CompEx Ex04 is the unit covering inspection of electrical equipment in explosive atmospheres for gas group IIC — which includes hydrogen, acetylene, and carbon disulphide. These are the most ignition-sensitive gases and require the most stringent Ex equipment. Ex04 builds on Ex03 and is held by electricians working on hydrogen production facilities, LNG terminals, coke oven plants, and any IIC-designated area.
What Makes IIC Different?
Gas group classification determines how easily a gas can ignite through a gap in an enclosure — measured by the Maximum Experimental Safe Gap (MESG). IIC gases have the smallest MESG values, meaning they can propagate through very small gaps. This has direct implications for equipment design and inspection:
| Gas Group | MESG | Typical Gases | Flamepath Limits (Ex d) |
|---|---|---|---|
| IIA | >0.9mm | Propane, methane, butane | Widest gaps permitted |
| IIB | 0.5–0.9mm | Ethylene, hydrogen sulphide | Intermediate |
| IIC | <0.5mm | Hydrogen, acetylene | Tightest gaps — most critical |
In IIC areas, every single component — glands, stopping plugs, enclosures, instruments, luminaires — must be certified and rated for IIC. An IIB rated component in an IIC area is a Category X deficiency. This is one of the most commonly tested points in the Ex04 assessment.
Additional Knowledge Required for Ex04
Ex04 requires all the knowledge of Ex03 plus specific understanding of:
- IIC gas group characteristics — MESG values below 0.5mm
- Ex d flamepath requirements for IIC — tighter gap limits per IEC 60079-1 Table 1
- IIC equipment certification — every component must be IIC rated
- Hydrogen-specific hazards — very wide flammability range (4–75% in air), low ignition energy
- Inspection of IIC barrier glands — barrier glands required for SWA in enclosures >2 litres
- ATEX/IECEx certification marking for IIC equipment
IIC Areas in Practice
IIC-designated areas are found at:
- Hydrogen production and storage facilities — electrolysers, compressors, storage tanks
- LNG terminals — boil-off gas handling, liquefaction units
- Coke oven plants — coke oven gas contains hydrogen and acetylene
- Battery manufacturing — hydrogen evolved during charging
- Chemical processing — various processes producing hydrogen as a by-product
- Acetylene production and storage
Practising Ex04 Inspection Scenarios
EX Academy's inspection simulator includes IIC scenarios — including Simulation 08 (Ex d LCS in IIC Zone 1) and Simulation 02 (Ex e junction box in IIC Zone 1). These require you to apply the additional IIC requirements, identify IIB components in IIC areas as Category X deficiencies, and correctly specify IIC-rated replacements.