1. The Complete Structure of an Explosion-Proof Marking (It Is Not Just d and T4)
A complete explosion-proof marking for a gas atmosphere:Ex d ⅡB T4 Gb— Ex (explosion protection) + d (flameproof enclosure) + IIB (gas group) + T4 (temperature class) + Gb (Equipment Protection Level, EPL). For dust atmospheres, the marking takes the form Ex t IIIC T80°C Db. When selectingevery segment must match the site conditions, and if any segment falls below the requirement, the unit cannot be used.
2. Gas Group: IIA/IIB/IIC (Dual Criteria: MESG + MIC)
Gas groups are classified by flame propagation capability, using two criteria:Maximum Experimental Safe Gap (MESG)andMinimum Ignition Current ratio (MIC)Dual criteria:
| Group | MESG | Typical gases | Typical applications |
|---|---|---|---|
| IIA | ≥ 0.9mm | Propane, methane, ammonia, acetone | Refining, wastewater treatment |
| IIB | 0.5–0.9mm | Ethylene, propylene, city gas | Petrochemical cracking, coal gas |
| IIC | ≤ 0.5mm | Hydrogen, acetylene, carbon disulfide | Hydrogen production, electrolysis, acetylene plants |
Downward compatible: IIC equipment can be used in IIB/IIA gas atmospheres, but not the reverse. Acetylene atmospheres require IIC.
3. Temperature Class: T1–T6 (Maximum Surface Temperature)
| Group | Maximum surface temperature | Typical gas (ignition temperature) |
|---|---|---|
| T1 | ≤ 450°C | Methane (537°C), hydrogen (560°C) |
| T2 | ≤ 300°C | Ethylene, acetylene (305°C) |
| T3 | ≤ 200°C | Gasoline, hydrogen sulfide |
| T4 | ≤ 135°C | Acetaldehyde, diethyl ether |
| T5 | ≤ 100°C | Carbon disulfide (≈100°C) |
| T6 | ≤ 85°C | Ethyl nitrate |
4. EPL — Equipment Protection Level (Ga/Gb/Gc correspond to zones)
Ga(very high protection, e.g. ia/ma) → Zone 0;Gb(high protection, e.g. d/e/p/mb) → Zone 1;Gc(normal protection, e.g. n) → Zone 2. For dust atmospheres the equivalent is Da/Db/Dc → Zone 20/21/22. When selecting, the zone sets the level, the gas sets the group, and the ignition temperature sets the T class.
5. Common scenarios — quick reference (match your site)
| Application | Atmosphere gas | Recommended marking |
|---|---|---|
| Gas station (submerged pump area) | Gasoline vapor (T3) | Ex d ⅡA T3 Gb |
| Hydrogen generation plant | Hydrogen (T1, IIC) | Ex d ⅡC T1 Gb |
| City gas pressure regulating station | Natural gas (T1, IIA) | Ex d ⅡA T1 Gb |
| Acetylene cutting shop | Acetylene (T2, IIC) | Ex d ⅡC T2 Gb |
| Baijiu distillery | Ethanol (T2, IIA) | Ex d ⅡA T2 Gb |
FAQ FAQ
How do I find the gas group and ignition temperature for my site?
Check the GB 3836.1 annexes and standard flammable gas property tables, or have the design institute / owner specify it on the hazardous area classification drawing.
Is there a problem if I select a higher rating than needed?
No safety issue — it is justmore expensive and heavier(IIC enclosures cost considerably more). Select for the actual gas; when in doubt, go higher rather than lower.
Can T4 equipment be used in a gasoline atmosphere?
Yes. Gasoline is a T3 atmosphere (ignition temperature on the order of 280°C). T4 equipment has a lower maximum surface temperature (stricter), so it is downward-compatible with T3; the reverse does not hold — T3 equipment cannot be used in a T4 gas atmosphere.
How is the rating marked for dust atmospheres?
Use Ex t + IIIA/IIIB/IIIC + the specific temperature (e.g. T80°C Db); the T1–T6 gas groups do not apply.
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