Maximum Surface Temperature for Combustible Dust
Find the maximum surface temperature allowed for equipment in a combustible dust area, from the dust cloud minimum ignition temperature and the 5 mm layer ignition temperature, following the IEC 60079-14 rules.
- Cloud limit = ⅔ × 430 = 286.7 °C
- Layer limit = 300 − 75 = 225 °C
- Allowed = lower of the two = 225 °C
Dust Surface Temperature Rules (IEC 60079-14)
Dust layer up to 5 mm: Tmax ≤ T5mm − 75 K
Permitted Tmax = the lower of the two
TCL is the minimum ignition temperature of the dust cloud and T5mm the minimum ignition temperature of a 5 mm dust layer, both from laboratory testing or dust property databases. Equipment for dust is marked with its maximum surface temperature in °C (for example T135 °C) rather than a gas T class.
Worked example
For a dust with a cloud ignition temperature of 430 °C and a 5 mm layer ignition temperature of 300 °C, the limit is the lower of 287 °C and 225 °C, so 225 °C. Equipment marked T135 °C is acceptable.
Frequently Asked Questions
Why are there two dust ignition temperatures?
A dust cloud and a dust layer ignite differently. A layer on a hot surface can smoulder and ignite at a much lower temperature than a cloud.
How is dust equipment marked?
With the maximum surface temperature in °C, for example T135 °C, along with the dust group (IIIA, IIIB, IIIC) and EPL.
What if the dust layer is thicker than 5 mm?
The allowable temperature must be reduced further, using the IEC 60079-14 curve for layer thickness or specific tests.
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