The basics of dust-explosion protection - STAHL - #24

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Figure8: Determining the Maximum Surface Temperature T_L Under a Dust Layer of Thickness L in mm

When determining the maximum permissible surface temperature (in relation to the surrounding dust), two figures should be noted:

1. Maximum permissible surface temperature when a dust cloud is present (calculation in accordance with 6.1 of EN 50281-1-2).

The ignition temperature of the dust cloud must be taken into account.

2. Maximum permissible surface temperature when a dust layer is present. When making the determination, one must take the ignition temperature of a dust layer (which is dependent on the thickness of the dust layer) into account:

> For dust deposits up to 5mm thick, the calculation must be made in accordance with 6.2.1 of EN 50281-1-2.

> For dust deposits >5mm-50mm, the curves in Figure 7 may be applied.

The redutions in temperature plotted here were determined empirically for dust layers of up to 50 mm on top of electrical equipment. The curves take into account both the reduction, specific to dusts, of the ignition temperature with increasing layer thickness, and also an ex­pected increase of the temperature of the electronic apparatus, due to the reduction of the heat flow. In addition, the curves include the usual safety reduction of 75K. Here it must be stated explicitly that this does not take into account an electrical apparatus submerged completely in dust.

> Determining a surface temperature for dust deposits >50mm: According to the currently valid requirements in Clause 6.3 of EN 50281-1-1, if dust deposits of excessive thickness are present, the equipment in question must be tested in a laboratory using the relevant dust (see Annex A of the above-mentioned standard).

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