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Fundamentals of dust explosion protection

Fundamental principles: The manufacturer of operating devices for areas rendered potentially explosive through dust must indicate the maximum surface temperature of all devices that dust can penetrate (usually expressed in °C – indication of the temperature class should be avoided here). This temperature is part of the dust Ex-designation. Designation examples: II 2 D T90°C IP64, II 2 D Ex td A21 T90°C IP64 (If the ignition protection type is based on the housing, the housing protection rating should also be stated as an IP Code) or II 2 D Ex iaD 21 T96°C(This device has been approved according to the IEC de-jure standard ”Intrinsic Dust Safety – ”iaD”. This de-jure standard specifies that the designation also contains the corresponding zone – in this case 21) Special case – Category 3 devices Combustion and explosion parameters for dusts depend on their condition. Parameters that affect combustion and explosion be-haviour include particle size, particle shape, water content, purity and where applicable the content of the flammable solvents. The particle size distribution and the mean value (value for average particle size) should also be known.In accordance with 1999/92/EG Directive (ATEX 137), the system operator /employer is obliged to makea hazard assessment and must therefore be aware of the minimum glow temperature of the dust.There are simple calculations to determine the two “ temperatures “ and they are carried out thus: a) Limit temperature 1 = 2/3 of minimum ignition temperature b) Limit temperature 2 = minimum ignition temperature* minus 75°KThese two limit temperatures must now be examined to confirm which guarantees the greater safety. Example 1: Minimum ignition temperature = +330°C, minimum glow temperature = +300°C: a) Limit temperature 1 = 2/3 x +330°C = +220°C b) Limit temperature 2 = +300°C - 75°K = +225°C Greater safety: Limit temperature (1) = +220°C Here a device with a max. surface temperature in the event of failure <= +220°C must be used. As stated, the device designation includes a corresponding value. Example 2: Minimum ignition temperature = +186°C, minimum glow temperature = +180°C: a) Limit temperature 1 = 2/3 x +186°C = +124°C b) Limit temperature 2 = +180°C - 75°K = +105°C Greater safety: Limit temperature (2) = +105°C Here a device with a max. surface temperature in the event of failure <= +105°C must be used. *The value for the glow temperature applies with a dust layer thickness of 5mm. The temperature safety distance must be increased for larger layer thicknesses. In contrast to Category 1 and 2 devices, potential hazards in the event of failure (e.g. short circuiting, connection break etc.) do not have to be considered for Category 3 devices (for use only in Zones 2 respectively 22). The device is evaluated only in respect of hazards during normal operation. It is relatively unlikely that the device should fail at the same time as a short-term explosive atmo-sphere is present. No EC Type Approval Test Certificate is therefore required for Category 3 operating devices. The manufacturer may confirm that the operating device complies with the relevant standard. Nonetheless, ecom Category 3 devices are still tested by a certified centre and ecom is then awarded a declaration of conformity.(- Of course, Category 2 operating devices also offer significantly greater safety in Zones 2 respectively 22..) Dust explosion protection – temperature: 10