The power of copper

Man first came to understand the value of copper around 10,000 BC. The Romans, Greeks, Aztecs and Egyptians were quite innovative in their use of copper and copper-tin alloys such as bronze and brass. The metal was readily accessible, very ductile and malleable, making it relatively easy to smelt. It also had a natural resistance to corrosion. The current extraordinary rate of development in China and India has significantly increased the demand on this resource, driving the cost of copper to record levels. As the lifeblood of all electrical systems, it is essential that designers and engineers are efficient in their use of copper to produce an economical solution without jeopardising the integrity of the electrical supply system.

Whatever the method of transmission, pure copper properties are constant:

• Electrical resistivity = 16.78 nΩm at 20°C
• Thermal Conductivity = (300 K) 401 W·m-1·K-1

The cross-sectional area of copper determines its current-carrying capacity, as does its installation environment. The exposed surface area, the ambient temperature and various types of insulation all affect the amount of current that copper can carry before it starts to melt. The fuse is the simplest example of using these copper properties to define a ‘breaking point’ to protect a circuit by forcing it to melt when exposed to preordained conditions. Due to its predictable nature, designers and engineers can select the appropriate size copper for applications and can be confident that it will perform as intended. It is important to note that copper properties do not change, irrespective of how the copper is formed into a current-carrying conductor. Cables, solid bars and flexible bars of similar cross-sectional areas tend to carry the same amount of current for similar conditions. The standards have been created to clearly define what copper sizes should be used to maintain maximum operating temperatures for specific load currents. The most commonly used standards for panel board designs or cabling installations are:

• AS/NZS3008.1.1:2009
• AS60890-2009 (currently under review), AS/NZS3439.1:2002 and AS/NZS3000 also contain references to these standards.

These standards contain worked examples and tables that identify the environmental factors that affect the final selection of a conductor. It is essential that the designer understands the significance of environmental conditions when selecting the appropriate copper conductor size for an application. Consideration must be given to the following variables: ambient temperatures; derating for connection to switchgear; conductor installation; and, eddy currents (high current installations).

AS/NZS3439.1:2002, Table 2 - ‘Temperature Rise Limits’ stipulates that the maximum permissible bus bar temperature rise for built-in components is 70°K. However, it should be noted that there is a fundamental difference between AS/NZS3439.1:2002 and IEC61439-1 when specifying maximum bus bar temperatures. IEC61439-1 allows a maximum temperature rise of 105°K. As a consequence, European ratings for bus bar connections to apparatus can be higher than Australian ratings for identical conductor cross-sectional areas.

The tables in AS60890-2009 are quite comprehensive and clearly define the maximum permissible current draw for a conductor of specific dimensions and cross-sectional area, within a specific ambient, for use with low-voltage apparatus. Copper has a watts loss value that can be calculated on a watts/meter basis for a specific load current for a specific cross-sectional area. The heat dissipated by the copper as well as the peripheral switchgear all contribute to the watts losses accumulated within an enclosure. It is then necessary to understand the heat dissipation characteristics of the enclosure to understand the steady state ambient temperature that can be expected within a panel board operating at its rated maximum current-carrying capacity. The operating temperature of the bar is determined by the heat dissipated by the enclosure compared to the heat being generated within the enclosure. If a copper conductor exceeds the maximum permissible temperature then there are really only three solutions: increase the amount of copper used to transmit current, alter the ventilation characteristics of the enclosure and/or reduce the load current.

Circuit breaker chassis are usually given a nominal current rating which is based on ‘free to air’ tests. Therefore, it is necessary for the designer to consider the impact of the ambient temperature within an enclosure and derate the nominal current value of the chassis accordingly. For normal power distribution configurations, a factor of diversity is often applied and this may allow the designer to use the chassis for derated applications. It is essential that the engineer is aware of the need to derate standard chassis nominal values to prevent overheating and possible premature failure of low-voltage circuit breakers fitted to the bus bar.

It is a fact that modern switchgear is significantly more compact than in previous generations, and as a consequence tends to operate at higher temperatures. It is not uncommon for air circuit breakers to demand a substantial cross-sectional area of copper bus to be connected to its main tags to perform at the rated current. In the case of moulded or air circuit breakers, the devices could fail to clear a fault if the contact temperature is not maintained within appropriate limits.

The manufacturer’s recommendations must be followed to ensure that the apparatus performs as intended and does not suffer from deteriorated lifespan or premature failure. The IEC60947 standard series defines the test regime for all low-voltage electrical apparatus.

Table 2 in IEC60947-1 states that 70°K temperature rise is permissible on silver- or nickel-plated terminals of low-voltage apparatus within an ambient of 35°C, thereby achieving the maximum permissible conductor temperature of 105°C. Once the ambient temperature within an enclosure is identified, the designer can select the conductor size from table B3 from AS60890- 2009 with confidence, knowing that the apparatus will perform as tested.

Conductor installation and insulation properties

The physical orientation of a conductor can affect its current-carrying capacity. For example, horizontally installed flat copper conductors tend to ‘trap’ hot air. In comparison, vertical flat copper conductors allow better airflow across the surface area. It is normal for the main connection tags of an air circuit breaker to operate at a lower temperature when mounted in a vertical configuration. Spacing between cables has a similar effect. Additional forms of mechanical protection, eg, conduits, create another thermal barrier to heat dissipation, thereby reducing the current-carrying capacity of the conductor.

Cable manufacturers offer a wide variety of insulation ratings for their cables. For example, V90 rated insulation allows a cable to operate at a higher temperature than a similarly sized cable with V75 insulation. The inference is that you can opt to use a smaller diameter cable for a similar load current. However, the switchboard designer must always consider the cable current-carrying capacities as listed in Table 6 of AS/NZS3008. This table specifies the cable cross-sectional areas that are used to complete the tests as defined in IEC60947 for low-voltage apparatus, while maintaining the maximum permissible temperature at the switchgear terminals. It is essential that the designer complies with the conductor cross-sectional area recommendation of this table to ensure the lifespan and intended operation of the apparatus, irrespective of the cable manufacturer’s insulation rating.

Eddy currents - high current installations

At very high currents, eg, 3500 A and above, circulating currents can be generated in closed paths within the body of a ferromagnetic enclosure, causing an undesirable heat loss. These eddy currents are created by the differences in potential existing throughout the body of the metallic enclosure owing to the action of the changing flux.

Experienced switchboard builders advise that greater clearances are required between current-carrying conductors and the enclosure openings between tiers for these applications. Ultimately, heat rise tests will determine how eddy currents affect the final design of a switchboard as this is not covered by the standard.

Conclusion

Copper has a broad range of uses in the modern world, and its value contribution to our way of life cannot be underestimated, particularly in power reticulation and communications. The significant increase in its commodity price has forced designers and engineers to look at economical solutions and methods of reducing the amount of copper used in installations and switchboards. While this is a necessity, it is essential that we abide by the Australian and International Standards to ensure that we do not compromise the integrity of a system design, and that the safety and longevity of an installation is maintained.