By Wu Peng, Senior Process Instrumentation Engineer · Last reviewed July 17, 2026
The Division system and the Zone system are the two ways the world classifies hazardous areas where flammable gas or dust can ignite. The Division system (NEC Article 500, North America) splits a hazardous area in two: Division 1 and Division 2. The Zone system (IEC 60079-10, used by most of the world and available in the US through NEC Article 505) splits the same reality in three: Zone 0, Zone 1, and Zone 2. Zones 0 and 1 together cover what Division 1 covers alone.
If your projects cross borders, you will meet both. A skid built for a Texas refinery arrives with CL I DIV 2 markings; the same instruments quoted for a plant in Malaysia need Ex markings with gas group IIC or IIB. The two languages describe the same physics, but the groups run backwards from each other and the certificates do not automatically transfer.
This guide gives the Zone definitions, the Zone to Division mapping, a gas group conversion table, the protection level ladder, and a side by side decode of both marking styles. For the North American side in depth, the companion guide on Class 1 Div 1 vs Div 2 covers divisions, gas groups A to D, and T-codes.
Contents
- Two classification systems
- The Zone system
- Zone to Division mapping
- Gas groups compared
- Protection levels
- Reading the markings
- Which system applies
- Instruments and certification
- FAQ
Two classification systems
Both systems answer the same question: how likely is ignitable material to be in the air at this spot, and how bad would ignition be. They just slice the answer differently.
The Division system classifies by Class (I gas, II dust, III fibers), Division (1 or 2), and Group (A to G). It is the traditional method of the NEC and the Canadian Electrical Code, and most existing North American plants are drawn this way.
The Zone system classifies by Zone number alone for likelihood (0, 1, 2 for gas; 20, 21, 22 for dust) and by equipment group for the material (IIA, IIB, IIC for gas; IIIA, IIIB, IIIC for dust). It comes from IEC 60079 and underpins ATEX in Europe and IECEx internationally. The NEC adopted it as an alternative in Article 505 (gas) and Article 506 (dust), and in Canada all new installations have been classified by Zones since 1998.
The Zone system
Zones grade an area by how often ignitable concentrations are present, with a finer split at the dangerous end than divisions have.
| Zone | Ignitable atmosphere is | Typical example |
|---|---|---|
| Zone 0 | Present continuously or for long periods | Vapor space inside a solvent tank |
| Zone 1 | Likely during normal operation | Around a vent or a sampling point |
| Zone 2 | Not likely in normal operation; short duration if it happens | Ventilated area with all-welded piping |
| Zone 20 / 21 / 22 | Same three probability steps, for combustible dust | Inside a silo / at a filling point / dust-settling areas |
A widely used engineering guide puts numbers on the steps: Zone 0 more than 1,000 hours per year of ignitable atmosphere, Zone 1 between 10 and 1,000 hours, Zone 2 under 10 hours. Treat those as guidance values for classification studies, not clauses you will find printed in the code itself.
Zone to Division mapping
Class I Division 1 is generally equivalent to Zones 0 and 1 combined, and Class I Division 2 to Zone 2. That single sentence answers most cross-reference questions, and the diagram shows why.
| Zone | Guidance presence | Division equivalent |
|---|---|---|
| Zone 0 | Over 1,000 h/yr, continuous | No direct equivalent; falls inside Division 1 |
| Zone 1 | 10 to 1,000 h/yr | Division 1 |
| Zone 2 | Under 10 h/yr, short duration | Division 2 |
The practical consequence sits at the dangerous end. Because Division 1 lumps the continuous case together with the intermittent case, equipment certified for Division 1 must handle the worst of both. The Zone system separates them, so Zone 1 equipment can be built (and priced) to a lighter standard than Zone 0 equipment. That finer resolution is a main reason new international projects classify by Zones.
Gas groups compared
Here is where the two systems trip people: the letters run in opposite directions. In the Division system, Group A is the most dangerous gas (acetylene). In the Zone system, IIC is the most dangerous group and IIA the mildest. The gases anchor the conversion:
| Representative gas | Division group | Zone group |
|---|---|---|
| Acetylene | A | IIC |
| Hydrogen | B | |
| Ethylene | C | IIB |
| Propane, methane, gasoline vapors | D | IIA |
So a Division Group D area corresponds to Zone group IIA, and Division Groups A and B together correspond to IIC. Equipment rated IIC covers IIB and IIA beneath it. On the dust side the parallel is looser: Division Group E (metal dusts) lines up with conductive-dust group IIIC, Groups F and G (carbonaceous dusts, grain, flour, plastics) broadly with non-conductive group IIIB, and Class III fibers and flyings with IIIA. Temperature codes T1 through T6 are the one element both systems share unchanged; the T-code table is in the Division guide.
Protection levels
The Zone system adds one more layer the Division system never had: a formal equipment protection level (EPL) printed on the nameplate. IEC 60079-0 defines Ga, Gb, and Gc for gas (Da, Db, Dc for dust), and each level pairs with a zone. ATEX expresses the same ladder as equipment categories.
| Zone | EPL (IEC 60079-0) | ATEX category | Intrinsic safety level |
|---|---|---|---|
| Zone 0 | Ga (very high protection) | 1G | Ex ia |
| Zone 1 | Gb (high protection) | 2G | Ex ib |
| Zone 2 | Gc (enhanced protection) | 3G | Ex ic |
The rule of use is the same as for divisions: equipment may always go into a less hazardous zone than its level allows, never the other way. A Ga device is welcome in Zone 2; a Gc device in Zone 1 is a violation. The protection concepts themselves also carry zone limits: flameproof Ex d and increased safety Ex e are Zone 1 methods, Ex n is a Zone 2 method, and energy-limited intrinsic safety splits into ia, ib, and ic as the table shows. The energy-limiting idea behind intrinsic safety is covered in our 4-20 mA current loop guide.
