Hazardous Area Classification for a Solvent Storage Room
A workshop is converting a store room into a solvent decanting area. The room is 6 m × 4 m × 3 m, with a single doorway and a low-level mechanical exhaust fan. Solvents with a flash point below ambient will be decanted from 20 L drums into smaller containers. The installer is asked to provide a 10 A general-purpose outlet, a fluorescent light fitting and a wall-mounted switch inside the room. The client has no hazardous area drawing or classification document. Existing wiring in the room is surface-mounted PVC conduit with standard IP20 accessories.
1. What must happen before the installer can select equipment for this room, and why can the installer not simply choose 'explosion-proof' fittings?
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The installer cannot select equipment until the hazardous area classification has been established. Classification defines the zone (Zone 0, 1 or 2 for gases and vapours), the extent of each zone, the gas group and the temperature class. These determine the required equipment protection level and marking. Without a classification document, the installer has no defensible basis for selecting equipment, and choosing 'explosion-proof' fittings by guesswork is not acceptable.
The classification should be prepared by a competent person, taking into account the flash point and vapour density of the solvents, the quantity handled, ventilation, openings and the location of the decanting activity. A low-level exhaust is appropriate for vapours heavier than air, but the classification must consider what happens if the ventilation fails, which may extend the zone or change its category. The zone around an open decanting point is typically more extensive than the zone near closed drums.
Only after the classification is documented can the installer select equipment with the correct Ex marking for the zone, gas group and temperature class, and confirm that the installation and wiring methods comply with the relevant hazardous area requirements (AS/NZS 60079 series and AS/NZS 3000 Section 7). The installer should record the classification document as part of the design and certification records.
2. Assuming a classification is later provided, how should the installer treat the proposed outlet, light and switch, and what installation issues arise from the existing wiring?
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Each item must be assessed against the zone in which it will be located. If the outlet, light or switch falls within a classified zone, it must be certified equipment with the appropriate Ex marking for that zone, gas group and temperature class, and installed in accordance with its certification conditions. A standard IP20 fluorescent fitting and a standard switch would not be acceptable in a classified zone. If an item can be located outside the classified zone, standard equipment may be used, but the boundary must be clearly established and documented.
Fluorescent fittings deserve particular attention because of sparking at lamp holders and control gear, and because surface temperature must be checked against the temperature class. Switching should preferably be located outside the hazardous area. Socket-outlets in hazardous areas are problematic because plugging and unplugging under load can produce arcs; interlocked or specially rated arrangements may be needed, or the outlet relocated outside the zone.
The existing surface PVC conduit and IP20 accessories are unlikely to be suitable within a classified zone. The installer must review cable types, conduit, sealing at boundaries, and earthing/bonding of metallic parts, and must ensure that the installation does not introduce ignition sources. Any existing equipment inside the new zone must be assessed and either removed, relocated or replaced. All of this should be recorded against the classification drawing.