Why Zone Classification Is the Foundation of DSEAR Compliance
The Dangerous Substances and Explosive Atmospheres Regulations 2002 (SI 2002/2776) — commonly known as DSEAR — require employers to classify places where explosive atmospheres may occur into hazardous zones. This zone classification is not a bureaucratic exercise. It directly determines which electrical equipment, mechanical equipment, and ignition-source controls are legally required in that area. Get it wrong and you face enforcement under Regulation 18 (improvement and prohibition notices) and potential prosecution under the Health and Safety at Work etc. Act 1974.
This step-by-step guide focuses specifically on storage areas for flammable liquids — the most common scenario in UK industrial and commercial sites.
Step 1: Identify the Dangerous Substances Present
DSEAR applies to any substance defined as a 'dangerous substance' under Regulation 2. For flammable liquids, the key classification criteria under the Classification, Labelling and Packaging (CLP) Regulation (EC) 1272/2008 (retained in UK law) are:
- Flammable liquid Category 1: Flash point below 23°C, initial boiling point at or below 35°C (e.g., diethyl ether)
- Flammable liquid Category 2: Flash point below 23°C, initial boiling point above 35°C (e.g., acetone, petrol)
- Flammable liquid Category 3: Flash point between 23°C and 60°C (e.g., diesel, white spirit)
Flash point is the critical parameter. A liquid stored below its flash point presents significantly lower explosive atmosphere risk. However, storage temperature must account for the full range of ambient conditions on your site including seasonal variation and proximity to heat sources.
Step 2: Determine Source Grade and Release Rate
The zone type is determined by the likelihood and persistence of a flammable atmosphere forming. This is assessed by identifying release sources and grading them:
- Continuous grade source: Atmosphere is present continuously or for long periods (e.g., open surface of a flammable liquid storage tank)
- Primary grade source: Release likely in normal operation (e.g., drum vent, pump seal during use)
- Secondary grade source: Release unlikely in normal operation but possible due to malfunction (e.g., flanged connections, closed containers in good condition)
Step 3: Assign the Correct Zone Classification
Zone classification maps directly to source grade:
| Zone | Definition | Source Grade | Typical Example |
|---|---|---|---|
| Zone 0 | Explosive atmosphere present continuously or for long periods | Continuous | Inside a vented storage tank, inside an IBC |
| Zone 1 | Explosive atmosphere likely during normal operation | Primary | Within 1 m of a drum vent; bunded area around storage tanks |
| Zone 2 | Explosive atmosphere not likely, but possible if abnormal conditions occur | Secondary | Area around a closed-head drum store; general area around Zone 1 perimeter |
| Non-hazardous | Explosive atmosphere not expected | None / negligible | Open storage of diesel at ambient temperature |
Step 4: Determine Zone Extent
Zone extent — the physical boundary of each zone — is calculated using IP Model Code of Safe Practice Part 15 (EI 15) or equivalent guidance such as EN 60079-10-1. For a typical 205-litre drum store with primary grade sources (drum vents, open bungs during use), Zone 1 typically extends:
- 1.5 m horizontally from the source
- Up to 1 m vertically above floor level (vapour density greater than air)
- Within the bunded area boundary where ventilation is restricted
Ventilation is the single most powerful variable. A naturally ventilated open-sided store will have far smaller zone extents than an enclosed room with the same substances. HSE guidance HSG176 provides specific worked examples for petroleum storage.
Step 5: Select Equipment for Each Zone
Once zones are classified, all electrical equipment installed within or near those zones must be certified to the appropriate Equipment Protection Level (EPL) under SI 2016/1107:
- Zone 0: EPL Ga (very high protection — Category 1G equipment)
- Zone 1: EPL Gb (high protection — Category 2G equipment)
- Zone 2: EPL Gc (enhanced protection — Category 3G equipment)
In GB, UKEX marking is required for new equipment placed on the market after 31 December 2024. Equipment installed before this date with valid ATEX marking remains lawful in service.
Step 6: Produce the Zone Drawing and Update Your DSEAR Assessment
DSEAR Regulation 7 requires employers to produce a document — often called a DSEAR assessment or explosion protection document — that includes the zone classification drawings. These must be reviewed whenever the storage arrangements change. Under Schedule 1 of DSEAR, the document must state the hazardous zones, the equipment installed, and the control measures in place to prevent ignition.
Zone drawings must show to scale the extent of each zone and the location of all identified release sources. They should be signed, dated, and reviewed at intervals not exceeding three years, or following any significant change.
Common Mistakes That Lead to Enforcement Action
- Classifying all flammable storage as Zone 2 without a systematic source-grade analysis
- Failing to include the interior of IBCs and storage containers in the zone drawing
- Using non-rated electrical equipment (standard LED fittings, standard sockets) within classified zones
- No ventilation analysis to justify zone extent reduction
- Zone drawings not updated after new substances or containers added
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