parameters." This includes:
- Start-up, steady operation, and shutdown
- Foreseeable process upsets within design limits
- Routine maintenance and cleaning activities
- Operator interventions within normal procedures
It does not include major equipment failures, disasters, or operations outside intended use.
Hierarchy of Controls
Before classifying zones, remember the employer's primary duty is to prevent explosive atmospheres where possible. Zone classification assumes that prevention isn't fully achievable—if a release can be eliminated by process change or substitution, that may be preferable to classifying and controlling a zone.
Competence Requirements
Zone classification requires competent persons who understand both the process being assessed and explosion protection principles. This often means combining process knowledge from operations/engineering staff with explosion safety expertise—whether internal specialists or external consultants. Poor classification—either over-conservative (expensive) or under-conservative (dangerous)—reflects inadequate competence in the assessment process.
Regular Review
Classification must be reviewed following significant changes to processes, equipment, materials, or operating procedures. What was correctly classified initially may become wrong after modifications. The Explosion Protection Document should establish triggers for classification review.