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Thinking inside the box 

Far from being ‘just a box around the important stuff’, enclosures can have a major impact on the performance of the electrical component they enclose, as Paul Haddlesey has been finding out 

With such a wide range of electrical components and assemblies being installed into enclosures, perhaps it’s hardly surprising that these ‘boxes’ are sometimes seen as little more than a ‘wrapping’ around the bits that do the important work. However, there are certain key considerations, as Robert Pratt of Lewden Electrical Industries notes: “Specifiers and contractors need to consider a number of issues relating to enclosures. These include whether there is a particular specification or IP rating that needs to complied with for that area, the best type of material to use for that enclosure and what entries are required.” 

There are also many occasions where the enclosure is a key element in the integrity and performance of the system, particularly in situations where higher ingress protection (IP) ratings are required for the enclosures. “When the enclosure has a low IP rating the thermal management isn’t usually a problem because there is plenty of ventilation,” observes Andrew Baker of Electrical Distribution Solutions (EDS). “But as IP ratings increase to accommodate, for instance, more stringent cleaning regimes, then by definition there will be less airflow and the components should be rated to reflect this.” 

Andrew Baker also points out that there are circumstances where the contents of the enclosure dictate a more rigorous approach to ventilation. “Transformers need good ventilation and some people think it’s sufficient to put louvres on the front of the enclosure,. However, our tests show that the air will then just circulate around the transformers rather than through them. You need to have ventilation at the top and bottom and use the stack effect to draw the air through the components and maximise the heat removal,” he warns. 

Tighter enclosures with less ventilation can be a concern for electronic assemblies as well. “This is an issue with high frequency communications systems using more sophisticated electronics operating at higher frequencies, as they are more vulnerable to electromagnetic interference,” observes Keith Reynolds of Schroff. “This means the enclosure has to create a tighter Faraday cage, thus restricting ventilation and potentially creating temperatures as high as 55°C. So in these systems cooling with chilled water may be necessary – though some of this heat can be recovered and used for other purposes, such as pre-heating hot water,” he adds. 

Thermal management can also be an issue with outdoor enclosures located in direct sunlight. “If the ambient temperature is around 20°C on a sunny day, the temperature in the enclosure can rise to 35-40°C, and above 35°C the life of the components will be reduced dramatically,” notes John Wilkins of Rittal. “A twin wall with an air space will act as a buffer, or it may be necessary to introduce cooling. We have found an energy-efficient way to do this is to utilise the lower temperatures found in soil at 3m or below, circulating air through a buried hose, using a solar powered fan,” he explains.

Maintaining standards 

There has been some confusion as to whether the new IEC 61439, which will eventually replace the IEC 60439 series of standards, changes the requirements for corrosion resistance of enclosures. However, as John Wilkins clarifies: “The text in the IEC 61439 standard simply reiterates the requirements of the previous standards, namely that compliance with corrosion resistance requirements is checked by the test of 10.2.2.” 

So it’s clear that the design and selection of appropriate enclosures depends heavily on the specifier’s understanding of the specific requirements of each project. To that end, Robert Pratt believes manufacturers have an important role to play. “It’s not enough to produce a catalogue, manufacturers needs to go back to basics and pro-actively educate their customers and help them to understand all of the implications,” he concludes.