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Explosion Protection

Inadequate Treatment of Dust Explosions in Atex?

| Author / Editor: Rolf K. Eckhoff / Marcel Dröttboom

Unfortunate Alignment of Standards

The absence of clarification of the basic differences between gas and dust in Atex directives 94/9/EC and 1999/92/EC has resulted in some unfortunate alignments and combinations of dust and gas standards for the design of electrical apparatus.

Pressurized Enclosures and Rooms

Neither the enclosure standard nor the room standard make much sense in the case of combustible dusts. This is because they rest on the erratic assumption that particles in a dust cloud that embrace an enclosure, by entering the enclosure though unintentional narrow holes and gaps, can accumulate as an explosive cloud inside the enclosure.

In industries being faced with potential dust explosion and fire hazards, attempts at applying these standards must inevitably cause considerable confusion and frustration.

Encapsulation by Moulding

Moulding is a type of protection by which electrical parts that can ignite an explosive gas/vapour atmosphere are moulded into a compound material in such a way that any external explosive atmosphere surrounding the moulded unit cannot make contact with these parts. The combined IEC moulding standard for gases and dusts is essentially a gas standard and is far too complicated and sophisticated to be relevant for dusts.

Intrinsically Safe Apparatus

Intrinsically safe electrical apparatus for use in areas with combustible dusts is covered by a voluminous IEC standard, which is again more or less a direct copy of the corresponding gas standard, with little relevance for dusts.

As discussed above, electrical circuits, switches, etc., to be used in areas containing combustible and/or electrically conductive dusts should always be kept inside dust tight or dust protected enclosures, which makes intrinsic safety superfluous.

However, in some very special cases there is a genuine need for intrinsically safe apparatus even in areas containing combustible powders/dusts, e.g. open capacitive level indicators for solid bulk materials stored in silos and bins.

One striking deficiency in the intrinsically safe apparatus standard for dust is that it does not at all differentiate between dusts of different ignition sensitivities by introducing 'dust groups' corresponding to the 'gas groups' in the gas standards. In stead, the gas group IIB requirements (minimum ignition energy, MIE = 0.06 mJ) are imposed on all dusts.

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