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Thermal Expansion’s effect on busbar 2013-08-02

The word busbar, derived from the Latin word omnibus ('for all'), gives the idea of a universal system of conveyance. In the electrical sense, the term bus is used to describe a junction of circuits, usually in the form of a small number of inputs and many outputs. 'Busbar' describes the form the bus system usually takes, a bar or bars of conducting material. In any electrical circuit some electrical energy is lost as heat which, if not kept within safe limits, may impair the performance of the system. This energy loss, which also represents a financial loss over a period of time, is proportional to the effective resistance of the conductor and the square of the current flowing through it. A low resistance therefore means a low loss; a factor of increasing importance as the magnitude of the current increases. The capacities of modern-day electrical plant and machinery are such that the power handled by their control systems gives rise to very large forces. Busbars, like all the other equipment in the system, have to be able to withstand these forces without damage. It is essential that the materials used in their construction should have the best possible mechanical properties and are designed to operate within the temperature limits laid down in BS 159, BS EN 60439-1:1994, or other national or international standards.

If the changes in length that occur in a conductor as it expands and contracts with temperature variations are not allowed for, undue forces will be set up in the conductor support system or in the equipment to which the busbar is connected. The coefficient of linear expansion for copper may be taken as 17 x 10–6 /°C (for temperatures from ambient up to 200°C) compared with 23 x 10–6 /°C for aluminium. The lower value for copper is of great importance when allowing for thermal expansion under both normal and transitory conditions, as up to 25% less expansion need be accommodated for a particular length of busbar. If a length of copper bar were to be kept from expanding or contracting, a force of nearly 2 N per mm2 of cross-sectional area would be developed for a temperature change of 1°C. In most cases the supports expand far less due to much smaller temperature changes and lower thermal expansion coefficients. It is therefore normal practice to allow for the full expansion using flexible conductor connections at suitable points.