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T6 … T1 or T6 / T5 / T4 / T3 / T2 / T1 ?
When an explosion-protected equipment is marked with a temperature class, the intention is to indicate the maximum surface temperature that the equipment may reach under the specified operating conditions. However, different manufacturers and certification bodies sometimes express the temperature class range in different ways, such as:
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T6 … T1
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T6 / T5 / T4 / T3 / T2 / T1
Although these markings may appear similar at first glance, they can imply different technical and certification approaches. Understanding the distinction is important for manufacturers, certification bodies, inspectors, and end users.
The lower the allowable surface temperature, the “higher” the safety level regarding ignition risk.
Therefore:
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T6 is the most restrictive/common highest protection level among T-classes.
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T1 is the least restrictive.
Meaning of “T6 … T1”
The notation:
T6 … T1
(or sometimes written as “T6 to T1”)
typically means that the equipment may be certified for different temperature classes depending on operating conditions such as:
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ambient temperature,
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load,
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power dissipation,
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configuration,
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cable entry arrangement,
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duty cycle,
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installation method.
However, this notation does not explicitly identify which intermediate temperature classes are actually applicable or assessed.
In practice, it is often interpreted as:
“The equipment may have any temperature class between T6 and T1 depending on conditions.”
This style of marking was historically used in many certificates and legacy equipment markings.
Meaning of “T6 / T5 / T4 / T3 / T2 / T1”
The notation:
T6 / T5 / T4 / T3 / T2 / T1
is more explicit.
It indicates that:
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each listed temperature class has been specifically considered,
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the equipment has defined operating limits for each class,
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the certificate or manufacturer documentation normally provides corresponding ambient ranges or operating conditions for each class.
For example:
Ambient Temperature |
Temperature Class |
|---|---|
-60 °C to +40 °C |
T6 |
-60 °C to +55 °C |
T5 |
-60 °C to +70 °C |
T4 |
Technical difference between the two markings
From a purely physical safety perspective, both expressions may ultimately refer to the same equipment performance capability.
The real difference is mainly:
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clarity,
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traceability,
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explicitness of certification scope,
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usability for inspectors and end users.
T6 … T1 approach
Advantages
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Shorter marking.
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Historically common.
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Simpler nameplate layout.
Disadvantages
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Ambiguous interpretation.
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Does not clearly identify intermediate classes.
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Less transparent for users and inspectors.
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May create uncertainty regarding assessed operating conditions.
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Some notified bodies or ExCBs may request clarification during certification updates.
T6 / T5 / T4 / T3 / T2 / T1 approach
Advantages
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Explicit and unambiguous.
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Better alignment with modern certification philosophy.
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Easier for:
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inspectors,
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end users,
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notified bodies,
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market surveillance authorities.
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Clearly demonstrates which temperature classes were evaluated.
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Better traceability between:
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thermal test results,
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ambient ranges,
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certificate schedules,
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marking.
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Disadvantages
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Longer marking.
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Requires more detailed certification documentation.
Which one is more useful for certification?
From a certification perspective, the explicit notation:
T6 / T5 / T4 / T3 / T2 / T1
is generally more useful and preferable.
Reasons
1. Better Technical Transparency
Certification bodies prefer clearly defined certification parameters.
Explicit listing reduces interpretation risk during:
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certification review,
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audits,
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factory inspections,
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certificate amendments.
2. Improved Traceability to Thermal Testing
Modern certification practice increasingly expects direct linkage between:
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thermal test results,
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ambient limits,
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declared temperature classes.
Explicit notation supports this more effectively.
3. Easier Future Certificate Modifications
When updating certificates, adding new ambient ranges, or revising thermal assessments, explicit temperature class listings simplify the evaluation.
For example, adding T4 capability becomes straightforward if the structure already explicitly lists classes.
4. Reduced Risk of Misinterpretation
Market surveillance authorities and inspectors may interpret “T6 … T1” differently.
Explicit notation avoids disputes regarding:
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whether intermediate classes were actually assessed,
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whether the manufacturer intended all classes,
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applicability under certain ambient conditions.
Is one format mandatory?
No.
IEC 60079 standards do not explicitly mandate one notation over the other.
However:
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certification practice has evolved,
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notified bodies increasingly prefer explicit declarations,
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modern certificates more commonly use the slash-separated format.
Therefore, the trend in the industry is toward:
T6 / T5 / T4 / T3 / T2 / T1
rather than:
T6 … T1
Practical recommendation for manufacturers
For new certifications and certificate updates, manufacturers are generally advised to:
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use explicit temperature class listings,
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clearly define ambient ranges for each class,
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ensure consistency between:
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marking,
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instruction manual,
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certificate schedule,
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thermal assessment report.
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Example of a modern preferred marking style:
Ex db eb IIC T6/T5/T4 Gb
with corresponding ambient conditions specified in the certificate.
Conclusion
Although the markings:
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T6 … T1
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T6 / T5 / T4 / T3 / T2 / T1
