Phoenix Journal · LEV Testing
When your extraction moves combustible dust, the ductwork itself becomes an explosion risk. Here is how ATEX and DSEAR shape the design, marking and testing of a safe system.
Extraction & explosive atmospheres
Most kitchen extraction deals with grease, heat and moisture - but the moment a system starts moving combustible dust, the ductwork itself becomes part of a fire and explosion risk that the law treats very differently.
Flour, sugar, custard powder, dried milk, spice, starch and cocoa are all everyday ingredients, and every one of them will burn fiercely as a fine airborne cloud. Bakeries, food-production units, central production kitchens and any prep space that sieves, tips, mixes or blends dry powder can generate enough dust to form an explosive atmosphere. Draw that dust into a hood and along a duct, and you have concentrated the fuel, suspended it in air and enclosed it - three of the five conditions a dust explosion needs, gathered inside one metal box.
This is the world of ATEX and DSEAR. It is a specialist corner of extraction, and getting it wrong is not a matter of a failed inspection - it is a matter of preventing a deflagration that can travel back through the system in a fraction of a second. If your process touches combustible dust, the design, marking, maintenance and testing of your extraction all sit under rules that go well beyond ordinary ventilation.
The legal framework
In Great Britain the rules on explosive atmospheres come from two European directives, brought into domestic law by the Dangerous Substances and Explosive Atmospheres Regulations 2002 - DSEAR for short. DSEAR covers the workplace duties: assessing the risk, classifying areas and choosing safe equipment. Alongside it sits the ATEX equipment directive, 2014/34/EU, which has governed how explosion-protected kit is designed and marked since it became mandatory for manufacturers on 20 April 2016. Together they are what people mean when they say a system needs to be "ATEX rated".
The first duty is a DSEAR risk assessment. Where a combustible dust could form an explosive atmosphere, you must classify the space into zones. For dusts the scheme runs Zone 20, where an explosive cloud is present continuously or for long periods, Zone 21, where one is likely occasionally in normal running, and Zone 22, where a cloud is not likely in normal operation and would only be brief if it did appear. Gases and vapours use the parallel scheme of Zones 0, 1 and 2. The inside of dust ductwork and the filter chamber of an extraction unit are commonly assessed as Zone 20 or 21, because that is precisely where a dense, continuous cloud is designed to travel.
Each zone dictates the category of equipment that may sit in it. ATEX Category 1D equipment is built for Zone 20, Category 2D for Zone 21 and Category 3D for Zone 22, the "D" marking the kit as suitable for dust rather than gas. Read an equipment plate and you will see this spelled out - a marking such as Ex II 3D tells you the group, category and that it is dust-rated, and for dust the maker must also declare a maximum surface temperature so you can check it stays safely below the ignition temperature of your particular powder. It is worth being clear on a point people often blur: ordinary ventilation, general extraction and a controlled LEV system are not the same thing, and only equipment carrying the right ATEX category belongs in a zoned dust atmosphere.
Why extraction concentrates the risk
A dust explosion needs five things at once, often drawn as the explosion pentagon: fuel in the form of fine dust, oxygen, an ignition source, dispersion of the dust into a cloud, and confinement. Ordinary housekeeping tackles the first and fourth by keeping settled dust off surfaces. An extraction system, by its nature, does the opposite inside itself - it deliberately disperses dust into a moving cloud and confines it within sealed metal. That is why extraction handling combustible dust is designed to a different standard from grease ductwork, and why the ignition source is the element you fight hardest to remove.
How aggressive an explosion would be depends on the dust. Materials are graded by their dust explosion class, or St value, derived from the Kst pressure-rise figure measured in a test sphere. St 1 covers many flours and grain dusts; St 2 takes in the likes of sugar, powdered milk and some starches; St 3 - the most violent - includes wood flour, cellulose and certain metal dusts. A woodworking shop feeding a central unit is a good example of an St 3 hazard, and the sizing and protection of that plant follows directly from it; our buyer’s guide to woodworking dust units walks through how that shapes the specification.
Removing ignition sources runs through everything. All ductwork, hoods and the extraction unit are earthed and bonded so a static charge cannot build and spark; filters are antistatic and fully grounded; motors and fans are chosen so no hot surface or spark sits in the airstream, which usually means the motor is mounted clear of the moving dust. Compressed air for cleaning is forbidden, because a jet of air will loft settled dust straight into a cloud. Where the assessment shows an explosion still cannot be fully prevented, the system is protected rather than just prevented - explosion relief venting sized to standards such as BS EN 14491 lets pressure blow off safely to a wall or roof, and isolation devices stop a flame front travelling back down the duct into the workspace. Good design also earns its keep by shrinking the problem: well-placed LEV that captures dust at source can reduce a zoned area so far that its extent becomes negligible, which is the outcome you always want.
Keeping it safe over time
ATEX equipment is only as safe as its condition on the day. A filter that has clogged and split, an earthing strap that has corroded, a fan drawing more current because dust has loaded its impeller, a relief panel painted over during a refurbishment - any of these quietly erodes the protection the design relied on. That is where scheduled examination matters, and for extraction systems two duties run in parallel.
The first is control of exposure. Under the Control of Substances Hazardous to Health Regulations, any local exhaust ventilation must be thoroughly examined and tested at least every 14 months. That figure is a legal maximum, not a target - higher-risk processes listed in COSHH Schedule 4 need testing far more often, and many dust systems are examined on a shorter cycle by choice. The examination, carried out to the HSE’s HSG258 guidance by a competent person holding a qualification such as BOHS P601, confirms the system still captures dust as it was designed to, and you must keep the records for at least five years. The second duty, under DSEAR, is to maintain the explosion safety measures - the earthing, the antistatic filters, the venting and isolation - so that the zoned area stays as safe as the assessment assumed. In practice a good examination looks at both together, because a system that has stopped controlling dust at source is usually the same system that is letting a cloud settle where it should not.
Regular, thorough cleaning of the ductwork sits underneath all of it. Settled dust inside a duct is stored fuel; keep the internal surfaces clean and you keep the pentagon broken. If you run extraction on any combustible-dust process, the sensible path is a DSEAR assessment that zones the space honestly, equipment marked for the zone it sits in, and a testing and cleaning regime that proves it all still works. Get an experienced eye on the system before an insurer, an enforcement officer or an incident does it for you.
Questions
Only if your process can create an explosive atmosphere, which for kitchens almost always means combustible dust rather than cooking grease. If you sieve, tip, mix or blend dry powders such as flour, sugar, custard powder or spice, a DSEAR risk assessment may classify the area and the inside of the ductwork as a dust zone. Standard grease extraction over ranges and fryers is not ATEX territory, but a bakery or food-production line handling fine powder often is.
They are the DSEAR classifications for areas where a combustible dust cloud may form, graded by how often that happens. Zone 20 is where an explosive cloud is present continuously or for long periods, Zone 21 where one is likely occasionally in normal running, and Zone 22 where a cloud is not likely and would only be brief. The zone decides the ATEX equipment category you must fit, from Category 1D for Zone 20 down to Category 3D for Zone 22.
Yes. Any local exhaust ventilation must be thoroughly examined and tested at least every 14 months under COSHH, and that legal maximum applies whether or not the system is ATEX rated. For dust extraction the examination checks that the system still captures dust at source, while a parallel DSEAR check confirms the explosion safety measures such as earthing, antistatic filters and relief venting are intact. Higher-risk processes are tested more often, and records must be kept for at least five years.
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