How Electrical Conductivity and Temperature-rise Limitations Impact LV Switchboards

Credit to Author: Pascal Lepretre| Date: Mon, 27 Aug 2018 13:00:27 +0000

The first post in this series explained why following IEC 61439 ensures quality switchboards, panels and assemblies. In short, adhering to the standard means that everything in an assembly works as it should now and will continue to do so for years to come. This will be true despite aging components and such environmental factors as heat, humidity and pollution.

This is particularly important because failures and problems in switchboards and assemblies can have serious consequences. For instance, electrical faults cause more than half of India’s industrial disasters. Worldwide, 80 percent of assemblies do not comply with IEC 61439.

Previous posts in this series have shown how the standard applies to the testing of protection devices, creepage and clearance distances, insulating materials, and withstand voltage. I’ve also gone over the impact of thermal stability and reliability.

This post, the last in the series, will look at the role played by and impact of electrical conductivity and temperature-rise limitations. The latter – temperature-rise limitations – are explicitly mentioned in Table 6 of IEC 61439-1. These are spelled out for busbars, individual components and external insulated conductors. Tests to ensure acceptable thermal performance are also detailed in the standard.

IEC 61439 also contains instructions related to electrical conductivity constraints. For instance, section 10.8 states that terminals for external conductors must be verified to meet the applicable rules regarding current carrying capability. As part of this, the verification must account for any bending and associated cross-sectional impact as well as any joint connection and other factors. After all, a 2-meter long conductor tested alone in free air with its own standard reference will not have the same performance as it will have when it is considered as part of an enclosed system.

The thermal constraint should consider both nominal rating and abnormal situations, with one example of the latter being a short circuit. According to the standard, even an abnormal circumstance cannot cause a change in design characteristics. So, a short circuit must not create conditions that will result in exceeding the rated thermal capacity (I²t) if this leads to a failure to meet requirements.

These constraints imply important features of the conductor. This is because the quality of the conductor is critical in determining overall system quality.

IEC 61439 does block innovation and so it does not state what type of conductor shall be used. Instead, the guidelines mean specify that if copper is used, it should be Cu-ETP H12 (Cu-ETP means its 99,99% pure) or the equivalent. This type of copper has the required conductivity, which means that busbars will not get too hot and will be able to avoid different types of electrical risk. It also has the needed malleability and mechanical properties, which means that the metal can be bent, drilled and cut without a problem.

For similar reasons, if aluminum is used, and it can be, it should be a grade between 55 and 61 percent conductivity as defined by the International Annealed Copper Standard. Thus, it should be grade 1350 or 6101, as specified by the ASTM B609 or ASTM B317 standard, respectively. It is also recommended that it should have the right surface treatment or coating to ensure better temperature performance. Raw, uncoated aluminum is not forbidden by the IEC 61349 standard, but the temperature limit for raw aluminum will be around 30° C lower as compared to coated aluminum. The cross section of the busbar shall be considered because this is required to limit the maximum, absolute temperature of the conductor to 90° C. Meeting these requirements ensures that the necessary conductivity is retained for a long time.

Now, no matter the type of conductor, limits on allowable temperature rise and, therefore, current density cannot be exceeded. Making sure that won’t happen requires calculations with consideration of such parameters including electrical load diversity factor, the ratio of the maximum individual component demands to the entire system demand.

Determining that conductivity and temperature limits are not exceeded can be a tricky task, with extensive calculations involving many different aspects of an assembly. There is conductor type, cross-section, bends, length, load, diversity factor and more that must be feed into the relevant equations. An alternative, which is allowed by IEC 61439, is to use a third-party certified solution. The second approach uses a panel, components and resulting assembly from a supplier, with this combination known to satisfy the standard.

For more about IEC 61439 and what it means to low-voltage switchboards, look here. Schneider Electric has a complete range of busbars, protection devices and other components that are third party certified to meet the standard in its catalog. There also are tools that will help and guide you in ensuring your assembly is compliant. For more information on Schneider Electric’s solutions for panel builders, click here.

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