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Gas groups, temperature classes and hazardous area zones are the three concepts buyers most often confuse when specifying an explosion-proof motor. Get one wrong and the motor can be rejected at inspection or, worse, become unsafe in service. This FAQ explains what each term means and how they work together in selection.
1. What Are Hazardous Area Zones?
Zones describe how often an explosive atmosphere is present. Under IEC/ATEX, gas atmospheres are classified:
| Zone | Presence of explosive gas | Typical location | Motor EPL / category |
| Zone 0 | Continuously or for long periods | Inside tanks, process vessels | Ga / Category 1G (rotating machines almost never used) |
| Zone 1 | Likely to occur in normal operation occasionally | Near flanges, valves, pump seals | Gb / Category 2G (Ex d, Ex e…) |
| Zone 2 | Not likely in normal operation; short duration if it occurs | Around well-sealed equipment | Gc / Category 3G (Ex nA…) at lower cost |
Dust atmospheres use the parallel zones 20, 21 and 22. Rule of thumb: Zone 1 requires Gb (Ex d or Ex e); Zone 2 allows Gc (Ex nA). If you see a "Zone 0" motor requirement, double-check — it is usually a misclassified Zone 1.
2. What Do Gas Groups IIA, IIB and IIC Mean?
Gas groups rank gases by how hard they are to ignite — from Group I (mining methane) and IIA (propane-like) through IIB (ethylene-like) to IIC (hydrogen and acetylene), the most demanding. A motor certified IIC is suitable for IIA, IIB and IIC atmospheres; an IIB motor is not suitable for IIC. Selecting a group below what the site needs is a common and dangerous error. Our IIB vs IIC gas group guide gives the full comparison table.
3. What Are Temperature Classes T1–T6?
The temperature class sets the maximum permitted surface temperature of the motor so it cannot ignite the gas:
| Class | Max surface temperature | Typical gases |
| T1 | 450 °C | Hydrogen, ammonia |
| T2 | 300 °C | Ethylene… (most industrial gases) |
| T3 | 200 °C | Gasoline, diesel |
| T4 | 135 °C | Ethyl ether… |
| T5 | 100 °C | — |
| T6 | 85 °C | Carbon disulfide |
Selection rule: the motor's T-rating must stay safely below the gas auto-ignition temperature — commonly a margin of at least 20%. For example, a T4 motor (135 °C) is chosen for an atmosphere with a 170 °C ignition temperature; T3 (200 °C) would be unsafe. Most explosion-proof motors are built to T4 or better, but always verify against the site gas.
4. How Do Zones, Groups and Temperature Classes Work Together?
They are read left to right on the marking. A typical flameproof motor marking Ex db IIB T4 Gb means: Ex db = flameproof protection; IIB = suitable for ethylene-class gases and above; T4 = max surface temperature 135 °C; Gb = EPL for Zone 1. Changing any element changes the motor's suitability — never "upgrade" one parameter without checking the whole specification against the site classification.
5. FAQ
Q: Can one motor cover all zones?A: The EPL determines zone suitability: Gb covers Zone 1 and 2, Gc covers Zone 2 only. A Gb motor is also acceptable in Zone 2, but a Gc motor is not permitted in Zone 1.
Q: Is a higher temperature class always better?A: Not necessarily — T6 limits surface temperature most strictly, which can reduce motor output for the same frame. Choose the class that gives adequate margin without over-restricting duty.
Q: Does the same logic apply in North America?A: No — North America uses Class/Division or Class/Zone with NEMA ratings. Our IEC Zone vs North American Division comparison explains the differences for dual-standard projects.
Unsure About Your Zone, Group or Temperature Class?
Send us the site gas, zone classification and temperature data — our engineers will confirm the correct motor marking and certification. Browse the Wolong explosion-proof motor product range or read What Makes an Explosion-Proof Motor Different for the fundamentals.
