What does a marking like T85 °C or T135 °C mean instead of T1-T6?
Abstract
It is the equipment's actual maximum surface temperature in °C. Dust equipment is always marked this way; gas equipment may use it for a value between two classes, above 450 °C, or alongside a class. Compare it directly with the ignition temperature data of your gas or dust.
It is the actual maximum surface temperature of the equipment, in degrees Celsius, rather than a temperature class band. T135 °C means no surface that matters will exceed 135 °C under the rated conditions. Equipment for dust (Group III) is always marked this way; gas equipment (Group II) may carry a plain value in °C instead of, or next to, a class from T1 to T6. Either way, compare the number directly with the ignition temperatures of your gas or dust.
Short answer
- Dust equipment: always a value in °C, for example
Ex tb IIIC T85 °C Db. There are no T1 to T6 classes for dust; the installer checks the value against the cloud and layer ignition temperatures of the dust. - Gas equipment: normally a class (T1 to T6), but IEC 60079-0 lets the manufacturer mark a value between two classes, alone or with the next hotter class in brackets, such as
350 °C (T1). - Above 450 °C: gas equipment hotter than the T1 limit can only be marked with its temperature, for example
600 °C. - Equipment for one specific gas need not carry a class or temperature at all.
- Non-electrical equipment whose temperature depends on the process may carry a range, such as
T6...T4or85 °C...150 °C, with details in the instructions.
Why some equipment is marked in degrees
A temperature class is a band: T4 covers every design whose hottest surface lies between 100 °C and 135 °C. For gases that is practical, because area classification asks only whether the surface stays below the autoignition temperature (AIT), and the AIT bands match the classes.
For dust a band would hide information the installer needs. A dust has a cloud ignition temperature and a layer ignition temperature, and the permitted surface temperature is derived from both with margins (two thirds of the cloud value, and the 5 mm layer value minus 75 K under IEC 60079-14). The real number is needed, so Group III equipment is marked with it. The International Electrotechnical Vocabulary defines the maximum surface temperature as the highest temperature any part or surface of the equipment reaches in service under the most adverse conditions within the specified tolerances; for dust equipment it is the outside of the enclosure, possibly with a defined dust layer. That is the value marked.
For gas equipment the degree marking is a choice. A heater whose hottest surface reaches 350 °C would be T1, the class for gases with AIT above 450 °C. Marking 350 °C tells the user it is in fact usable for any gas with an AIT above 350 °C, which is more useful in practice.
How to read the common forms
| Marking | Meaning | How to check |
|---|---|---|
T4 | Gas equipment, at most 135 °C | AIT of every gas present above 135 °C |
T135 °C on gas-only equipment | At most 135 °C, the same limit as T4 (the public editions show the gas form without "T", such as 350 °C) | Same check as for T4 |
350 °C (T1) | At most 350 °C; nearest class shown for convenience | AIT of every gas present above 350 °C |
600 °C | Hotter than any class; no class possible | Only for gases with AIT above 600 °C |
T85 °C on dust equipment | Enclosure surface at most 85 °C | At most 2/3 of the cloud ignition temperature and the 5 mm layer value minus 75 K |
T500 320 °C on dust equipment | At most 320 °C when buried in a 500 mm dust layer | Only where a layer of that depth is the assessed case |
The layer marking places the layer depth in millimetres after the T and then the temperature, as in the T500 320 °C example of IEC 60079-0. The 2026 edition (edition 8) clarifies that equipment for EPL Da uses a T200 layer prefix, and it makes the ambient temperature range a required part of every Ex marking, so the value is always tied to a stated ambient range.
Example: A filter fan for wood dust carries Ex h IIIB T135 °C Db. The dust data are 410 °C for the cloud and 300 °C for a 5 mm layer (hypothetical). The limits are 2/3 × 410 = 273 °C and 300 - 75 = 225 °C, so the lower, 225 °C, governs. 135 °C is below it: acceptable, provided layers stay within 5 mm.
T85 °C is not "T6 for dust"
T85 °C uses the same number as the T6 limit, and T135 °C the same as T4, which is why many nameplates for both atmospheres show a pair such as T4 and T135 °C. They are checked against different data: the class against the AIT of the gas, the degree value against the dust ignition temperatures with their margins. A dust with a low layer ignition temperature can make even a T135 °C device unacceptable, as explained in What does T135 °C on dust equipment mean?.
Common mistakes
- Treating a degree value as a class. Compare the number itself with the substance data, not the nearest class.
- Checking only the gas part of a dual marking. A gas class says nothing about dust, and vice versa.
- Ignoring the ambient range. The marked temperature is valid only up to the rated maximum ambient.
- Rejecting
350 °C (T1)equipment in a T2 area by reflex. If every gas present has an AIT above 350 °C, it may be acceptable; the AIT, not the class name, decides.
Related on excodex
- Temperature classes T1 to T6: matching surface temperature to autoignition temperature
- Surface temperature limits for dust: the two-thirds cloud rule and the 75 K layer margin
- What does T135 °C on dust equipment mean, and is it a temperature class?
- How do North American T-codes such as T2A or T3C differ from IEC T-classes?
- Tools: Ex marking check; IEC 60079-0 standards space
Standards and further reading
- IEC 60079-0, equipment general requirements (temperature classes, degree marking, Group III marking; edition 8, 2026). Edition 8 change list: cdn.standards.iteh.ai
- IEC 60079-0:2004, public copy with the
350 °C (T1)and600 °Cmarking rules: law.resource.org - IS/IEC 60079-0:2007 (identical to IEC 60079-0:2007), public text with the Group III and
T500 320 °Clayer marking: archive.org - ISO 80079-36:2016, non-electrical equipment, marking of temperature ranges for process-dependent equipment: law.resource.org
- IEC 60079-14, selection of equipment for gas and dust atmospheres.
- International Electrotechnical Vocabulary 426-01-04, maximum surface temperature: electropedia.org
This article is general technical information prepared by the excodex editorial team. It does not replace the applicable standards, the equipment certificates and instructions, national regulations or the assessment of a competent person.