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Risk awareness in demand

The future of dust explosion protection
Risk awareness in demand

Standards based on the EU explosion protection directives (94/9/EC and 1999/92/EC) are currently being prepared to cover various aspects of dust explosion protection. These standards have not yet been ratified. They will, however, undoubtedly reflect the growing importance attached to dust explosions by experts.

Rainer Lumme

In the past, manufacturers of explosion-proof equipment and operators of machines and plants installed in potentially explosive atmospheres had to acquaint themselves with new EC directives. The “manufacturer’s directive“ 94/9/EC (Atex 95) is now fully valid following the end of the transitional period in July 2003. The “user directive“ 1999/92/EC (ATEC 137) is still being implemented. This directive is enforced by the Betriebssicherheitsverordnung (BetrSichV = Operational Safety Order) in Germany and by national legislation in other EU states.
One significant new feature of the two directives is the increased importance attached to dust explosion protection, which in the old regulations was considered no more than an appendage of gas explosion protection. This realignment is unquestionably a more accurate reflection of reality: expert opinion is that far more explosions are caused by dust than by gas. The number of unreported cases is thought to be very high, so that the true figure is probably much more alarming than official statistics would have us believe.
Many different factories and industries are at risk from explosive dusts: whenever organic dusts are processed or dust accumulates as a result of processing organic materials (wood, grain, foodstuffs, fodder, etc.) dust explosions can occur as soon as the air contains a defined dust minimum concentration and an ignition source is present, for example an electrostatic discharge or a spark produced by the actuation of a conventional switchgear.
Standards harmonisation moves on
However, the implementation of Atex 95 and Atex 137 on a national level merely marks the beginning of standards harmonisation in the area of dust explosion protection. These directives provide no more than a basis for standards currently being prepared to cover various aspects of dust explosion protection.
From European to international level
The two Atex directives are in force within the EU member states. Progress is, however, also being made with regard to standardisation on an international – in other words IEC – level. Fortunately, it appears that many of the Atex regulations will be adopted. The zones envisaged in the draft of IEC standard 61241-10, for instance, correspond to the EU zones in EN 50281-3. Trade barriers will thus be dismantled not only in Europe (as intended by the EU directives) but also worldwide.
Marking of electrical equipment
Users have meanwhile become accus-tomed to the new rules for marking electrical equipment that is approved for installation in hazardous areas containing dust. In contrast with the Ex markings for gas, the IP degree of protection and the maximum permissible surface temperature are indicated rather than specifying the type of protection explicitly. The reason for this is that a leakproof enclosure – documented by the IP degree of protection – and a surface temperature limit normally guarantee sufficient explosion protection.
In future there will be four types of protection on based on IEC 61241 and designated tD (IP degree of protection), pD (pressu-rised enclosure), iD (intrinsic safety) and mD (encapsulation), whereby the D stands for dust in each case. At IEC level, there will be a separate standard (specifying the respective requirements) for each of the four degrees of protection.
Cleanliness is a must
Since dust deposits can swirl up and create potentially explosive atmospheres, there is a direct link between cleanliness and explosion protection. The above-mentioned future IEC standard 6124110 (zoning) will also take account of dust layers in the same way as EN 50281-3: if the user keeps the environment of the explosion-proof machine clean and makes sure that there are no deposits of explosive dusts, this will have a positive effect on the zone in that the explosion protection requirements decrease as the degree of cleanliness increases.
Risk awareness and conduct of personnel
Practical experience confirms that risk awareness among users and personnel is an essential prerequisite for preventing dust explosions. The explosion protection document required by Atex 137 therefore represents a useful instrument for minimising hazard potential. The documentation duty imposed on the user (immediately for new plants or by the end of 2005 at the latest in the case of existing plants) includes a detailed analysis of the prevailing risks and a list of measures implemented to curtail the explosion hazard.
The conduct of personnel on site is just as important as user awareness of risks. Careless actions, such as the use of non-explosion-proof equipment or naked flames during repair work, can have serious consequences. Once again, the relevant directives and regulations provide precise instructions: the identification of potentially explosive atmospheres is stipulated along with clear operating instructions for personnel.
These measures make good sense, because people working in a production plant with coffee, or with aluminium or plastic dusts, do not necessarily expect to witness an explosion. The risk of a dust explosion is, in other words, more abstract for personnel than the risk of a gas explosion in, say, the chemical industry. The increased importance now attached to dust explosion protection reflects not only the theoretical risk potential but also actual accident statistics.
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www. steute.de

Wide choice of explosion certified switchgear
It is an undisputed fact that electrical equipment used in areas exposed to a dust explosion hazard has to be selected extremely carefully – after all, sparks produced by switching operations in conventional switchgear could escape and ignite the atmosphere. Explosion certified switchgear guarantees that this cannot happen.
Users can choose between Atex-approved switchgear for all kinds of requirements and applications in potentially explosive atmospheres containing dust. Steute’s range of products with gas Ex or dust Ex approval includes position switches, foot switches, contactless proximity switches and control switches as well as a broad spectrum of safety switchgear for potentially explosive, dusty atmospheres, such as safety interlock switches, emergency pull-wire switches and safety switches with various operating elements.
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