What does T135 °C on dust equipment mean, and is it a temperature class?

T135 °C on dust equipment is the measured maximum surface temperature of the equipment, 135 °C, not a temperature class. Equipment for explosive dust atmospheres (Group III) is marked with its real temperature in degrees Celsius, and the user checks that number against the ignition temperatures of the actual dust. The gas temperature classes T1 to T6 are not used for dust.

Short answer

• T135 °C means that no external surface of the enclosure exceeds 135 °C under the most adverse rated conditions, within the marked ambient temperature range.
• It is not a class. A temperature class (T1 to T6) is a band defined for explosive gas atmospheres; the dust marking is a single value, which can be any number, such as T85 °C, T120 °C or T200 °C.
• To accept it, two checks under IEC 60079-14: 135 °C must not exceed two thirds of the dust cloud's minimum ignition temperature, and, where layers up to 5 mm can form, must not exceed the 5 mm layer ignition temperature minus 75 K.
• Thicker layers need a larger margin, and equipment that can be buried in dust needs a value established for that layer, marked with the layer depth after the T.
• T135 °C is the same number as the upper limit of gas class T4 (135 °C), which is why both often appear on one nameplate, but each part is checked against its own atmosphere.

Why dust uses a temperature and not a class

The International Electrotechnical Vocabulary defines a temperature class as a classification of equipment by maximum surface temperature, related to the explosive gas atmosphere it is meant for. Gases have one ignition property that matters here, the autoignition temperature, so a few bands are enough.

Dusts ignite on hot surfaces in two different ways, as a cloud and as a layer, and the layer value falls as the layer gets thicker. A band system would hide the margin the installer needs, so the IEC system marks Group III equipment with the actual temperature and leaves the comparison to the user.

There is also a difference in where the temperature is taken. The same vocabulary notes that for gas equipment the maximum surface temperature may occur on an internal component or on the outside of the enclosure, depending on the type of protection, while for dust equipment it is the external surface of the enclosure, possibly with a defined dust layer on it. The enclosure keeps dust out (dust-tight, or dust-protected where the standard allows it, for example level tc, and level tb for group IIIA), so the outside is what the dust touches.

How to check T135 °C against your dust

1. Get the dust data: the minimum ignition temperature of the cloud and of a 5 mm layer, from tests on a representative sample (see Which dust tests do you need?).
2. Cloud limit: two thirds of the cloud ignition temperature, in °C.
3. Layer limit: the 5 mm layer ignition temperature minus 75 K (75 K is the same interval as 75 °C).
4. The lower of the two is the highest permitted surface temperature. The marked value must not exceed it.
5. If layers thicker than 5 mm can form, apply the further reduction in IEC 60079-14 or obtain test data for that thickness.

Dust B shows why the number matters: a nameplate value that looks low can still be too hot for a dust whose layer ignites easily. Dust B would need equipment marked, for example, T100 °C or T85 °C.

Reading the whole dust marking

A typical IEC-style dust marking is Ex tb IIIC T135 °C Db:

Under the ATEX directive the marking also carries the group and category, for example II 2D. The temperature applies within the rated ambient range. Edition 8 of IEC 60079-0 (2026) makes the ambient range a required marking on every Ex marking, so a nameplate to this edition always shows it. The same edition's change list also clarifies that equipment for EPL Da is marked with a layer prefix of the form T200 (the layer depth in mm) before the temperature, because that level is assessed with the equipment under a dust layer.

Common mistakes

• Reading T135 °C as "T4 for dust". It is a measured value to compare with dust data, not a band.
• Checking only the cloud limit. For many organic dusts and coals the layer limit is lower and governs.
• Ignoring thick layers. The 75 K margin covers layers up to 5 mm only.
• Taking dust data from a different product. Ignition temperatures depend on the sample's fineness and composition.

Related on excodex

• Surface temperature limits for dust: the two-thirds cloud rule and the 75 K layer margin
• How to read an Ex marking: decoding II 2 G Ex db IIC T4 Gb step by step
• What is the difference between dust cloud ignition temperature and layer ignition temperature?
• Can IIIC-marked equipment be used for IIIA and IIIB dusts?
• Check a marking with the Ex marking tool

Standards and further reading

• IEC 60079-0, general requirements for Ex equipment (maximum surface temperature and Group III marking; edition 8, 2026).
• IEC 60079-14:2024 (edition 6), design, selection and installation of electrical installations (cloud and layer temperature limits for dust).
• IEC 60079-31:2022, equipment dust ignition protection by enclosure "t".
• International Electrotechnical Vocabulary, maximum surface temperature (426-01-04): electropedia.org and temperature class (426-01-05): electropedia.org
• INERIS, explosive atmospheres poster with gas and dust marking and the cloud and layer rules: ineris.fr (PDF)

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.