Gambica's Controlgear Group Technical committee has warmly welcomed the long-awaited publication of IEC 61439, the new series of international standards for low-voltage switchgear and controlgear assemblies, which will soon be implemented in the UK as BS EN 61439. Gambica warns, however, that manufacturers working to the new standard still need to exercise a high level of skill and caution in relation to the ratings they adopt for components used in their assemblies.
The root of Gambica's concern is that the nominal current rating of circuit breakers and other components – including, for example, contactors, overload relays and even electronic devices such as variable speed drives – relate to tests carried out in ‘free air'. For these tests, the component is mounted on an open framework with air space all around it, which helps to dissipate excess heat.
Unfortunately, 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. In effect, nominal component ratings should be considered only as a starting point for making sound engineering decisions relating to PSC design and construction.
Gambica notes reputable manufacturers of PSCs will be fully aware of these issues, and will willingly undertake the testing needed to determine ratings that are appropriate to the use of particular components in their enclosure systems. Alternatively, the manufacturers may choose to adopt another approach permitted by IEC 61439, which relies on calculations that incorporate generous safety margins.
Gambica stresses specifiers and purchasers of PSCs must insist on receiving guarantees from their suppliers that issues relating to component ratings have been fully addressed. If they have not, the consequences can be serious. PSCs operating in excess of their limiting temperatures may suffer progressive insulation breakdown ultimately leading to short circuits, users may be at risk of burns, and the possibility of the equipment catching fire is significantly increased. Further, the equipment cannot be declared as compliant with IEC 61439.
For users and manufacturers of PSCs needing further information and guidance on component ratings, the Gambica technical guide: Current rating of low-voltage electrical switchgear assemblies, July 2009, is available as a free download from http://www.gambica.org.uk/MCBFOT47319.