Every piece of equipment in a classified area is chosen against the same starting document: the area classification. It fixes the zone, the gas or dust group and the temperature class — and those three numbers, not the brochure, decide what may be installed.
Buyers usually arrive with a zone and stop there. But zone alone does not specify equipment: a Zone 1 hydrogen line and a Zone 1 petrol line need very different enclosures. Here is how the three blocks are derived, and what to send a supplier so the quotation comes back correct the first time.
What the area classification document contains
Area classification is a process-safety activity, normally produced by the process engineer with the electrical engineer, following IEC 60079-10-1 for gas and IEC 60079-10-2 for dust. The output is a drawing or schedule that divides the plant into zones and records, for each zone:
- the zone number (0, 1, 2 for gas; 20, 21, 22 for dust);
- the gas group or dust group present;
- the temperature class or the ignition temperature of the worst-case substance;
- often the required equipment protection level (EPL), and sometimes the ambient range.
Everything downstream — luminaires, junction boxes, glands, drives, instruments — is then selected to match that schedule. If you are procuring and you do not have it, ask for it before quoting anything.
Gas zones: 0, 1 and 2
| Zone | Definition | Where it usually appears | Typical EPL |
|---|---|---|---|
| Zone 0 | Explosive atmosphere present continuously, for long periods, or frequently | Inside tanks, inside pipes, inside vapour space above a volatile liquid | Ga |
| Zone 1 | Explosive atmosphere likely to occur during normal operation | Around pump seals, valve stems, sample points, filling nozzles, vent outlets | Ga or Gb |
| Zone 2 | Explosive atmosphere not likely, and if it occurs only briefly | Outside Zone 1 by a defined distance, around flanged joints in well-ventilated areas | Gc (or Ga / Gb) |
The distinction that trips people up is “normal operation”. A pump seal that weeps is normal operation, so the area around it is Zone 1. A flange that only leaks after a gasket failure is abnormal, so the area may be Zone 2.
Dust zones: 20, 21 and 22
Dust classification runs in parallel. The numbering mirrors the gas zones but the physics is different: settled dust layers, not clouds, are usually what ignites — a 5 mm layer on a hot luminaire housing can smoulder and then throw the whole volume into a cloud.
| Zone | Definition | Typical EPL |
|---|---|---|
| Zone 20 | Combustible dust cloud present continuously or frequently | Da |
| Zone 21 | Dust cloud likely during normal operation | Da or Db |
| Zone 22 | Dust cloud not likely, or only briefly | Dc |
Dust also carries its own group letters. IIIA covers combustible flyings, IIIB non-conductive dust, and IIIC conductive dust such as aluminium or carbon black powder. Conductive dust is the strictest, because a layer bridging two live parts can short them.
Many real installations are both. A mill or a bagging plant often has a gas marking and a dust marking on the same nameplate — for instance the BBJ alarm, marked Ex db IIC T6 Gb; Ex tb IIIC T80°C Db. Both must be satisfied.
Gas group: IIA, IIB or IIC
The group is derived from the substance, not from the process. It is based on how readily the gas ignites, measured by the maximum experimental safe gap (MESG) and the minimum igniting current ratio (MIC).
| Group | MESG | Substances that put you in this group |
|---|---|---|
| IIA | > 0.9 mm | Methane, natural gas, propane, butane, ammonia |
| IIB | 0.5 – 0.9 mm | Ethylene, coke oven gas, hydrogen sulphide, diethyl ether |
| IIC | < 0.5 mm | Hydrogen, acetylene, carbon disulphide, hydrogen sulphide mixtures |
Pick the group for the worst substance that can be present, including trace components. A refinery stream that is mostly IIA but carries hydrogen sulphide is an IIC area at the point where that stream can be released.
Temperature class: the part most often under-specified
The equipment’s maximum surface temperature must stay below the ignition temperature of whatever atmosphere is present. IEC 60079 divides the permissible surface temperatures into six classes.
| Class | Max surface temperature | Example substances and their approximate ignition temperature |
|---|---|---|
| T1 | 450 °C | Methane ≈ 580 °C, hydrogen ≈ 560 °C |
| T2 | 300 °C | Ethylene ≈ 425 °C |
| T3 | 200 °C | Petrol / gasoline ≈ 280 °C |
| T4 | 135 °C | Acetaldehyde ≈ 140 °C |
| T5 | 100 °C | Substances igniting between 100 and 135 °C |
| T6 | 85 °C | Substances igniting below 100 °C |
Two consequences follow, and both are frequently missed on site.
The stricter class wins when a substance falls between two limits. A gas igniting at 95 °C needs T6, because T5’s ceiling of 100 °C sits above the ignition temperature. You cannot round to the nearest class.
Dust has no T-class. Dust markings state the surface temperature directly in degrees Celsius, and it is usually far lower than the gas class on the same item — T80°C alongside a gas marking of T6 is normal. Always take ignition temperatures from the current edition of IEC 60079-20-1 rather than a table copied from an old catalogue.
Putting the three blocks together
A worked example. A solvent handling area has a Zone 1 boundary around a pump seal, the solvent is toluene, and the site reaches 45 °C in summer.
- Zone 1 → EPL Gb or better.
- Toluene vapour → group IIA (check the MESG value, but toluene sits in IIA).
- Toluene ignition temperature ≈ 480 °C → T1 would be permissible, but the standard practice is to specify a class with real margin. Most projects write T3 or T4 for solvent areas to cover mixed streams.
- Ambient 45 °C exceeds the default 40 °C → the equipment must be certified for an extended ambient range.
The resulting requirement is something like Ex db IIB T4 Gb, ambient −20 °C to +50 °C. That is the sentence to send a supplier — not “explosion-proof light”.
What to send us (or any supplier)
- Zone (gas and dust, if both apply)
- Gas group and/or dust group
- Temperature class or the ignition temperature of the worst-case substance
- Ambient range and whether the site is indoor, outdoor, coastal or wash-down
- Cable entry thread on the existing installation, if you are replacing equipment
- Mounting and supply details for luminaires: height, beam requirement, voltage and whether emergency operation is needed
With those six lines, a supplier can confirm the marking and quote the correct product. Without them, any quotation you receive is a guess.
Common questions
Can a Zone 0 area use equipment marked Gb?
No. Zone 0 requires EPL Ga, which in practice means an intrinsically safe circuit (Ex ia) or a specially certified enclosure. Zone 0 normally covers the inside of tanks and pipes, and equipment is usually kept out of it altogether rather than made safe inside it.
Does a Zone 2 area need certified equipment at all?
Yes. Zone 2 still requires certified equipment — typically EPL Gc, for example Ex n or Ex ic. What it does not require is Zone 1-grade enclosures, which is where the cost saving lies.
What is the difference between Class I Division 1 and Zone 1?
They come from different systems. Division 1 splits hazardous areas in two (Div 1 and Div 2) under the North American approach, while the IEC and ATEX systems split gas areas into three zones. The mapping between them is approximate, and equipment certified under one system is not automatically accepted under the other. See ATEX vs IECEx vs Class/Division.
Who produces the area classification document?
The plant owner’s process and electrical engineering team, or a specialist consultant. Equipment suppliers work from it; they do not produce it. If your project has no classification drawing, that is the first thing to commission — every downstream procurement decision depends on it.
Send us the classification and we will confirm the marking that fits. Zone, group, temperature class and ambient range — we will check it against our lighting and electrical apparatus ranges.
