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 that 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 that 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.
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Editor’s note: GAMBICA’s variable speed drives group membership includes countless notable engineers, scientists and expert spokespersons that are willing to provide journalists with impartial evidence, opinion and statistics relating to energy usage and efficiency in UK and global industry, commerce and buildings.
Electronic copy: Get an electronic copy of this release by mail richards@stonejunction.co.uk or download from www.gambica-pr.blogspot.com
Press enquiries: Richard Stone
Stone Junction, 33 Kirkdale,
Sydenham, London, SE26 4PN
Telephone: +44 (0) 20 8699 7743
Fax: +44 (0) 20 8699 7743
e-mail: richards@stonejunction.co.uk
www: www.stonejunction.co.uk
Blog: www.stone-junction.blogspot.com
For further information contact: Steve Brambley, Deputy Director
Association for Instrumentation, Control, Automation & Laboratory Technology (GAMBICA), Broadwall House, 21 Broadwall, London SE1 9PL
Telephone: +44 (0)20 7642 8080
www: www.gambica.org.uk
About GAMBICA: GAMBICA is the Trade Association for Instrumentation, Control, Automation and Laboratory Technology in the UK. It has a membership of over 200 companies including the major multinationals in the sector and a significant number of smaller and medium sized companies.
Ref: GAM007/02/10
