Examples of the assignment of gases and vapours to the respective temperature classes and explosion sub-groups:

Ignitition temperature from dusts (layer and cloud)

For different types of dust, the method for determining the ignition temperature has also been unified and coded in document IEC 61241-2-1. Please note that dust in its deposited form (layer) has a different ignition temperature than in its stirred form (cloud).

The permissible surface temperature for those parts of the systems, equipment and components accessible to the dust is determined by subtracting 75 K (Tperm L= Tmin L - 75 K) from the value determined for the dust layer and by multiplying by 2/3 (Tperm C = 2/3Tmin C) the value determined for the dust cloud.

The smaller of the 2 values determined in this way corresponds to the lowest permissible surface temperature of the equipment (Tperm L > Tperm < Tperm C). The surface is the area accessible to the dust, temperature classes are not defined for dust, so that a concrete type of dust must always be assumed. The parameters are made available in comprehensive tables, laboratories determine the values on request, and a small, non-official overview is contained in the following table (page 13).

Explosion sub-group - Conductivity of the dust

The IEC 61241-2-2 contains the test method for determining the specific electrical resistance of dust. The various types of dust are divided into 3 sub-groups according to this resistance:
  • IIIA - combustible flyings
  • IIIB - non-conductive combustible dust, specific electrical resistance > 10³ Ω
  • IIIC - conductive combustible dust, specific electrical resistance < 10³ Ω