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.
How to Read IECEx Equipment Markings (Ex db IIC T6 Gb Explained)
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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 Table
The table below summarizes the IEC temperature classes.
| Temperature Class | Maximum Surface Temperature |
|---|---|
| T1 | 450°C |
| T2 | 300°C |
| T3 | 200°C |
| T4 | 135°C |
| T5 | 100°C |
| T6 | 85°C |
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
A useful rule is:
T6 equipment may generally be used where T1 through T5 equipment is acceptable.
T5 equipment may generally be used where T1 through T4 equipment is acceptable.
T4 equipment may generally be used where T1 through T3 equipment is acceptable.
The final determination should always consider the ignition temperature of the hazardous substance.
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.
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.
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
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