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Warm welcome for IEC 61439 and also a warning

Nominal component ratings are no substitute for sound engineering judgement when it comes to installing drives and switchgear in enclosures in real world application environments.

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 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. Those responsible organisations that spend an appropriate proportion of their time and other resources in validating the performance of their equipment rigorously don't do so for the exercise. It is done because they know that not to do so places them and their customers at risk, and exposes them to potentially onerous legal issues if something goes wrong.

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.

High power loads
In its guidance on current ratings of low-voltage electrical switchgear and assemblies, GAMBICA notes that electronic devices and control products with power switching capabilities, such as soft starters and variable speed drives, are increasingly being used to control high power loads, from motors to furnaces and more. Significant amounts of heat can be generated by the passage of currents through such devices. Thyristors, for example, generate about 1W of heat per ampere of current passing through them, so a soft starter with thyristors in each phase feeding a motor load drawing 330A generates about 1kW of heat, continuously. Variable speed drives have even greater heat dissipation per amp of current in each phase. Commonly, such devices are fitted with fan assisted heat sinks which serve to pump excess heat into the surrounding environment - fine when installed in free air but adding to the mounting thermal load inside a switchboard when enclosed.

Where electronic power and control products are used, the switching assembly designer needs to be especially vigilant and might need to consider using forced cooling systems or even lowering the IP rating of the enclosure. Consideration of the potential effects on other installed equipment such as electronic overload protection relays in starter assemblies for example, or on the operation of computer equipment that might also have to be made. Separation of heat-sensitive devices from major heat sources can be necessary. 

This isn't to say, however, that a warm switchboard is necessarily running into problems. BS EN 61439-2 sets a maximum temperature rise limit for accessible external metallic surfaces of +30K. From an ambient start point of, say, 30°C, it infers that an external temperature of 60°C at maximum temperature rise is acceptable. Of course, to attain such a temperature, installed equipment may well be running considerably higher, with other potential consequences, efficient use of the available power being one.

Although the standards themselves are at pains to point out that compliance with a British Standard does not of itself confer immunity from legal obligations, GAMBICA notes that it is an accepted position that compliance with the relevant standards, European Directives and other relevant legislation (Electricity at Work Regulations, for example) goes a long way to protecting the diligent engineer from the unwanted consequences of accidental damage or injury (or worse) to users, or others, of the switchgear assembly he has designed or specified.
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