Installations in hazardous areas for the presence of combustible dust

In the last two newsletters, we analyzed the risks arising from the presence of combustible dust and, subsequently, we considered the...

In the last two newsletters, we analyzed the risks arising from the presence of combustible dust and, subsequently, we considered the method of classification of hazardous areas. In this newsletter, we will describe what types of protection can be used and how the installations are performed. In order to design an electrical system respecting the principles of safety against explosion in hazardous areas for the presence of combustible dust, the new EN 60079-14-2 standard is the reference. This standard is part of the whole new standard group resulting directly from IEC requirements which replaced the EN 50281 series standards. 

The main feature of hazardous installations due to the presence of combustible dust is to prevent the contact between the hazardous substances and the electrical components which can spark or are able to reach, under normal use conditions, trigger temperatures. This is achieved using junction boxes that have a particular degree of protection, as the case may be, of IP6X or IP5X.

This electrical installation is suitable for:

  • Zone 20
  • Zone 21
  • Zone 22

in which the danger is combustible dust, but it can not be used in areas with the presence of gases and vapors.

The electrical apparatus that can be used in these environments are the same used for installations where there is the presence of gas, vapors and mists; so, all the equipment built according to a standardized way of protection are allowed, providing that the equipment are also tested and certified for protection against the entrance of dust, as required by the ATEX Directive 94/9/EC for dust. At the present time, some specific rules were created in order to protect against dust, such as EN 60079-1-2 for proof equipment. In recent years, the regulatory block is continuously updating with new rules for specific types of protection against dust. Equipment certified for gas equipped with gaskets to prevent entry of dust is no longer used, as once.

The electrical apparatus shall be marked with the sign of the specific type of protection. In practice, depending on the areas of use, we have the following situation:  

The electrical apparatus must be protected against external factors (e.g. mechanical and thermal stress, chemical aggressiveness) to which may be exposed. Those measures, however, should not, under any circumstances, interfere with the box protection and with its normal heat dissipation.

Cables

The power cables, in addition to being directly buried, can be put into tubes, or with other means to achieve in any case, the minimum degree of protection. In order to ensure a suitable mechanical protection, up to 2.5 meters high and in areas where a damage is possible, the pipes must be metal, conforming to UNI 8863 standard, and they must be protected against corrosion by galvanizing or coating with flame-retardant. 

In system parts not subject to dangers of mechanical damage, the tubes can be metallic, in accordance with EN 50086-1 (CEI 23-39 Classification) and EN 50086-2-1 (CEI Classification 23-54) standards or it’s sufficient that the tubes are made in non-combustible material. Cables subject to movement during use must be enclosed in flexible tubes with equivalent characteristics to those of the rigid tubes.

Intrinsically safe cables can be used against mechanical damage and dust-tight, such as: 

  • thermoplastic or elastomeric insulated cables, shielded or with armor, in PVC, PCP or similar sheath;
  • cables encased in a seamless aluminum sheath, with or without armor;
  • mineral insulated cables, but used at reduced power in order to limit the surface temperature 

Cables should be positioned in order to prevent frictions and electrostatic charges due to the passage of dust. When the cables are placed in conduits, ducts or tunnels, it’s necessary to avoid passage or storage of combustible dust in those places. You must also consider the possibility of dust accumulation on the cable and, thus, a decrease in heat dissipation, using cables with higher capacity in order to reduce the risk of fire caused by overheating. 

Conclusion

As we have seen, in plants where powders are worked, especially those in the food industry, the dangers arising from the accumulation of dust or clouds are very high and the effects can be devastating explosions, as has happened in the past and continues today. The application of new regulations, therefore, must be seen as a step in the path to security for all those working in industry and liv

Fecha de publicación: 01-11-2010

Tema: Additional information