Temperature Classes Explained: T1 vs T2 vs T3 vs T4 vs T5 vs T6
Temperature class indicates the maximum surface-temperature limit assigned to equipment for explosive gas atmospheres.
When reviewing explosion-protected equipment, engineers need to check more than the hazardous-area zone and gas group. The equipment's temperature class is another important parameter because a hot surface can ignite a flammable gas or vapour even when no electrical spark occurs.
Under the IEC 60079 system, temperature classes T1 through T6 specify maximum surface-temperature limits for Group II equipment intended for explosive gas atmospheres. The higher the T-number, the lower the permitted maximum surface temperature.
Temperature Classes at a Glance
| Temperature class | Maximum surface temperature |
|---|---|
| T1 | 450°C |
| T2 | 300°C |
| T3 | 200°C |
| T4 | 135°C |
| T5 | 100°C |
| T6 | 85°C |
These are the standard temperature-class limits for Group II equipment in explosive gas atmospheres. The actual equipment must meet the applicable requirements under its certified operating conditions. IEC 60079-0 specifies general requirements for Ex equipment, including temperature classification and marking.
What Does Temperature Class Mean?
A temperature class identifies the maximum surface temperature permitted for equipment under the applicable specified conditions. It helps determine whether the equipment's surface could become an ignition source in the intended explosive gas atmosphere.
The relevant temperature is not simply the ambient temperature or the normal operating temperature of an internal component. It is the maximum surface temperature determined under the applicable equipment requirements, including relevant operating conditions and external sources of heating where applicable.
Temperature class is therefore one part of equipment suitability. It does not independently establish that a product is suitable for every hazardous area.
Understanding Each Temperature Class
T1 — Maximum surface temperature of 450°C
T1 is the highest maximum surface-temperature limit in the T1–T6 series. Equipment marked T1 may reach a maximum surface temperature of 450°C under the applicable specified conditions.
It is suitable only when that temperature limit is acceptable for the hazardous substance and the complete equipment selection requirements are met.
T2 — Maximum surface temperature of 300°C
T2 limits the maximum surface temperature to 300°C. This is lower than the T1 limit, but it may still be too high for some gases or applications.
Confirm the required temperature class from the actual hazardous substance and the project documentation rather than assuming that T2 is suitable based on the zone alone.
T3 — Maximum surface temperature of 200°C
T3 limits the maximum surface temperature to 200°C. It is used where the applicable equipment and hazardous-substance requirements permit this limit.
A T3 marking does not indicate the gas group or the level of protection against an explosive atmosphere. Those parameters must be assessed separately.
T4 — Maximum surface temperature of 135°C
T4 limits the maximum surface temperature to 135°C and is commonly encountered in industrial hazardous-area specifications.
Do not treat T4 as a universal default for oil and gas facilities. The correct class depends on the hazardous substance, the project requirements and the equipment certification.
T5 — Maximum surface temperature of 100°C
T5 limits the maximum surface temperature to 100°C. It is more restrictive than T4 because the permitted surface temperature is lower.
Equipment with a T5 rating may satisfy a less restrictive temperature-class requirement, provided its full certification and other application parameters are suitable.
T6 — Maximum surface temperature of 85°C
T6 is the lowest maximum surface-temperature limit in the T1–T6 series. The permitted maximum surface temperature is 85°C under the applicable specified conditions.
T6 does not automatically mean that equipment is suitable for every gas or hazardous area. Gas group, zone, Equipment Protection Level (EPL), protection concept, ambient conditions and certification must still be checked.
Can a Lower Surface-Temperature Rating Replace a Higher One?
For preliminary equipment selection, the general temperature-class hierarchy is:
T6 → T5 → T4 → T3 → T2 → T1
Equipment with a lower maximum surface-temperature limit can generally meet a less restrictive temperature requirement, subject to its complete certification and application conditions.
For example, T6 equipment has a maximum surface temperature of 85°C, which is lower than the 135°C limit for T4. It may therefore be acceptable where T4 is specified, provided the remaining equipment requirements are also met.
The reverse does not apply automatically. T4 equipment should not be selected where T6 is required, because its permitted maximum surface temperature is higher.
This hierarchy is a screening principle, not a substitute for checking the actual product certificate and marking.
Temperature Class vs Gas Group
Gas group and temperature class describe different equipment-selection parameters.
- Gas group: identifies the applicable gas subgroup, such as IIA, IIB or IIC.
- Temperature class: identifies the maximum surface-temperature limit for equipment intended for explosive gas atmospheres.
