Page 603 - Dust Explosions in the Process Industries

570 Dust Explosions in the Process Industries

Figure 8.7 Three models of a capacitive surface
level indicator. The model to the left, with a metal
rope carrying a weight at the end, is for powders
and granulates in silos. The two other models,
with coaxial and plain metal rods respectively,
are for liquids in tanks (Courtesy of G. Klotz-
Engmann, Endress i Hauser, Germany).

return of the reflected pulse from the point where the rod or rope becomes immersed
in the powder.

8.5.3
THE NEW IEC EX"iD" STANDARD FOR DUSTS

The voluminous, detailed draft Ex"iD' standard for dusts, produced by IEC (2002h),
is to a large extent based on the corresponding IEC (1999~) and CENELEC (2001) stan-
dards for gases. In view of the very limited applications for intrinsically safe electrical
apparatuses in areas containing dusts, the proposed draft standard does not seem an opti-
mal solution. Furthermore, in spite of entering into minute details on a number of aspects,
the draft standard provides no guidance for differentiating between dusts of different
ignition sensitivities, such as by introducing dust groups corresponding to the gas groups
in the gas standards. Instead, the gas group IIB requirements are imposed on all dusts. One
argument in favor of this approach is that the same electrical apparatuses can be used for
almost all combustible gases and vapors and for all dusts. However, whereas this approach
clearly simplifies production, stocking, and sale of electrical apparatuses, it is not an accept-
able basis for a safety standard. The basic objective of any safety standard should be to define
borderlines between safe and unsafe conditions, with reasonable safety margins.
In view of this, the possibility of producing a completely independent, intrinsic safety stan-
dard for dusts should be given careful consideration. Such a standard should focus directly
on the very limited segment of intrinsic safety technology relevant to combustible dusts and