Feature Hazardous Area Equipment Understanding the standards
internal explosive mixture is ignited, it is not permitted for this to transmit through the flame-proof joints to the outer external atmosphere.
Flame propagation depends on the upper ambient temperature of the enclosure/equipment and on the internal geometry. When exceeding the given ambient temperature range stated by the manufacturer, the equipment will typically no longer be safe. The same is true for the mechanical integrity of the enclosure when exceeding the lower ambient temperature. Modifying the internal dimensions, especially when reduc- ing the cross-sectional area, will have a similar result.
Toni Ott of Cooper Crouse-Hinds concludes his two part feature (part one in Electrical Engineering - April) on how important it is that engineers fully understand the implications of modifying enclosures to house electrical apparatus for use in explosive atmospheres
W
hen modifying enclosures it is important that the relevant standards are consulted. EN/IEC 60079-19 provides end users with technical instructions on the repair, overhaul and modifica- tion of equipment designed for use in explosive atmospheres. For design, selection and erection of electrical installations in explosive atmos- pheres, EN/IEC 60079-14 is the rele- vant standard to be used.
After an Ex d enclosure has been installed correctly, EN/IEC 60079-17 outlines the principles that the operator should adhere to in terms of mainte- nance (inspection, service and repair) of the equipment, including advice on the issue of safe work permits.
Certification tests
Buyers of Ex d enclosures should also be aware of the certification tests involved. The tests to be performed are defined in EN/IEC 60079-0 (general requirements) and in EN/IEC 60079-1 for the type of protection ‘flame-proof enclosure’, known as ‘Ex d’. For all Ex d equipment, thermal tests are conducted to determine the maximum surface temperature of the complete equipment (enclosure and inbuilt parts) under maximum service conditions with a safety margin added.
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Additional thermal endurance of heat and cold (non-metallic enclosures or parts of enclosures) are tested and impact tests are always carried out on critical parts. These tests or definitions made to prevent an electrostatic igni- tion source from becoming effective, are also performed.
For Ex d enclosures (EN/IEC 60079- 1) additional tests that are normally carried out involve Reference Pressure, Overpressure and Flame Transmission. Additional tests are necessary, for example, for light trans- mitting parts made from glass or breather plugs.
The Reference Pressure tests involve filling the enclosure with an explosive mixture of gas and igniting it. The reference pressure depends on the lower ambient temperature (the lower the temperature, the higher the reference pressure) of the enclosure and of the geometry to be found within the enclosure. Following these tests, a hydrostatic overpres- sure test is conducted to verify the mechanical strength of the enclosure. Following overpressure tests, the enclosure is also subjected to a series of flame transmission tests, using a similar explosive mixture of gases as before. The enclosure is also placed in an explosive atmosphere. When the
Above: control panel comprising multiple Ex d enclosures
For enclosures that have an ambi- ent temperature range higher or lower than -20°C to 40°C, the refer- ence pressure overpressure and flame transmission tests may be conducted at the extremes of low and high ambi- ent specified by the manufacturer. Sometimes, complete Ex d assem- blies are put onto the market with only a ‘U’ at the end of the certificate number. This is not permitted because this shows that only the empty enclosure has been certified, not the complete assembly. Typically, the temperature class is not shown on the type label because the heat loss of inbuilt components is not known by the manufacturer of an empty Ex d enclosure.
Conclusion
Before starting any work on an Ex d enclosure, end users and installers should first ensure that they under- stand and recognise the relevant standards, and read the instruction manual(s) provided by the manufac- turer. If you are unsure, always buy complete Ex d equipment from the company that owns the certification.
Cooper Crouse-Hinds
www.crouse-hinds.com T: 0247 630 89 30
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MAY 2013 Electrical Engineering
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