Reading the markings
Set the two nameplate styles side by side and the systems stop looking foreign. Take a flameproof transmitter housing certified both ways:
CL I, DIV 1, GP C, D, T6 versus Ex d IIC T6 Gb
- CL I, DIV 1 states where the device may live: a flammable-gas area, gas present in normal operation. Ex d states how the device protects: a flameproof enclosure. The Zone marking states the where too, in the EPL letter at the end.
- GP C, D and IIC both name the gas group, in opposite alphabets. IIC is the hardest group; GP C, D covers the milder half.
- T6 means the same 85 °C surface limit in both systems.
- Gb is the EPL: fit for Zone 1 and Zone 2. The Division marking has no equivalent field; the division itself carries that meaning.
US installations classified under Article 505 use a hybrid marking with an AEx prefix, for example Class I Zone 2 AEx nA IIC T5: NEC zone classification, IEC protection concept, US certification. If you meet a plate like that, read it left to right as location, concept, group, T-code.
Which system applies
Geography decides most of it. In the United States both systems are legal; an area is classified under Article 500 (divisions) or Article 505 (zones), and equipment must match the system on the classification drawing. Most existing US plants stay with divisions, while new chemical and petrochemical projects increasingly use zones. In Canada, new installations have been zone-classified since 1998; existing division-classified plants may remain. Europe requires ATEX. International projects generally specify IECEx certificates alongside the IEC zone classification.
Two practical rules keep projects out of trouble. First, do not mix systems within one area: a single space is classified once, under one system, on one drawing. Second, certificates do not transfer automatically. The same physical transmitter may hold a UL listing for Division service, an ATEX certificate for Europe, and an IECEx certificate for a project in the Gulf; each is a separate piece of paper, and the one your authority accepts is the one that counts.
Instruments and certification
For instrument selection the workflow is identical under both systems: read the classification drawing, match the protection concept and group, check the T-code, then verify the certificate is one your site accepts. On the product side, our explosion-proof pressure transmitter carries an Ex d IIC T6 flameproof housing, the YX18-F explosion-proof pressure switch covers flameproof setpoint service, and the ultrasonic level switch is rated Ex d IIC T6 Gb, the exact marking decoded above. Non-contact level in classified tanks is covered by the explosion-proof ultrasonic level transmitter, and the wider range of Ex d and Ex ia builds sits under pressure instruments. State the required certification scheme (ATEX, IECEx, or North American) in the inquiry so the quoted build carries paper your authority accepts.
Application example
Process plant instrumentation package. An inquiry for a multi-instrument package specified the area up front: Zone 1, gas group IIB, with pressure transmitters required in flameproof Ex d execution, 0 to 50 bar g, IP65, alongside differential pressure transmitters for the same unit. Because the classification arrived complete (zone, group, protection concept), matching the build was a selection exercise rather than a guessing game: Ex d rated transmitters covering IIB, quoted against the stated process connections. That is the shape of a good hazardous-area inquiry, and the reason we ask for the classification line before anything else.
The IP code on the same nameplate is a separate system again: it grades dust and water ingress, not explosion protection. Our guide to IP ratings decodes those two digits.
FAQ
What are zone 1 and zone 2 hazardous areas?
Zone 1 is an area where ignitable gas or vapor is likely to be present during normal operation, such as around vents or sampling points. Zone 2 is an area where it is not likely and would persist only briefly after an upset, with common guidance putting it under 10 hours per year. Zone 1 equipment (EPL Gb) may be used in Zone 2, but Zone 2 equipment (Gc) may not be used in Zone 1.
What is zone 20 21 and 22 hazardous area classification?
Zones 20, 21, and 22 apply the same three probability steps to combustible dust instead of gas: dust cloud present continuously (inside a silo), likely in normal operation (at a filling point), or only abnormally (where settled dust could be disturbed). Equipment for these zones carries dust EPLs Da, Db, and Dc.
What is gas group IIA, IIB, IIC?
They are the Zone-system gas groups, ranked by how easily an explosion transmits and ignites. IIA covers propane-class gases, IIB ethylene-class, and IIC the hardest gases, hydrogen and acetylene. Equipment certified IIC may be used in IIB and IIA atmospheres. In Division terms, IIA corresponds to Group D, IIB to Group C, and IIC to Groups A and B.
What is class 1 div 2 group c and d?
It is a Division-system rating: Class I (flammable gas), Division 2 (gas present only under abnormal conditions), Groups C and D (ethylene-class and propane-class gases). In Zone language the nearest equivalent service is Zone 2 with gas groups IIB and IIA. The full Division-side definitions are in our Class 1 Div 1 vs Div 2 guide.
Request a quote
Send the classification line from your drawing, Zone or Division, gas group, T-code, plus the measurement itself: medium, range, process connection, output, and the certification scheme your site accepts. We will match a flameproof or intrinsically safe build and quote it. Reach our application engineers or use the form below.
Written and technically reviewed by Wu Peng and the Instranova engineering team.