Area classification zones are the graded categories that describe how likely a flammable atmosphere is to be present at a given location. They turn a fuzzy question - how dangerous is this spot - into a precise label that dictates which equipment may be installed. Understanding the zones is the foundation for selecting compliant instruments and electronics. This guide explains what the zones mean and where they fit in oil and gas.
Area Classification Zones in one line: Area classification zones grade a location by how often a flammable atmosphere is present. For gases and vapors: Zone 0 means an explosive atmosphere is present continuously or for long periods, Zone 1 means it is likely in normal operation, and Zone 2 means it is unlikely and only briefly. For combustible dusts, the equivalents are Zones 20, 21, and 22. The zone dictates the equipment protection required.
The zone system, defined in the IEC 60079 series and used internationally under ATEX and IECEx, classifies gas and vapor hazards by the persistence of an explosive atmosphere. Zone 0 is the most severe: a flammable atmosphere is present continuously, for long periods, or frequently, such as the vapor space inside a fuel tank. Zone 1 covers areas where a flammable atmosphere is likely to occur in normal operation, for example around vents, sample points, or frequently opened connections.
Zone 2 is the least severe: a flammable atmosphere is not likely in normal operation and, if it occurs, persists only briefly, typically as a result of an abnormal event like a leak. Much of the general process area of a well-ventilated facility falls into Zone 2. The grading is deliberately conservative because the whole point is to keep ignition sources out proportionate to the real likelihood of a flammable cloud.
For combustible dusts, a parallel set of zones applies: Zone 20 (dust cloud present continuously or frequently), Zone 21 (likely in normal operation), and Zone 22 (unlikely and short-lived). The logic mirrors the gas zones, and the numbers 20, 21, 22 intentionally rhyme with 0, 1, 2 to make the correspondence easy to remember.
The zone alone does not fully specify equipment; it must be paired with the gas group and temperature class. Gas groups IIA, IIB, and IIC rank how easily a specific gas ignites, with hydrogen and acetylene in the demanding IIC group. The temperature class, T1 through T6, caps the maximum surface temperature equipment may reach so a hot surface cannot ignite the atmosphere. Certified equipment must match all three: zone, gas group, and temperature class.
North American practice historically uses a different vocabulary, the Class and Division system from the National Electrical Code, where Class I Division 1 roughly corresponds to Zones 0 and 1, and Class I Division 2 corresponds to Zone 2. Modern NEC articles now permit the zone system as well, and internationally the zones dominate. The two systems describe the same physical reality with different labels, and a site should not mix them within one area.
In oil and gas, the zone assigned to a location drives every downstream decision about field equipment. It determines the protection concept allowed - intrinsic safety, flameproof, increased safety, or pressurization - and, combined with gas group and temperature class, the exact certification a transmitter, junction box, or motor must carry. Misjudging a zone, or installing equipment rated for a less severe zone than the actual conditions, defeats the entire protection strategy.
Zone assignment is an engineering exercise, not a guess. Standards such as IEC 60079-10-1 for gases and 60079-10-2 for dusts, along with industry guidance like API RP 505 and the NFPA 497 and 499 recommended practices, provide the methodology. Engineers consider the source of release, its grade (continuous, primary, or secondary), the ventilation available, and the properties of the material to determine both the zone type and its physical extent.
The result is documented on hazardous-area classification drawings that map each zone's boundaries across the facility. Those drawings become the authoritative reference for equipment selection, and they must be revisited whenever the process, layout, or ventilation changes, because a modification that adds a new release source can enlarge a zone and invalidate equipment that was previously compliant.
They grade how often a flammable gas atmosphere is present. Zone 0 means it is present continuously, for long periods, or frequently, such as inside a tank vapor space. Zone 1 means it is likely during normal operation, for example near vents or sample points. Zone 2 means it is unlikely in normal operation and only present briefly if it occurs, typically from an abnormal leak. More severe zones demand more protective equipment.
The North American Division system and the international Zone system describe the same hazards differently. Roughly, Class I Division 1 corresponds to Zones 0 and 1 combined, and Class I Division 2 corresponds to Zone 2. Modern NEC articles now also allow the zone system directly. The correspondence is approximate, and a facility should not mix the two systems within a single classified area to avoid ambiguity.
No. The zone tells you the likelihood of a flammable atmosphere and therefore the required protection concept, but you also need the gas group (IIA, IIB, or IIC) and temperature class (T1 to T6). Certified equipment must match all three. A device rated for the correct zone but the wrong gas group or with too high a surface temperature is still non-compliant and unsafe for that location.
This page references the vendor products and their official documentation published by the organizations below. Editions, product capabilities, and documentation change over time - confirm current requirements and specifications directly with the source.
Last reviewed: July 27, 2026. Merobix is not affiliated with, endorsed by, or sponsored by these organizations; their names are used only to identify the standards and products discussed.
Merobix reads your field devices into a cloud SCADA - the real thing behind these terms, live in days from any browser.