Motors for hazardous dust environments

One of the main benefits of the ATEX Directive is the focus that it provides on industrial dust — a potential source of explosions. However, notes Andy Glover of WEG UK, the technical evaluation of safety measures to avoid the risks of dust explosions is both complex and extensive.

There are many factors involved in describing the explosion risk posed by dust, including the areas that are affected by combustible dust and the type, character and particle size of the dust.

According to Glover, it is also important to ask "Is the dust electrically conductive or non-conductive? Will it build up on the product in layers or be present as a cloud surrounding the product? What is the ignition temperature of the dust? — this must be ascertained in order to ensure that motor surface temperatures always remain below that of the explosive mixture."

Once the dust has been characterised, an examination needs to be made of the industrial processes concerned. This, said Glover, should take account of possible ignition sources, explosive volumes, operating temperatures and an assessment of the possibility of a dust explosion under given conditions.

Following this risk assessment, the plant areas affected by dust can be classified into zones, each one supported by detailed documentation and inspection schedules to ensure safety compliance. Helpfully for engineers involved in safety evaluations of dust-laden atmospheres, the IEC Standard simplifies explosion protection by classifying risk areas into three zones: 20, 21 and 22 (see table).

The extremely hazardous nature of Zone 20 means that motors cannot be used there, however products complying with the ATEX Directive 94/9/EC can be used in Zones 21 and 22.

Zone 20, the most critical of the three, is an area in which an explosive atmosphere in the form of a cloud of combustible dust in air is present continuously, or for long periods, or frequently. Typically, these conditions would be encountered on the inside of containers or pipelines and enclosed conveying equipment.

Zone 21 is a place in which an explosive atmosphere in the form of a cloud of combustible dust in air is likely to occur in normal operation occasionally for example when discharging and filling equipment. In this instance the certification covers both conductive and non-conductive dusts with electric motors obliged to have a minimum protection degree against ingress of solid foreign bodies of IP65.

Zone 22 is a place in which an explosive atmosphere in the form of a cloud of combustible dust in air is not likely to occur in normal operation but — if it does occur — will persist for a short period only. Areas in which dust escapes and forms deposits are included in this category. Here the certification covers non-conductive dusts for motors having minimum ingress protection IP55 and for conductive dusts IP65.