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Temperature rise within lv assemblies has always been a significant performance criterion. The pursuit of more compact assemblies, component optimisation and conservation of materials all lead to higher temperatures within assemblies.

Quality manufacturers appreciate and manage this complex issue. Users and specifiers must also be fully aware of the intended capability and environment for the assemblies they specify/purchase.

The lv assembly standard IEC 60439-1 is deficient in areas relating to temperature rise verification of modern systems. BEAMA Installation’s Engineered Systems Product Group (ESPG) is calling for change. It wants to see clarity of understanding and means of temperature rise performance verification, plus consistency in approach.

Tighter definition will enable all those involved with assemblies to have the same expectations of loading capability, while users will be assured of defined performance.

Modern lv assemblies

Rarely are two assemblies identical. In construction, modern lv assemblies tend to be modular. Designs accommodate an almost infinite array of arrangements, constructed from a limited number of well defined ‘blocks’.

Often, busbar systems of essentially the same construction use different numbers of laminations for alternative ratings. Frequently, frame sizes for functional units cover more than one rating.

Temperature rise verification of assemblies is more involved than testing mass-produced products. It is impracticable to test every different set-up in a modular system.

However, differing interactions occur due to connections to other assembly components. The position and loading of adjacent circuits for all arrangements within the assembly should be considered. Manufacturers must have a practical method of verifying a modular system’s capability. Where applicable, limitations must be established.

IEC 60439-1

Designs have advanced, but IEC 60439-1 has hardly changed its approach to temperature rise testing over 30 years. Its method is geared towards a specific assembly, similar to that for single-circuit devices.

Therefore, manufacturers’ designs have to consider the system configuration for each application. There is lack of clarity around the derating of components due to their enclosure conditions, diversity and the temperature limits of an assembly and its parts.

The standard is not clear and lacks the means of verification of temperature rise performance, as required by the manufacturers and users of a modern lv assembly.

Subsequently, to overcome its limitations, manufacturers have adapted the test procedures within the standard. Lack of confidence in the standard means there are users insisting upon – and paying the price for – testing each assembly.

Progress

What can be done? ESPG is pressing the IEC for significant advances in the temperature rise testing of assemblies. The outcome is not yet assured, but there is support for change within the IEC committee. Representatives of other nations have also identified these shortcomings, resulting in similar approaches to those of UK manufacturers to overcome the standard’s weaknesses.

It is anticipated the future IEC standard for power switchgear and controlgear assemblies will take a more enlightened approach to temperature rise verification. The proposal includes an unambiguous definition of diversity and comprehensive temperature rise verification of a modular system.

So that designers can use their initiative and components can be used to their maximum capability, the standard’s proposed changes will remain performance-based for matters within the assembly. The responsibility for an assembly’s thermal performance in relation to the defined criteria within the proposed standard rests with the designer and manufacturer.

A more analytical approach is proposed where customisation is essential and testing undesirable due to cost/time constraints. Safety margins ensure an equivalent performance to tested units.

The proposals outlined relate to power switchgear and controlgear assemblies (general assemblies as complying with IEC 60439-1) of a modular construction. Elements of the restructured IEC 60439 series covering specific products (eg consumer units to IEC 60439-3) have yet to have their requirements reviewed.

The standards for specific products may continue calling for testing of a particular arrangement as the only means of temperature rise verification.

User assurance

Assuming the IEC adopts it, the proposed new approach will overcome the present standard’s limitations. It will provide a means of temperature rise verification consistent with the design/application needs of modular assemblies. Significantly, proposed procedures will enable verification of all makes of the modular system in the same manner and within the standard’s scope.

Proposed temperature rise verification will enable design optimisation without compromising the standard’s defined performance. The tighter definition of diversity factor will enable all those involved with assemblies to have the same expectations of loading capability. Users can rest assured that their assemblies are capable of a defined performance in service.