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The Gambica Controlgear Group Technical Committee has welcomed the publication of the IEC 61439 series of international standards for low-voltage switchgear and controlgear assemblies.

This new series of standards will soon be implemented in the UK as BS EN 61439.

The company has warned, 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.

Gambica’s concern centres around 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 – relating 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.

These test conditions are very different from the conditions in which the component will operate when installed in a power switchgear or controlgear (PSC) assembly.

As a result, substantial de-rating 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.

According to the company, reputable manufacturers of PSCs will be fully aware of these issues and will undertake the testing needed to determine ratings that are appropriate to the use of particular components in their enclosure systems.

Alternatively, manufacturers may choose to adopt another approach permitted by IEC 61439, which relies on calculations that incorporate generous safety margins.

Gambica said 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.

In addition, the equipment cannot be declared as compliant with IEC 61439.

For users and manufacturers of PSCs needing further information and guidance on component ratings, a Gambica technical guide, entitled ‘Current Rating of Low-Voltage Electrical Switchgear Assemblies’, is available as a free download from the company’s website.

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