Everything you need to know about copper busbars
03 July 2014
Authors: David Chapman and Prof Toby Norris
Busbars are used within electrical installations for distributing power from a supply point to a number of output circuits. They may be used in a variety of configurations ranging from vertical risers and carrying current to each floor of a multi-storey building, to bars used entirely within a distribution panel or within an industrial process.
The issues that need to be addressed in the design of busbar systems include temperature rise due to energy losses; energy efficiency and lifetime cost; short-circuit current stresses and protection; jointing methods and performance; and maintenance.
With regard to materials for busbars, the properties of a conductor material are essential to achieving a long and reliable service life at the lowest lifetime cost. These include low electrical and thermal resistance; high mechanical strength in tension, compression and shear; high resistance to fatigue failure; low electrical resistance of surface films; ease of fabrication; high resistance to corrosion; and competitive first cost and high eventual recovery value.
As a result, busbars are generally made from either copper or aluminium. For conductivity and strength, high conductivity (HC) copper is far superior to aluminium. The only disadvantage of copper is identified at its higher density, which results in higher weight.
The greater hardness of copper compared with aluminium gives it better resistance to mechanical damage, both during erection and in service. Copper bars are also less likely to develop problems in clamped joints due to cold metal flow under the prolonged application of a high contact pressure.
The higher modulus of elasticity of copper gives it greater beam stiffness compared with an aluminium conductor of the same dimensions. The temperature variations encountered under service conditions require a certain amount of flexibility to be allowed for in the design. The lower coefficient of linear expansion of copper reduces the degree of flexibility required.
There are, however, inherent jointing and corrosion problems associated with contacts between dissimilar metals: for example between an aluminium busbar and terminals and switch contacts, which are generally always produced from copper or a copper alloy. The root of the problem is that an exposed aluminium surface rapidly forms a hard insulating film of aluminium oxide. On the contrary, the oxide film that forms on the surface of copper is conductive, which is another reason for using copper as material for busbars rather than aluminium.
CURRENT-CARRYING CAPACITY OF BUSBARS
The current-carrying capacity of a busbar is limited by the maximum acceptable working temperature of the system. The bar is heated by the power dissipated in it by the load current flowing through the resistance, and cooled by radiation to its surroundings and convection from its surfaces. At the working temperature, the heat generation and loss mechanisms, which are highly temperature and shape dependent, are balanced.
The heat generated in a busbar can be dissipated through convection, radiation and conduction. In most cases, convection and radiation heat losses determine the current-carrying capacity of a busbar system. In a simple busbar, conduction plays no part since there is no heat flow along a bar of uniform temperature.
Conduction is only taken into account where a known amount of heat can flow into a heat sink outside the busbar system, or where adjacent parts of the system have differing cooling capacities. Conduction may be important in panel enclosures. There is also an extensive range of shape and proximity factors for typical configurations, such as single solid rods, single tubes, rectangular bars and parallel bars, when estimating the working current and temperature of busbars.
In the context of electrical installation design, life-cycle costing (LCC) is used to inform design choices. It means that all the costs, including investment cost, running costs and end-of-life costs, are compared for design options. Since the objective is to minimise the lifetime cost rather than to determine it, everything that remains the same for each design option can be ignored, simplifying the analysis and allowing greater focus on the differences.
The problem in assessing the lifetime cost is that the setting up costs occur at the beginning of the project while the costs of energy losses occur over a long period of time, so it is necessary to add together costs which arise, and have to be paid for, at different times. This is where the ‘time value’ of money must be included in the calculation.
When it comes to calculations of LCC of electrical installations, factors must be considered such as the covering installation design, installation costs, recurring costs, maintenance costs, energy and end-of-life costs.
Like all electrical circuits, busbars need to be protected against the effects of short-circuit currents. The open construction of busbars increases the risk of faults, for example by the ingress of foreign bodies into air gaps, and the risk of consequent damage is high due to their high normal operating currents and the amount of energy available.
Very high currents lead to rapid and extreme overheating of the bars with consequent softening of the material and damage to the support structure. At the same time, the electromagnetic forces generated will distort the softened conductors that may break free from their supports. Resonance effects may make the situation worse.
In practice, what is important is that the final temperature of the bar remains lower than the limiting design temperature throughout the short-circuit event. The limiting temperature for copper busbars is determined by the temperature resistance of the support materials but should not exceed ~200°C. The maximum circuit breaker tripping time is 200/Dtr seconds.
An important remark is that busbars that have been subject to short-circuit should be allowed to cool and inspected before being returned to service to ensure that all joints remain tight and that the mountings are secure. Although the heating time – the duration of the fault – is quite short, the bar will remain at high temperature for a considerable length of time. Also, because of the very high thermal conductivity of copper, parts of the bar beyond the fault will also have become hot.
Busbar profiles are being increasingly used in distribution panels and switchboards, where the design considerations are significantly different from long vertical and horizontal busbars. This is a growing market because busbar profiles offer distinct advantages, such as material savings, lower assembly time, reduced complexity and lower scrap.
Having said that, profiles are more difficult and costly to manufacture than flat bars due to technical constraints, including more complex design, more elaborate machine set-up, more delicate production process, and more complicated packaging. However, these constraints are now mostly under control and currently several fabricators produce hundreds, or even thousands, of different shapes for electrical applications. Many electrical parts, which are often cut from sheet, can be economically produced by slicing a profiled bar.
There are a number of advantages associated with using profiles, such as reduced skin effect, weight and cost savings, integrated fixings and mountings, retention of intellectual integrity, and the economic benefits achievable. In practice, during manufacturing process, there are some key design problems to avoid, such as sharp corners, deep narrow channels and hollow chambers.
With regard to the jointing of copper busbars, there are two types of joints: linear joints required to assemble manageable lengths into the installation; and T-joints required to make tap-off connections. Joints need to be mechanically strong, resistant to environmental effects and have a low resistance that can be maintained over the load cycle and throughout the life of the joint.
Efficient joints in copper busbar conductors can be made very simply by bolting, clamping, riveting, soldering or welding. Bolting and clamping are used extensively on-site. Shaped busbars may be prefabricated by using friction stir welding. The required bolting arrangements should always be calculated for the circumstances of the installation.
Degradation mechanisms must also be considered (oxidation, corrosion, fretting, creep and stress relaxation, thermal expansion) and the quality of busbar joints is crucial to the long-term reliability of a busbar system. It is therefore important to take care over the choice of joint design, the tightening torques, bolt types and the effect of temperature to ensure reliability. In-service maintenance should ideally include thermal imaging of joints so that any problems can be found before failure occurs.
The Copper Development Association first published its Copper Busbars: Guidance for Design and Installation back in 1936. Its latest edition includes a significant new chapter on busbar profiles, and simplified formulae for busbar configurations. It will be of particular benefit to design engineers of electrical distribution systems seeking to design efficient, economic and reliable busbar systems. It is now available for free download and is a complete overhaul of previous editions.
The authors have greatly simplified the calculation of current-carrying capacity of busbars by providing exact formulae for some common busbar configurations and graphical methods for others. The new chapter on profiles addresses the increasing trend to use busbar profiles in distribution panels and switchboards, and addresses the design considerations for these applications. Many other sections have been significantly updated and modified to reflect current practice.
Authors of the book
David Chapman was the electrical programme manager for the Copper Development Association in the UK, where his main interests included power quality and energy efficiency. He was an author and chief editor of the LPQI Power Quality Application Guide.
Prof Toby Norris is an electrical engineer who has worked in industry and at university. A central interest has been electromagnetic theory, especially in electric power plant and including superconductors.
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