The hazardous area classification study should be conducted in the early stages of the design of the plant and adapted/updated as the plant is being constructed, to reflect the “as built” plant. If the plant has been established for some time, then reviewing the classification is just as important to ensure the details are still correct.
The area classification is mandatory as noted in the wiring rules (AS/NZS 3000) under section 7.7.
In this section it refers to two main classification standards used in Australia.
Part 10:1 Classification of areas
Explosive gas atmospheres
(IEC 60079-10-1, Ed.1.0(2008) MOD)
Part 10:2 Classification of areas
Explosive dust atmospheres
Different methods can be used to classify plant and equipment which are referred to in the standards.
Method by Example
Source of Release
When using the method by examples there are several examples in Annex Za. However, this method is conservative and sometimes overly large. However, is accepted within the industry and identifies Zone 1 and Zone 2 areas distinctly.
Where, through lack of detailed data or operating experience, it is not possible to identify and assess individual sources of release in a plant, a generalised method may be used.
Generalised methods require judgements to be made, usually for quite large sections of the plant, on whether the overall hazard is high (Zone 0 or Zone 1) or low (Zone 2).
This Annex provides examples of area classification for areas associated with the production, processing, handling and storage of flammable liquids and gases in refineries and major processing plants.
Although the method by calculation is more precise, and potentially reduces the zones, it finds a hypothetically, volume Vz which represents the volume over which the mean concentration of flammable gas or vapor will typically be either 0.25 or 0.5 times the LEL. This calculation of Vz is only intended to assist in assessing the degree of the ventilation and is not directly related to the size of the hazardous area.
Modeling is more precise, and the result is potentiality smaller. This can replicate the conditions on site more accurately and provide detail on how a leak might behave.