Risk due to temperature
When choosing electrical and mechanical equipment for use in potentially explosive atmospheres, there are several factors to consider. In this article, we will explore the risk posed by the temperature generated by the equipment to prevent it from becoming a source of ignition.
There are two distinct parameters for assessing the temperature of instruments, depending on whether they are devices for gases, in which case we refer to temperature class, or dust, in which case we refer to maximum surface temperature.
Temperature class
For atmospheres containing gases and vapours, equipment classification is based on a series of temperature classes (from T1 to T6), which identify the maximum permissible surface temperature.
Temperature class | Maximum surface temperature (°C) |
T1 | 450 |
T2 | 300 |
T3 | 200 |
T4 | 135 |
T5 | 100 |
T6 | 85 |
Each flammable substance has its own minimum ignition temperature: to be safe, the equipment must have a surface temperature below that value. Below is a table containing some examples of temperature class and permitted product combinations:
Temperature class | Max surface temperature (°C) | Examples of gases |
T1 | 450 | Hydrogen (≈ 500 °C) |
T2 | 300 | Acetylene (≈ 305 °C) |
T3 | 200 | Pentane (≈ 240 °C) |
T4 | 135 | Ethyl ether (≈ 180 °C) |
T5 | 100 | Highly flammable solvents |
T6 | 85 | Carbon disulphide (≈ 90 °C) |
Maximum surface temperature
For atmospheres with dust, two distinct scenarios must be distinguished (as reported in technical standard EN/IEC 60079-31:2014):
- Ignition of dust clouds: in this case, the maximum surface temperature of a piece of equipment must not exceed two thirds of the minimum ignition temperature of the combustible dust cloud. (Tcl – cloud temperature) Tsurface < 2/3 * Tcl
- Ignition of dust layers: if there is a layer of dust thicker than 5 mm, the surface temperature of the equipment must be at least (75K = 75°C) lower than the minimum ignition temperature of the layer. (T5mm – ignition temperature for a 5 mm layer) Tsurface < T5mm – 75 °C
Once these two values have been calculated, the most cautious solution is to take the lower of the two values as a reference and choose equipment with a maximum surface temperature lower than this value. Below is a practical example involving sugar dust:
Tcl = 350 °C à Tcl, max = 2/3 * Tcl = 2/3 * 350 °C = 233,3 °C
T5mm = 490 °C à T5mm, max = T5mm – 75°C = 490 – 75 = 415 °C
The minimum temperature between Tcl, max e T5mm, max is 233,3 °C, therefore equipment with a maximum surface temperature lower than this value must be selected.
Practical example of ATEX marking
The parameters described above are indicated in the ATEX marking on the device. Below is a practical example of the Ex d case label that we use for the installation of grounding electronics:
The temperature class T6 is highlighted in red (the highest achievable temperature class, also for gases with a lower ignition temperature), and the maximum surface temperature of 85°C is highlighted in blue (suitable for dusts whose Tcl, max = 2/3 * Tcl e T5mm, max = T5mm – 75°C alues are lower than 85°C).