For example, an equipment marking may contain both IIC and T6. IIC identifies the gas subgroup, while T6 identifies the temperature class.
Equipment that meets the required gas group may still be unsuitable if its temperature class does not meet the specification.
See Gas Groups Explained: IIA vs IIB vs IIC.
Temperature Class vs Hazardous-Area Zone
Zone classification and temperature class must not be confused.
- Zone 0, Zone 1 and Zone 2 describe the likelihood and duration of an explosive gas atmosphere being present.
- T1 through T6 describe the maximum surface-temperature class of equipment for explosive gas atmospheres.
A project may specify equipment for Zone 1 with a particular gas group and T-class. Each requirement must be checked independently, together with the applicable EPL and protection concept.
For additional background, see Hazardous Area Classification Explained and Zone 0 vs Zone 1 vs Zone 2.
Temperature Class and Auto-Ignition Temperature
The temperature class is used to assess whether equipment's maximum surface temperature is suitable for the ignition characteristics of the hazardous gas or vapour.
For equipment selection, the permitted surface temperature must remain suitable for the actual substance under the applicable standards and installation conditions. Engineers should use reliable substance data and project documentation rather than selecting a T-class from a generic assumption.
For example, knowing that an area contains a particular gas is not enough to approve equipment solely because its marking says T4 or T6. The gas group, area classification, EPL, protection concept and certificate conditions must also be verified.
How to Read Temperature Class on an Ex Marking
Consider this example:
Ex db IIC T6 Gb
- Ex db: identifies the flameproof protection concept and its level of protection.
- IIC: identifies the gas subgroup.
- T6: identifies the temperature class, with a maximum surface-temperature limit of 85°C.
- Gb: identifies the Equipment Protection Level for gas atmospheres.
The complete marking must be reviewed as a whole. A correct T-class does not compensate for an unsuitable gas group, EPL, protection concept or certificate.
See How to Read IECEx Equipment Markings and Equipment Protection Level (EPL) Explained.
Does T6 Always Mean Better Equipment?
T6 has a lower maximum surface-temperature limit than T4, but that does not make it the automatic choice for every application.
The correct temperature class is the one that satisfies the hazardous-substance requirements and the project's equipment specification. A higher T-number should not be used as a substitute for checking gas group, EPL, zone, environmental limits and certification.
When comparing vendor offers, record the required T-class and compare it with the complete marking and certificate of each offered model. If the documentation does not demonstrate compliance, request clarification before accepting the offer.
Practical Equipment-Selection Checklist
Before approving equipment for an explosive gas atmosphere, verify the following:
- Identify the hazardous substance. Confirm which gas or vapour may be present.
- Confirm the area classification. Check the applicable zone and required EPL.
- Check the gas group. Confirm that the equipment is suitable for the required gas subgroup.
- Determine the temperature requirement. Check the required T-class against the hazardous substance and project specification.
- Review the complete Ex marking. Verify the protection concept, gas group, T-class and EPL.
- Verify the certificate. Confirm that it applies to the exact model and configuration offered.
- Check operating conditions. Review ambient-temperature limits and any special conditions of use.
- Confirm installation requirements. Check cable entries, accessories and other requirements relevant to the certified protection method.
This checklist is a preliminary review aid. Final equipment acceptance must follow the applicable standards, certificate conditions and project requirements.
Frequently Asked Questions
What does T6 mean on Ex equipment?
T6 is the temperature class with a maximum surface-temperature limit of 85°C under the applicable specified conditions.
What is the difference between T4 and T6?
T4 has a maximum surface-temperature limit of 135°C, while T6 has a limit of 85°C.
Can T6 equipment be used where T4 is required?
Generally yes, if the complete certification and other application requirements are suitable. T6 has a lower maximum surface-temperature limit than T4.
Can T4 equipment be used where T6 is required?
Not automatically. T4 permits a higher maximum surface temperature than T6 and therefore does not, by itself, satisfy a T6 requirement.
Does T6 mean the equipment is suitable for hydrogen?
No. Temperature class alone does not establish hydrogen suitability. The required gas group, zone, EPL, protection concept and certification must also be checked.
Does the T1–T6 system apply to combustible dust?
The T1–T6 temperature-class system is used for explosive gas atmospheres. Equipment for combustible dust uses its applicable maximum surface-temperature marking and dust-related requirements. Do not transfer gas temperature classes to dust applications without checking the relevant standard and equipment documentation.
References and Further Reading
The following resources provide starting points for further technical review:
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