Introduction
Electrical area classification (EAC) is the process of defining zones around potential sources of flammable gas or vapour, and specifying what electrical and instrumentation equipment may be installed within those zones. It is a fundamental safety discipline for any oil and gas facility — onshore or offshore — and a regulatory requirement under ATEX (Europe), IECEx (international), and equivalent national standards.
The goal is straightforward: prevent the ignition of flammable atmospheres by controlling the sources of ignition that electrical equipment can present. But the discipline requires engineering judgement as well as standards compliance. A classification that is too conservative covers large areas in Zone 1 classification, forces the use of expensive Ex-rated equipment everywhere, and adds weight and cost without improving safety. A classification that is too liberal leaves ignition sources in areas where flammable gases can accumulate.
This article explains the zone definitions, the factors that determine zone boundaries, and the practical decisions engineers make when producing a hazardous area classification drawing.
The Zone Definitions
IEC 60079-10-1 (for gases and vapours) defines three zones based on the likelihood and duration of a flammable atmosphere being present:
Zone 0 — A place where a flammable gas/air mixture is continuously present, or present for long periods. In practice this is inside vessels, tanks, pipes and sumps that contain flammable liquid or gas. Zone 0 is almost always confined to the interior of process equipment. Equipment certified for Zone 0 (Category 1G) is the most stringent and expensive — in practice, most instrumentation within Zone 0 is either pneumatic, intrinsically safe to Category 1G, or eliminated entirely.
Zone 1 — A place where a flammable gas/air mixture is likely to occur in normal operation. This covers the immediate vicinity of potential release sources: flanged connections, valve glands, pump seals, instrument connections, open drains, and vent outlets. Zone 1 is the zone that most EAC studies spend the majority of time defining and justifying.
Zone 2 — A place where a flammable gas/air mixture is not likely to occur in normal operation but may occur in abnormal conditions, and if it does, will persist only for a short time. Zone 2 surrounds Zone 1 areas and represents a secondary release scenario — a seal failure, a flange joint leaking under cyclic loading, a drain valve left open.
Outside the classified zones, the area is unclassified (safe area) — standard electrical equipment can be used without special precautions.
What Creates a Zone Boundary
Zone boundaries are defined by two factors: the grade of release from the source, and the ventilation available to dilute and disperse the release.
Grade of Release
IEC 60079-10-1 defines three grades:
| Grade | Description | Resulting zone |
|---|---|---|
| Continuous | Release is continuous or expected for long periods | Zone 0 |
| Primary | Release is likely in normal operation | Zone 1 |
| Secondary | Release is not expected in normal operation; possible in abnormal conditions | Zone 2 |
A flanged pipe joint in hydrocarbon service is a secondary grade release source — it should not leak under normal operation, but may do so under cycling, corrosion, or mechanical damage. A pump seal on a light hydrocarbon service is a primary grade release source — small continuous leakage is expected and designed for. An open sump or drain pit where liquid can accumulate and evaporate is a continuous grade source.
Ventilation
Ventilation is the dominant factor in determining zone extent. The IECEx standard introduces a ventilation effectiveness parameter — availability (degree of dilution) and availability of ventilation — that modifies the zone type:
- Good ventilation (outdoor, natural air movement): dilutes releases quickly. Zones are smaller in extent and may be downgraded — a primary release in good outdoor ventilation may produce only a Zone 2 rather than Zone 1.
- Fair ventilation (partially enclosed, mechanical ventilation): moderate dilution. Zones are standard extents per the standard nomographs.
- Poor ventilation (enclosed spaces, inadequate air changes): releases persist. Zones are larger and may be upgraded — a secondary release in a poorly ventilated room may produce a Zone 1 rather than Zone 2.
For offshore topsides, where equipment decks are open to the prevailing wind, ventilation is generally classified as good, which allows smaller zone radii. For enclosed modules — gas compressor rooms, pump rooms — ventilation is often the dominant engineering challenge, and the area classification study must be accompanied by a ventilation design confirmation.
