Gas Group IIC — Hydrogen and Acetylene Equipment Guide
Gas group IIC covers the most ignition-sensitive flammable gases — hydrogen, acetylene, and carbon disulphide. These gases have the smallest MESG values (below 0.5mm) and the lowest MIC ratios, meaning they can ignite through very small gaps with very small electrical sparks. IIC areas require the most stringent Ex equipment and the most careful installation and inspection practices.
IIC Properties — Why It Matters
| Property | Hydrogen | Propane (IIA — comparison) |
|---|---|---|
| MESG | 0.29mm | 0.92mm |
| MIC Ratio | 0.23 | 1.0 (reference) |
| Flammability range in air | 4–75% | 2.1–9.5% |
| Minimum ignition energy | 0.017 mJ | 0.25 mJ |
In IIC areas, every single electrical component must be IIC rated — glands, stopping plugs, enclosures, instruments, luminaires, junction boxes. A single IIB component in an IIC area is a Category X deficiency. There are no exceptions. This is the most commonly tested point in CompEx Ex04 assessments.
IIC in Practice
IIC areas are found at hydrogen production and storage facilities, LNG terminals, coke oven plants, battery manufacturing (hydrogen evolved during charging), acetylene filling plants, and any facility where hydrogen or acetylene is processed, stored, or handled.
IIC Equipment Requirements
For Ex d equipment in IIC areas, the flamepath gap limits are the tightest of any gas group. IEC 60079-1 Table 1 specifies maximum gaps as low as 0.10mm for large volume IIC enclosures. This requires precision machining and careful assembly — which is why IIC Ex d equipment is more expensive and why flamepath inspection is critical.
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