Zones, Equipment Categories & Marking
How Zone 0/1/2 and 20/21/22 are defined, what Category and EPL ratings mean, gas and dust groups, temperature classes, and how to read an Ex nameplate.
This series
Once a site has been assessed for explosion risk (Guide 1), that assessment gets expressed as a handful of codes: a zone, a category or EPL, a gas or dust group, a temperature class. Every one of those codes has to line up between the area and the equipment going into it. This guide works through each code in turn, then puts them back together on a real nameplate.
Zone classification
Zones describe how often a hazardous atmosphere is expected to be present, not how dangerous it would be if it ignited. Gas, vapour and mist atmospheres use Zones 0, 1, 2; combustible dust atmospheres use the equivalent Zones 20, 21, 22.
Continuous or long-period
A hazardous atmosphere is present continuously, or for long periods, or frequently - e.g. inside a fixed-roof tank.
Likely in normal operation
Likely to occur occasionally during normal operation - e.g. around a loading connection or vent.
Not likely, brief if it occurs
Not expected in normal operation and, if it does occur, only briefly - e.g. the general area around a Zone 1 source.
Continuous or long-period
A dust cloud in air is present continuously, for long periods, or frequently - e.g. inside a powder hopper.
Likely in normal operation
Likely to occur occasionally in normal operation - e.g. near a filling or discharge point.
Not likely, brief if it occurs
Not expected in normal operation and, if it does occur, only briefly - e.g. around a well-sealed process line.
The exact zone boundaries on your site are set by a hazardous-area classification study (per IEC 60079-10-1 for gas, IEC 60079-10-2 for dust), not by this page - treat the definitions above as a shortlisting starting point.
Equipment categories & EPL
Equipment is rated for the zone it's allowed into. Europe's ATEX directive uses Categories 1/2/3; the international IEC 60079-0 standard uses the equivalent Equipment Protection Level (EPL) system. Both appear side by side on most certificates sold into the GCC.
| Zone | ATEX category | EPL (gas) | EPL (dust) | Level of protection |
|---|---|---|---|---|
| 0 / 20 | 1 (G/D) | Ga | Da | Very high |
| 1 / 21 | 2 (G/D) | Gb | Db | High |
| 2 / 22 | 3 (G/D) | Gc | Dc | Enhanced |
A key rule of thumb: equipment rated for a higher-safety category or EPL is always acceptable in a lower-risk zone (Category 1 / Ga equipment can go in Zone 2), but never the other way round.
Gas & dust groups
Within Group II (surface industries, as opposed to Group I for mining), gases and dusts are further split by how easily they ignite and how energetically they burn - this sets how robust a given protection method has to be.
| Group | Applies to | Typical representative |
|---|---|---|
| IIA | Gas, lower risk | Propane |
| IIB | Gas, medium risk | Ethylene |
| IIC | Gas, highest risk | Hydrogen, acetylene |
| IIIA | Combustible flyings | Textile or wood flyings |
| IIIB | Non-conductive dust | Flour, sugar, most organic dusts |
| IIIC | Conductive dust | Metal powders |
Group IIC equipment is automatically suitable for IIB and IIA atmospheres - the grouping is cumulative downward, same logic as category and EPL.
Temperature classes
The maximum surface temperature a piece of equipment can reach must stay below the ignition temperature of the gas it might meet. Six temperature classes cover the range.
| Class | Max. surface temp. | Covers gases with ignition temp. |
|---|---|---|
| T1 | 450°C | Above 450°C |
| T2 | 300°C | Above 300°C |
| T3 | 200°C | Above 200°C |
| T4 | 135°C | Above 135°C |
| T5 | 100°C | Above 100°C |
| T6 | 85°C | Above 85°C |
A T6-rated fixture is the most restrictive and also satisfies T1-T5 requirements - equipment rated for a lower number always covers every class above it. Combustible dust isn't split into temperature classes the same way: instead, both the dust cloud's ignition temperature and the dust layer's much lower glow temperature have to be checked against the equipment's actual surface temperature.
Reading an Ex nameplate
Every code above shows up on the equipment's own marking. Take a typical ATEX/IECEx marking for a piece of Zone 1 flameproof equipment:
Read left to right, that's: Group II equipment, Category 2 for a gas atmosphere, explosion-protected, flameproof-enclosure protection, rated for the most demanding gas group (IIC), a 135°C maximum surface temperature, high-level protection suitable for Zone 1. The certificate number, test body and specific standards edition sit alongside this string on the nameplate and should always be checked against the equipment in hand, not just the paperwork.
North America uses a parallel but different system (NEC/CEC Class/Division or Class/Zone, with its own temperature-code letters) - covered in Guide 3 alongside how ATEX and IECEx certificates are typically referenced on UAE/GCC projects.