Premise
In systems built in potentially explosive atmospheres, large spaces are classified as Zone 2, i.e., an area in which the formation of an explosive atmosphere during normal operations is not likely, and if it does occur, it is of short duration. [1]
Over the years, ‘Ex n’ equipment has been increasingly used to meet the need for products specifically designed for Zone 2. This type of protection was developed in Great Britain in the 1970s and subsequently incorporated into European regulation through IEC/EN 60079-15 and the respective national standards from the early 2000s.
It must be said that this standard was born as a container of different approaches to explosion protection, and that these concepts have undergone different developments over the years: some have remained in the IEC/EN 60079-15 Standard, others have been integrated into the standards for Zone 1 devices.
In Figure 1, we see in detail the migration of some explosion protection methods included in IEC/EN 60079-15 Standard and their final arrival as “de-rated” versions ‘Ex ec’, ‘Ex dc’ and ‘Ex ic’.
Among these, the most important, due to the number of applications, is the protection method for non-sparking ‘Ex nA’ devices, which have now become increased safety equipment ‘Ex ec’ in the IEC 60079-7:2015 Standard.
Figure 1 – Incorporation into other standards of protection methods initially foreseen by IEC 60079-15
Restricted breathing apparatus ‘Ex nR ‘
In addition to non-sparking devices, which are mainly used in the field of switchboard components, many applications of this standard are found in ‘Ex nR’ restricted breathing equipment.
The concept behind these devices is to limit the phenomenon of breathing; in this regard, we refer to the article: “The phenomenon of breathing in an electrical device”. In short, this phenomenon allows the surrounding atmosphere to enter the electrical device after a series of on and off cycles.
The restricted breathing apparatus is designed and constructed in such a way as to prevent this phenomenon from leading to the entry of an explosive atmosphere above the flammability limit.
To achieve this goal, the closures and seals are made in such a way as to prevent the passage of air and to demonstrate their compliance, a specific test is carried out.
After having fulfilled all the requirements of the general standard EN/IEC 60079-0, including ageing cycles on non-metallic materials and impacts, a specific limited breathing test is performed: air is removed from inside the device with a vacuum pump to reach 0.3 kPa below atmospheric pressure and then the time taken for this pressure to halve is timed. [2]
Lighting fixtures, switches and actual electrical panels are made with this type of protection. For example, the FLOWEX-MN series lighting fixture by Cortem Group is made with the “Ex nR” protection method. This series is characterised by:
- Constant lighting efficiency of real 151 l/W, high-power LED plates, a lumen output ranging from 4,784 lm to 27,961 lm.
- Available in three sizes and with a wide range of voltages and powers.
- The finned body, patented with Heat technology Destroyer, is made of high-quality die-cast aluminium and acts as a great heat sink for the ring-shaped LED plate. The central heat sink element allows the fresh outside air to disperse the heat by lapping the surfaces in an ascending air motion that exploits natural convection.
Fig. 2 FLOWEX-MN series lighting fixture
Conclusions
Cost-effective device solutions are increasingly widespread. In addition to the economic advantages, limited breathing devices can also boast improved performances such as less weight, better luminous efficiency, and higher heat dissipation.
Finally, it should be remembered that, as for any equipment that can be installed in a potentially explosive atmosphere, an important role is played by the use and maintenance manual with specific requirements relating to maintaining protection and the correct choice of cable entry devices.
Reference standards and bibliography
[1] DIRECTIVE 1999/92/EC – ANNEX 1 (2) [2] CEI EN 60079-15:2012-01 par. 23.2.3.2.1