A hazardous-area drawing is an output of an assessment—not its starting point. Defensible classification depends on the technical basis, site evidence, assumptions and competent review behind every zone and extent.
01 / PRACTICAL GUIDANCE
Define the substance and hazardous scenario
First establish whether the concern is flammable gas, vapour, mist or combustible dust. Record the relevant material properties from credible data and understand the process conditions under which a hazardous atmosphere could form.
Gas and vapour classification and combustible-dust classification use related principles but different assessment details. The applicable method must match the hazard.
- Material identity and composition
- Flash point or relevant explosibility properties
- Process temperature and pressure
- Dust particle behaviour and layer formation
- Normal and abnormal operating scenarios
02 / PRACTICAL GUIDANCE
Identify and characterise release sources
Equipment presence alone does not define a hazardous zone. The assessment considers where a flammable substance or combustible dust may be released, the grade or likelihood of release, potential release rate and the conditions influencing dispersion or accumulation.
Photographs are useful, but they should be connected to equipment identifiers, areas, drawings and field observations so the evidence can be traced.
03 / PRACTICAL GUIDANCE
Assess ventilation, housekeeping and geometry
For gas or vapour hazards, ventilation availability and effectiveness can strongly influence persistence and extent. For dust hazards, containment, extraction, deposits, housekeeping and disturbance mechanisms are central considerations.
Walls, floors, roofs, pits, openings, obstacles and elevated equipment affect how a release may travel or collect. This is why a universal fixed radius is rarely a defensible substitute for assessment.
04 / PRACTICAL GUIDANCE
Record assumptions and unresolved information
Where operating data, material data, ventilation performance or equipment details are missing, the gap should be visible. A provisional conclusion must be labelled as provisional and linked to the evidence needed for confirmation.
Relevant parts of IEC 60079 and other applicable regulatory, client or industry guidance may inform the method. The selected basis should be recorded for the project rather than applied by habit.
05 / PRACTICAL GUIDANCE
Retain competent-person review
Software can organise evidence, calculations, drawings, decisions and approvals. It cannot assume the accountability of the competent specialist who evaluates the scenario and signs the technical conclusion.
Classification, Ex equipment suitability, inspection and certification should remain within the competence and authority required by the assignment and jurisdiction.
KEY TAKEAWAYDo not draw a zone and then search for evidence to justify it. Establish the release, material, ventilation, geometry and operating basis first; let the conclusion follow the evidence.
QUESTIONS PROFESSIONALS ASK
Frequently asked questions
Is there a standard 1-metre hazardous-zone radius?+
No universal distance is suitable for every release. Extent depends on the applicable method and factors such as release characteristics, material properties, ventilation, geometry and operating conditions.
Can an area be provisionally classified?+
A preliminary or provisional conclusion can support controlled progress when clearly labelled, bounded and linked to outstanding evidence and competent review. It should not be presented as final approval.
This article provides general professional guidance, not legal advice, certification or a site-specific technical conclusion. Requirements and suitable methods depend on the jurisdiction, evidence, operating context and agreed scope.