Design of electrical equipment for areas containing combustible dust: Why dust standards cannot be extensively harmonised with gas standards

Over recent years, the idea has emerged within the IEC (International Electrotechnical Commission), as well as within the standardisation system of the European Union, that it may be beneficial to harmonise design concepts for electrical equipment for areas containing combustible dusts, with those for areas containing combustible gases and vapours. The harmonisation idea has been encouraged by the European Union “ATEX 100a” Directive, which suffers from insufficient differentiation between combustible dusts, combustible mists, and combustible gases/vapours. This deficiency probably originates from focusing on the extensive similarity of combustible dust clouds, mist clouds and gas/vapour clouds when it comes to ignition and burning properties. However, these similarities are of little significance unless there is an explosible cloud in the first place. And this is where dusts, mists and gases/vapours differ substantially, as discussed in detail in the present paper. It is suggested, therefore, that the idea of extensive harmonisation of design concepts for dusts with those established for gases/vapours be put aside (e.g. IEC Committee draft standards for “Ex i” and “Ex p” for dusts, as well as a proposal for a new “Ex m” dust standard). Instead, the safe design of electrical equipment for areas containing combustible dusts should mainly be based on two simple concepts, use of enclosures that keep the dust out to the required extent, and measures that keep the temperature of any surface in contact with dust clouds or layers sufficiently low to effectively prevent ignition. This is in full accordance with the current philosophy in European standardisation as expressed clearly in EN 50281-1-1 and -2: “The ignition protection is based on the limitation of the maximum surface temperature of the enclosure, and on the restriction of dust ingress into the enclosure by the use of “dust tight” or “dust protected” enclosures”. The same philosophy has been prevailing in USA for quite some time. It is indeed to be hoped that Europe will also maintain this sensible approach, and revise the “ATEX 100a” directive accordingly.