The basics of dust-explosion protection - STAHL - #22

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7. Equipment Selection

The "Essential Health and Safety Requirements" from Directive 94/9/EC deal with the problem of dust deposits in Annex II, Paragraph 1.2.4. In addition to requiring the removal of dust layers, this states that the surface temperature of equipment and equipment parts be well below the ignition temperature of the dust layer. An accumulation of heat must be expected and should be countered with using temperature limitation. Dust accumulations should, if possible, be limited or avoided entirely. For the equipment manufacturer, this means the equipment must be produced in such a way that dust deposits do not arise and/or the equipment is easy to clean.

> EN 50281-1-2 deals wih the selection of electrical equipment and specifies that independent of classification into zones, the possibility of the equipment being covered or completely submerged by dust must be taken into consideration, unless this situation can be avoided.

EN 50281 does not currently regulate the question of how dust deposits on electrical equipment influence the safety level. For equip­ment from Category 3D, there is no requisite consideration of possible faults. This would mean that equipment from Category 3D would be subject to a thermal assessment - in ex­treme cases when totally submerged in dust -however that common faults need not be taken into consideration.

Zone 21 presents a similar problem. Is the inside of containers in which combustible dust is stored also a part of this zone? The situation is clearer in the case of Zone 20, as due to the definition complete sumerging of the equipment must be taken into account.

After assessment of the installation and its possible risk, the user should consider the following criteria when selecting electrical equipment:

> Determination of the equipment category in accordance with the hazardous zone.

> Assessment of the properties of existing dust.

> Maximum permissible surface temperature of the equipment, taking into consideration

the type of dust, the ignition temperature of the dust cloud and, if dust deposits cannot be excluded, the ignition temperature of the dust layer.

The selection of the equipment category can be carried out as described in Table13. This defines the design of the enclosure in accordance with the requirements of Clause 4,5 and 6 of EN 50281-1-1. The dust-tightness tested using the procedure described in EN 60529 for Category 1D, must be taken into account.

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