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Controlgear assemblies `could pose a fire risk`

20 February, 2010

Gambica, the trade body representing the UK’s control and automation sector, is warning that a new series of IEC standards covering low-voltage switchgear and controlgear assemblies will not eliminate the need to exercise skill and caution when choosing the ratings of components used in assemblies. If care is not taken, the assemblies could overheat, possibly causing injuries or leading to fires.

Gambica’s Controlgear Group technical committee has welcomed the long-awaited publication of IEC 61439 – soon to be implemented in the UK as BS EN 61439. But it is worried that the nominal current ratings of circuit-breakers and other components – such as contactors, overload relays and even variable speed drives – relate to tests carried out in “free air”. For these tests, the component is mounted on an open framework with air all around it, which helps to dissipate excess heat. The photograph below shows an MCCB being tested in this way.



Unfortunately, the committee says, these test conditions are very different from the conditions in which the component will operate when installed in a power switchgear or controlgear assembly (PSC). As a result, substantial derating may be needed to ensure safe operation and to comply with the temperature rise requirements of IEC 61439.

If PSCs operate above their limiting temperatures, they can suffer progressive insulation breakdown, leading ultimately to short circuits. Users could suffer burns and the risk of the equipment catching fire is increased significantly. Furthermore, the equipment cannot be declared as compliant with IEC 61439.

Gambica’s committee warns that nominal component ratings should be considered only as a starting point for making sound engineering decisions relating to PSC design and construction.

It points out that reputable PSC manufacturers are aware of these issues and will undertake the testing needed to determine ratings that are appropriate to the use of particular components in their enclosures. Alternatively, the manufacturers may adopt another approach permitted by IEC 61439, which relies on calculations that incorporate generous safety margins.

Gambica stresses that specifiers and purchasers of PSCs must insist on guarantees from their suppliers that issues relating to component ratings have been addressed.

Gambica has published a free technical guide, called Current Rating of Low-Voltage Electrical Switchgear Assemblies, which is available as a download from www.gambica.org.uk.




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