Temperature Classes Explained: T1 vs T2 vs T3 vs T4 vs T5 vs T6

Temperature classes define the maximum surface temperature permitted for hazardous area equipment and help prevent ignition of flammable gases.

When reviewing hazardous area equipment markings, engineers often focus on gas groups such as IIA, IIB, and IIC. However, another critical part of the certification marking is the temperature class.

Temperature classes help ensure that equipment surfaces do not become hot enough to ignite a surrounding explosive atmosphere.

For example, in the marking:

Ex db IIC T6 Gb

the "T6" indicates the maximum surface temperature permitted under specified operating conditions.

What Is a Temperature Class?

A temperature class defines the maximum surface temperature that equipment can reach during operation.

The maximum surface temperature must always remain below the ignition temperature of the hazardous gas or vapor present in the area.

This requirement helps prevent equipment from becoming an ignition source.

Why Temperature Classes Matter

Even if equipment does not generate sparks or arcs, a hot surface can still ignite a flammable atmosphere.

Examples include:

• Motors

• Lighting fixtures

• Heaters

• Junction boxes exposed to heat sources

• Electrical enclosures

Temperature classification helps ensure that equipment operates safely within hazardous environments.

Temperature Class Summary

The table below summarizes the IEC temperature classes.

As the T-number increases, the maximum permitted surface temperature decreases.

Understanding T1

T1 equipment may reach a maximum surface temperature of 450°C.

This classification is suitable for gases with ignition temperatures above 450°C.

Understanding T2

T2 equipment may reach a maximum surface temperature of 300°C.

The equipment must remain below this temperature under specified operating conditions.

Understanding T3

T3 equipment may reach a maximum surface temperature of 200°C.

Many industrial applications require T3 or higher protection levels.

Understanding T4

T4 equipment may reach a maximum surface temperature of 135°C.

T4 is one of the most commonly specified temperature classes in oil and gas facilities.

Understanding T5

T5 equipment may reach a maximum surface temperature of 100°C.

This classification provides additional protection where lower ignition temperatures are present.

Understanding T6

T6 equipment may reach a maximum surface temperature of only 85°C.

Because of the lower temperature limit, T6 equipment often requires more restrictive designs and is frequently associated with higher-cost certified equipment.

Equipment Compatibility

Equipment with lower maximum surface temperatures may generally be used where higher temperature classes are acceptable, but not the reverse.

Temperature Classes and Auto-Ignition Temperature

Temperature classes are based on the ignition temperature of the hazardous gas or vapor present in the area.

As a general rule, the maximum equipment surface temperature must always remain below the auto-ignition temperature of the hazardous substance.

Engineers should always verify the actual ignition temperature data for the hazardous substance before selecting equipment.

Reading Equipment Markings

Temperature classes appear directly within hazardous area certification markings.

Examples include:

• Ex db IIC T4 Gb

• Ex eb IIB T5 Gb

• Ex db IIC T6 Gb

The temperature class forms an important part of equipment selection and compliance verification.

Common Temperature Class Selection Mistakes

Confusing Gas Groups and Temperature Classes

Gas groups and temperature classes address different ignition hazards and must be evaluated separately.

Assuming T4 Is Always Acceptable

Equipment selection must always be based on the actual ignition temperature of the hazardous substance.

Ignoring Auto-Ignition Temperature Data

Temperature class selection requires verification of ignition temperature information.

Selecting Equipment with Excessive Surface Temperatures

The equipment temperature class must always remain below the ignition temperature of the hazardous gas.

Technical Guidance for Temperature Class Selection

When selecting hazardous area equipment, engineers should evaluate temperature class requirements together with gas groups, EPL ratings, area classification, and process operating conditions.

• Verify the ignition temperature of the hazardous gas.
• Check equipment certification markings.
• Confirm compatibility with T1 through T6 requirements.
• Review process temperature conditions.
• Verify gas group and EPL requirements.

Frequently Asked Questions

Is T6 better than T4?

T6 provides a lower maximum surface temperature than T4 and may be suitable for a wider range of applications. However, equipment selection should always be based on the actual hazardous substance present.

Why is T6 equipment often more expensive?

Achieving lower maximum surface temperatures often requires additional engineering controls, thermal design considerations, and certification testing.

Can T4 equipment replace T6 equipment?

No. Equipment must meet the temperature class requirements specified for the application.

Can T6 equipment be used in T4 areas?

Yes. T6 equipment generally provides a lower maximum surface temperature and may be used where T4 equipment is acceptable.

What is the difference between T4 and T6?

T4 permits a maximum surface temperature of 135°C, while T6 limits surface temperature to 85°C.

Why are temperature classes important?

Temperature classes help ensure equipment surfaces do not become ignition sources in hazardous atmospheres.

Conclusion

Temperature classes help prevent hazardous area equipment from becoming an ignition source due to excessive surface temperatures.

Understanding the differences between T1, T2, T3, T4, T5, and T6 classifications is essential when interpreting equipment markings, reviewing certifications, and selecting equipment for explosive atmospheres.

References

• IEC 60079-0

• IEC 60079 Series

• IECEx System