Hazardous area classification: zones, divisions, gas groups and Ex markings
Deskely Completions team Singapore
Hazardous area classification divides a plant into zones by how likely an explosive atmosphere is to be present, so that the electrical and mechanical equipment in each zone can be chosen so it cannot ignite it. It decides what can be installed where, and every item installed there has to be inspected against it.
This guide covers the IEC zone system and the North American class and division system, gas groups and temperature classes, the Ex protection types, how to read an Ex marking, and the inspection records a project keeps.
- Zone 0, 1 and 2 (gas) and Zone 20, 21 and 22 (dust) rank how often an explosive atmosphere is present.
- North America also uses Class I, II, III with Division 1 and 2, now alongside zones.
- Equipment must suit the zone (EPL), the gas group (IIA, IIB, IIC) and the temperature class (T1 to T6).
- Every Ex item is inspected before start-up and recorded against its tag, in an Ex register.
Zones for gas and dust
The IEC system, used internationally and in Europe under ATEX, classifies areas by how often an explosive atmosphere is present. Gas zones follow IEC 60079-10-1 and dust zones IEC 60079-10-2.
| Zone | Explosive atmosphere is present | Typical examples |
|---|---|---|
| Zone 0 | Continuously, for long periods or frequently | Inside tanks and vessels above the liquid |
| Zone 1 | Likely to occur occasionally in normal operation | Around vents, pump seals and sample points |
| Zone 2 | Not likely in normal operation, and only for a short time if it does | Around flanges and valves in process areas |
| Zone 20 | Combustible dust cloud continuously, for long periods or frequently | Inside silos, hoppers and mills |
| Zone 21 | Dust cloud likely occasionally in normal operation | Around filling and emptying points |
| Zone 22 | Dust cloud not likely, and only briefly if it does | Areas near dust-handling equipment |
Areas outside these zones are non-hazardous (safe areas). The extent of each zone around a source of release is set by the area classification study and drawn on the hazardous area classification drawings.
Classes and divisions (North America)
In the US and Canada the traditional system, in NEC Article 500 and the Canadian Electrical Code, uses classes for the type of material and divisions for how likely it is to be present. NEC Articles 505 and 506 also allow the zone system.
| Class and division | Meaning | Closest zones |
|---|---|---|
| Class I, Division 1 | Flammable gas or vapour present in normal operation | Zone 0 and Zone 1 |
| Class I, Division 2 | Flammable gas or vapour present only in abnormal conditions | Zone 2 |
| Class II | Combustible dust, Division 1 or 2 | Zones 20, 21 and 22 |
| Class III | Ignitable fibres and flyings | Dust zones for fibres |
Gas groups
Gases are grouped by how easily their flames pass through gaps and how little energy ignites them. Equipment rated for a more demanding group can be used in a less demanding one: IIC equipment is suitable for IIB and IIA.
| IEC group | Typical gas | NEC group |
|---|---|---|
| IIA | Propane | Group D |
| IIB | Ethylene | Group C |
| IIC | Hydrogen | Group B |
| IIC | Acetylene | Group A |
| I | Methane in underground mines (firedamp) | Mining |
| IIIA, IIIB, IIIC | Combustible flyings, non-conductive dust, conductive dust | Class II and III |
Temperature classes
The temperature class gives the highest surface temperature the equipment can reach. It must stay below the auto-ignition temperature of the gas in the area.
| Temperature class | Maximum surface temperature |
|---|---|
| T1 | 450 °C |
| T2 | 300 °C |
| T3 | 200 °C |
| T4 | 135 °C |
| T5 | 100 °C |
| T6 | 85 °C |
A T4 item can go anywhere a T3, T2 or T1 item can. Hydrogen, for example, needs only T1, while carbon disulphide needs T6.
Equipment protection levels and ATEX categories
| Zone | Equipment protection level (EPL) | ATEX category |
|---|---|---|
| Zone 0 | Ga | 1G |
| Zone 1 | Gb (or Ga) | 2G |
| Zone 2 | Gc (or Gb, Ga) | 3G |
| Zone 20 | Da | 1D |
| Zone 21 | Db (or Da) | 2D |
| Zone 22 | Dc (or Db, Da) | 3D |
In Europe, equipment for hazardous areas is placed on the market under the ATEX equipment directive 2014/34/EU, and workplaces are classified under directive 1999/92/EC. Internationally, the IECEx scheme certifies equipment to the IEC 60079 series.
Ex protection types
| Code | Protection type | How it works |
|---|---|---|
| Ex d | Flameproof enclosure | Contains an internal explosion and stops it spreading outside |
| Ex e | Increased safety | Prevents arcs, sparks and hot surfaces by design |
| Ex i (ia, ib, ic) | Intrinsic safety | Limits the energy in the circuit below what can ignite the gas |
| Ex p | Pressurised enclosure | Keeps the gas out with clean air or inert gas at overpressure |
| Ex m | Encapsulation | Seals the ignition source in compound |
| Ex o | Liquid immersion | Immerses the parts in a protective liquid |
| Ex q | Powder filling | Fills the enclosure with powder, such as quartz sand |
| Ex t | Protection by enclosure (dust) | Keeps dust out and limits surface temperature |
How to read an Ex marking
A typical IECEx or ATEX marking reads Ex db IIB T4 Gb:
- Ex: equipment for explosive atmospheres.
- db: protection type, here flameproof enclosure at level b.
- IIB: gas group, suitable for IIB and IIA gases.
- T4: temperature class, surface temperature not above 135 °C.
- Gb: equipment protection level, suitable for Zone 1 and Zone 2.
ATEX equipment also carries the Ex hexagon with the group and category, such as II 2G, and the certificate number. The installation must match all of these to the zone, gas group and temperature class on the area classification drawing.
Ex inspection and the Ex register
Installed Ex equipment is inspected before it is put into service and then periodically, following IEC 60079-17. The initial inspection is usually a detailed inspection of every item: the right certificate and marking for the location, cable glands and entries, enclosure condition, earthing and, for intrinsically safe circuits, the loop documentation.
Each inspection is recorded against the tag in an Ex register: the list of every item of equipment in a hazardous area, with its location, zone, marking, certificate and inspection history. Faults become punch items, and A items stop the system from being energised. See the ITR guide and the punch list template.
In Deskely, Ex inspections are set up as your own inspection record templates and filled in against each tag like any other ITR, so open Ex findings show on the subsystem and the certificates they hold back.
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Frequently asked questions
What is the difference between Zone 1 and Zone 2?
In Zone 1 an explosive gas atmosphere is likely to occur occasionally in normal operation. In Zone 2 it is not likely in normal operation, and if it occurs it lasts only a short time.
What is Class I Division 1?
The North American classification for areas where flammable gas or vapour is present in normal operation. It corresponds roughly to Zone 0 and Zone 1.
What does T4 mean on Ex equipment?
Temperature class T4: the maximum surface temperature of the equipment does not exceed 135 °C.
Which gas group is hydrogen?
IIC in the IEC system, Group B in the NEC system. Acetylene is also IIC (Group A).
What does Ex d mean?
Flameproof enclosure: the enclosure can contain an explosion inside it and stop it from igniting the atmosphere outside.
What is an Ex register?
The list of all equipment installed in hazardous areas, with location, zone, Ex marking, certificate and inspection history for each item.
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