Zone Extents: Typical Radii
The standard does not mandate fixed zone radii — it requires engineering calculation based on release rate, gas density, ventilation, and source geometry. However, IP Model Code of Safe Practice (IP15) and similar guides provide typical zone radii as a starting point:
| Source | Zone 1 extent | Zone 2 extent |
|---|---|---|
| Flanged joint (secondary) | — | 1.5 m radius |
| Instrument connection / vent | 0.5 m | 3 m |
| Pump seal (primary, light HC) | 1.5 m | 3 m |
| Compressor seal (primary) | 3 m | 5 m |
| Open drain / sump | 1.5 m (Zone 0 in pit) | 3 m |
| Relief valve outlet (to atmosphere) | 3 m | 5 m |
These are starting points only. The engineering judgement lies in combining overlapping zones from multiple sources, handling grade transitions at building walls, and making decisions about areas where zone extents from different sources overlap.
The Area Classification Drawing
The deliverable from an EAC study is a set of hazardous area drawings — plan and elevation views of every area of the facility, showing zone boundaries as lines or volumes. Each zone is colour-coded or hatched to the zone type.
The drawing package should be accompanied by a hazardous area classification document that records, for each source:
- Tag number and description
- Grade of release and justification
- Gas group (IIA, IIB, IIC — based on the most hazardous material present)
- Temperature class (T1–T6 — autoignition temperature of the material)
- Zone type and extent
- Reference standard and any derogations
The gas group and temperature class are used to select electrical equipment. IIC is the most stringent group (covers hydrogen and acetylene). T6 is the lowest surface temperature class (85°C maximum). A facility handling hydrogen would require IIC T1 (or T2) rated equipment in classified areas — significantly more expensive and limiting than IIA T3, which covers most petroleum facilities.
Equipment Selection in Classified Zones
Once the zone drawing is complete, every piece of electrical and instrumentation equipment installed within a classified zone must be certified to the appropriate protection concept. Common concepts:
| Protection concept | Code | Suitable zones | Typical application |
|---|---|---|---|
| Intrinsic safety | Ex ia | Zone 0, 1, 2 | Transmitters, sensors, analysers |
| Flameproof enclosure | Ex d | Zone 1, 2 | Junction boxes, motors, actuators |
| Increased safety | Ex e | Zone 1, 2 | Terminal boxes, luminaires |
| Purged/pressurised | Ex p | Zone 1, 2 | Control panels, analysers |
| Non-sparking | Ex n | Zone 2 only | General equipment in Zone 2 |
The equipment selection is documented in an Ex equipment register — a database listing every item of electrical and instrument equipment in a classified area, its protection concept, equipment group, temperature class, and certificate number.
Common Pitfalls
Over-classifying enclosed areas. A common conservative assumption is that any room containing hydrocarbon equipment is Zone 1 throughout. This is rarely justified if the room has adequate ventilation. Classify by source, not by room.
Not considering gas density. Heavy gases (LPG, refrigerants) sink to floor level — the zone extent at low level may be larger than at instrument elevation. Light gases (hydrogen, methane at elevated temperature) rise. The geometry of the zone must reflect the gas behaviour.
Forgetting to update the drawings after modifications. Process modifications that add or change release sources must trigger a review of the area classification drawing. This is a Management of Change (MoC) requirement, but it is frequently overlooked when the modification is considered minor.
Inconsistency between the EAC drawing and the equipment schedule. Every item of Ex equipment in the facility must appear in the Ex register with its certification traceable to a current certificate. Uncertified or improperly certified equipment in a classified zone is a regulatory non-compliance and an insurance issue.
Conclusion
Electrical area classification is an engineering discipline, not a paperwork exercise. The zone boundaries you draw determine the cost, weight, and operational flexibility of every electrical and instrumentation installation in your facility for its entire life. The standard gives you the methodology; engineering judgement and a thorough knowledge of your release sources and ventilation conditions determines whether the result is genuinely fit for purpose. A good EAC study is reviewed by process, EC&I, and safety disciplines together — because the inputs come from all three.