Gases/vapours temperature class Many factors such as size, shape, type and surface quality have an influence on the ignition temperature. ISO, IEC and CENELEC have agreed on a method for gases and vapours defined in EN-ISO/ IEC 80079-20-1 „Method of test for auto ignition temperature (AIT)”. This method is defined in such a way, that a value very close to the lowest practically possible, is determined. By means of this method, gases and vapours are divided into temperature classes. According to these temperature classes, the surface temperatures in explosion protected equipment and other technological objects is designed in such a way that ignition by the surface is not possible. In the standard, permissible excess values and necessary safety margins below these standard values are defined in detail. Ignition temperature from dusts (layer and cloud) For different types of dust, the method for determining the ignition temperature has been unified and coded in document EN-ISO/IEC 80079-20-2. Please note that dust in its deposited form (layer) has a different ignition (read: smouldering) temperature than in its stirred form (cloud). The permissible surface temperature for those parts of systems and equipment is determined by subtracting 75 K (Tmax.= T5 mm- 75 K) from the smouldering temperature value determined for the 5 mm dust layer and by taking 2/3 (Tmax.= 2/3 TCloud) of the ignition temperature value determined for the dust cloud. The permissible surface temperature of the equipment shall always be smaller than the lowest outcome of the Tmax. values determined by using above mentioned formulas. Temperature classes are not defined for dust, so a concrete type of dust must always be considered. The parameters are made available in comprehensive tables, laboratories determine the values on request,
