What if the dust layer on equipment is thicker than 5 mm?
Abstract
The usual limit, 5 mm layer ignition temperature minus 75 K, no longer applies. Thicker layers ignite at lower temperatures, so IEC 60079-14 lowers the permitted surface temperature up to 50 mm; beyond that, test the actual layer and use equipment marked for it.
Then the normal selection rule no longer covers the installation, and the permitted surface temperature must be lowered or the layer prevented. The usual rule, a maximum surface temperature at least 75 K below the ignition temperature of a 5 mm layer, assumes layers no thicker than 5 mm. Thicker layers insulate the surface and ignite at lower temperatures, so IEC 60079-14 gives a reduced limit for layers up to 50 mm; beyond that the ignition temperature of the actual layer has to be found by test and the equipment must be suitable for that layer.
Short answer
- Up to 5 mm: maximum surface temperature no higher than the 5 mm layer ignition temperature minus 75 K (and no higher than two thirds of the dust cloud ignition temperature).
- Above 5 mm up to 50 mm: IEC 60079-14 contains a graph that relates the maximum permissible surface temperature to the layer thickness. The thicker the layer, the lower the limit. The graph has limits of validity stated in the standard.
- Above 50 mm, or where equipment can be buried in dust: the layer ignition temperature for the actual thickness must be determined by laboratory test, and the equipment should be tested and marked for a stated layer thickness (often called the TL marking).
- Often better: keep the layer from forming, through housekeeping, sloped tops, relocation or enclosure of the equipment.
Why a thicker layer is more dangerous
A dust layer on a hot surface is heated from below and loses heat from its top. Inside the layer the dust oxidises slowly and produces heat of its own. A thin layer loses that heat easily; a thick layer insulates itself and the surface beneath it, so the heat stays in, the temperature inside the layer climbs and it starts to smoulder at a lower surface temperature.
Published test data show how large the effect is. ASTM E2021, the US hot-surface test for dust layers, cites results for a pulverised bituminous coal dust:
| Layer thickness | Hot-surface ignition temperature |
|---|---|
| 6.4 mm | 300 °C |
| 9.4 mm | 260 °C |
| 12.7 mm | 240 °C |
| 25.4 mm | 210 °C |
Quadrupling the layer thickness lowered the ignition temperature by 90 K for this sample. The same standard warns that a result at one thickness may not apply to all industrial situations, and that tests at several thicknesses may allow extrapolation to thicker layers. HSE guidance in the UK makes the same point: thicker layers can ignite at much lower temperatures than thin ones, the electrical equipment standards assume deposits will never exceed 5 mm, and where accumulations cannot be kept to that thickness, advice should be sought from the equipment supplier.
What IEC 60079-14 requires
The installation standard IEC 60079-14 (edition 6.0, 2024) sets three regimes for selecting equipment by surface temperature in dust atmospheres:
| Layer that can form on the equipment | How the permitted maximum surface temperature is found |
|---|---|
| None, or up to 5 mm | Lower of two thirds of the cloud ignition temperature and the 5 mm layer ignition temperature minus 75 K |
| More than 5 mm, up to 50 mm | Reduced further using the standard's graph of permissible surface temperature against layer thickness, within its stated limits of validity |
| More than 50 mm, or complete covering by dust | Laboratory determination of the ignition temperature of the actual layer; equipment tested and marked for a stated layer thickness |
The graph is a figure in the standard ("correlation between the maximum permissible surface temperature and thickness of dust layers"), and the reduction it gives depends on the 5 mm layer ignition temperature of the dust. Read the values from the current edition; do not extrapolate the curves outside the range they cover. Where the dust's properties fall outside the stated limits of validity, testing is again the route.
The cloud limit, two thirds of the cloud ignition temperature, always continues to apply as well.
Equipment marked for a dust layer
Group III equipment is normally marked with a maximum surface temperature determined without a dust layer, for example Ex tb IIIC T135 °C Db. Equipment can also be tested with a specified layer thickness, in a specified orientation, and the resulting temperature is then marked together with that thickness. This is the right choice where the equipment will sit in or under thick deposits by design, for example a level sensor in a silo, a motor in a dusty pit or a luminaire that will be covered between cleaning rounds.
The installer compares the marked layer temperature with the ignition temperature of the actual dust at that thickness, determined by test, and keeps an appropriate margin. A plain temperature marking without a layer thickness says nothing about thick layers.
Practical case
A motor in a feed mill basement sits under a conveyor that leaks meal; between weekly cleans, about 15 mm builds up on top of the motor. The meal has a cloud ignition temperature well above 400 °C and a 5 mm layer ignition temperature of 290 °C, so the 75 K rule gives 215 °C. Because the layer exceeds 5 mm, the engineer reads the IEC 60079-14 graph for 15 mm, which gives a substantially lower limit than 215 °C, and checks it against the motor's marking. In parallel, the conveyor leak is repaired, a sloped cover is fitted over the motor and the cleaning interval is shortened. Fixing the cause is better than relying on a lower temperature rating.
Common mistakes
- Applying the 75 K rule to any thickness. It is valid only for layers up to 5 mm.
- Using a T135 °C marking as proof of safety under thick layers. That value is determined without a layer.
- Extrapolating the graph beyond 50 mm. Above that, or where the equipment is buried, only tests give a valid limit.
Related on excodex
- Surface temperature limits for dust: the two-thirds cloud rule and the 75 K layer margin
- Dust layers and housekeeping: how thick is too thick?
- What is the difference between dust cloud ignition temperature and layer ignition temperature?
- What does T135 °C on dust equipment mean and is it a temperature class?
- Standards space: IEC 60079-14
Standards and further reading
- IEC 60079-14:2024, explosive atmospheres, design, selection, erection and initial inspection of electrical installations (selection by dust ignition temperature, figure relating permissible surface temperature to layer thickness): standard summary
- IEC 60079-0, equipment general requirements (marking of Group III maximum surface temperature, with or without a dust layer).
- ASTM E2021-15 (reapproved 2023), hot-surface ignition temperature of dust layers (layer thickness data): astm.org
- HSE, HSG103 Safe handling of combustible dusts (layers thicker than 5 mm): hse.gov.uk (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.