The new IEC standard for pressurised electrical equipment enclosures for areas containing combustible dust. A self-contradiction?

Abstract
During the last years the International Electrotechnical Commission (IEC) has been working towards producing a series of new standards for electrical equipment to be used in areas containing combustible dusts (ExpD, ExiD, ExmD). A main objective is that the new standards shall be "harmonized" with the corresponding existing standards for explosive gas atmospheres (Exp, Exi and Exm). The ExpD standard IEC 61241-4 is the first of the new harmonized standards that has been issued. The main purpose of the present paper is to show that this new standard is self- contradictory, and that this can be traced back to basic deficiencies in the European "Atex I 00a" Directive", which does not account for the substantial differences in the physical behaviour of gas molecules and dust/powder particles, and to corresponding deficiencies in the current standards for classification of areas containing combustible dusts.