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Text version of the page
| explosion protection | ||||||||||||||||
| 4. technical principles | ||||||||||||||||
| 18 | ||||||||||||||||
| Combustible dusts are not divided into temperature classes. The minimum ignition temperature of the dust cloud has to be compared with the max. surface temperature of the apparatus. In doing so, a safety factor has to be considered. The max. surface temperature of the apparatus must not exceed 2/3 of the minimum ignition temperature of the dust cloud. Since dust can also deposit on apparatus, the minimum ignition temperature of the dust layer must also be taken into account. This temperature is the lowest temperature of a hot surface on which a dust layer of 5 mm can be ignited. The max. surface temperature of the apparatus has to be adjusted using a safety factor of 75 K. The thicker the layer, the higher the heat insulation. For this reason, the dust layer is already ignitable at low temperatures, which is why a reduced surface temperature is admitted on the apparatus. It is determined according to the scheme (fig. 4) (EN 61241-14). If the layer is thicker than 50 mm, the ignition temperature has to be determined by laboratory tests. This applies also to layers thicker than 5 mm when the ignition temperature at 5 mm is lower than 250 °C. Laboratory testing is also necessary when the apparatus are completely covered with combustible dust. | ||||||||||||||||
| 4.3 Ignition Temperature and Temperature classes | ||||||||||||||||
| The ignition temperature of a flammable gas, vapour, or combustible dust is the lowest temperature of a heated surface at which the gas/air or vapour/air mixture ignites. It represents virtually the lowest temperature at which a hot surface can ignite a respective explosive atmosphere. Flammable gases and vapours are classified into temperature classes according to their inflammability. The maximum surface temperature of electrical apparatus should always be lower than the ignition temperature of the gas/air or vapour/air mixture in which it is used. Of course, equipment classified in a higher temperature class (e.g. T5) may also be used for applications in which a lower temperature class is required (e.g. T2 or T3). In North America there is a system incorporating further classification according to temperature subclasses. | ||||||||||||||||
| Fig. 4: Determination of the max. Surface Temperature of Dust Layers of 5 mm to 50 mm | ||||||||||||||||
| R.STAHL explosion protection | ||||||||||||||||