By John Benson, SST Sensing
IIA, IIB, and IIC classify hazardous locations by gas explosiveness: IIA (least volatile, e.g., propane), IIB (moderate, e.g., ethylene), and IIC (most volatile, e.g., hydrogen or acetylene). This determines the protection level required for electrical equipment in hazardous environments.
- IIA: This group includes gases like propane and has a lower explosion risk
- IIB: Includes gases like ethylene with moderate explosion risk.
- IIC: Includes gases like hydrogen and acetylene, with the highest explosion risk.
Key differences
- Minimum Ignition Energy (MIE): The main factor is MIE, which is the energy required to ignite the gas.
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- IIA gases have the highest MIE
- IIB gases have a moderate MIE
- IIC gases have the lowest MIE making them the most easily ignited
Equipment compatibility: Equipment must be rated for the specific gas group present to ensure safety. Equipment approved for a higher gas group (e.g., IIC) is also suitable for lower groups (IIB or IIA). However, equipment rated only for a lower group (e.g., IIA or IIB) must not be used in higher group areas such as IIC, as it may not provide adequate protection.
Safety implications: Equipment must be chosen based on the most hazardous gas present in the area to ensure safety. IIC equipment is more expensive and has tighter manufacturing tolerances due to the high risk involved
A temperature class categorizes equipment for hazardous locations by the maximum surface temperature, using six main classes T1-T6. These classes ensure equipment surfaces remain below any flammable gas or vapor’s auto-ignition temperature. The higher the temperature class number, the lower the maximum surface temperature, increasing safety. Always use equipment with a temperature classification equal to or lower than the area rating.
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Temperature Class
|
Maximum Surface Temperature (°C)
|
| T1 | 450°C |
| T2 | 300°C |
| T3 | 200°C |
| T4 | 135°C |
| T5 | 100°C |
| T6 | 85°C |
