Gas Groups Explained: IIA vs IIB vs IIC

Gas groups such as IIA, IIB, and IIC are identified in hazardous area equipment markings and determine where certified equipment can be safely installed.

When selecting equipment for hazardous areas, area classification is only part of the process.

This is where gas groups become important.

Gas groups classify flammable gases according to their ignition characteristics. The gas group assigned to an area directly affects which equipment can be safely installed and what certification markings are required.

What Are Gas Groups?

Not all flammable gases behave the same way.

Some gases ignite more easily than others, while some produce more severe explosion pressures.

To account for these differences, IEC 60079 classifies gases into groups that help determine equipment design and certification requirements.

For most industrial applications, the relevant gas groups are:

• IIA

• IIB

• IIC

Why Gas Groups Matter

Gas group classification affects:

• Equipment certification

• Flamepath design

• Explosion protection requirements

• Hazardous area equipment selection

Using equipment certified for the wrong gas group may compromise safety and regulatory compliance.

Gas Group IIA

IIA gases are generally considered the least demanding group among the three.

Common examples include:

• Propane

• Methane

• Ammonia

Many oil and gas facilities contain substances classified within the IIA group.

Equipment certified for IIA may only be suitable for IIA applications.

Gas Group IIB

IIB gases present more demanding ignition characteristics than IIA gases.

Common examples include:

• Ethylene

• Coke oven gas

Equipment certified for IIB can generally be used in IIA applications as well.

However, the reverse is not true.

Gas Group IIC

IIC represents the most demanding gas group classification.

Common examples include:

• Hydrogen

• Acetylene

Equipment intended for IIC environments typically requires the highest level of protection.

Because of this, equipment certified for IIC can generally be used in IIA and IIB environments.

The table below summarizes the differences between IIA, IIB, and IIC gas groups.

Gas Group Comparison Table

Equipment Compatibility

A simple way to understand compatibility is:

• IIC equipment may be used in IIA, IIB, and IIC areas

• IIB equipment may be used in IIA and IIB areas

• IIA equipment may only be used in IIA areas

This is one of the most important rules when reviewing hazardous area certifications.

Reading Equipment Markings

Gas group information is commonly shown in equipment markings.

Examples include:

• Ex db IIA T4 Gb

• Ex eb IIB T4 Gb

• Ex db IIC T6 Gb

The gas group designation forms part of the complete certification marking.

Frequently Asked Questions

Is IIC more hazardous than IIA?

IIC gases generally require more stringent equipment design and protection measures because of their ignition characteristics.

Can IIC equipment be used in IIA areas?

Yes. Equipment certified for IIC can generally be used in IIA and IIB applications.

Is propane IIA or IIB?

Propane is typically classified as IIA.

Conclusion

Gas groups help engineers understand the ignition characteristics of different flammable gases and select appropriate hazardous area equipment.

Understanding the differences between IIA, IIB, and IIC classifications is essential when reviewing equipment markings, selecting certified products, and maintaining compliance with IECEx and IEC 60079 requirements.

References

• IEC 60079 Series

• IEC 60079-0

• IEC 60079-1

• IECEx System