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.
A conductor material should therefore have the following properties if it is to be produced efficiently and have low running costs from the point of view of energy consumption and maintenance:
a) Low electrical and thermal resistance
b) High mechanical strength in tension, compression and shear
c) High resistance to fatigue failure
d) Low electrical resistance of surface films
e) Ease of fabrication
f) High resistance to corrosion
g) Competitive first cost and high eventual recovery value
This combination of properties is met best by copper. Aluminium is the main alternative material, but a comparison of the properties of the two metals shows that in nearly all respects copper is the superior material.
Busbars can be sub-divided into the following categories, with individual busbar systems in many cases being constructed from several different types:
a) Air insulated with open phase conductors
b) Air insulated with segregating barriers between conductors of different phases.
c) Totally enclosed but having the construction as those for (a) and (b)
d) Air insulated where each phase is fully isolated from its adjacent phase(s) by an earthed enclosure. These are usually called 'Isolated Phase Busbars'.
e) Force-cooled busbar systems constructed as (a) to (d) but using air, water, etc. as the cooling medium under forced conditions (fan, pump, etc.).
f) Gas insulated busbars. These are usually constructed as type (e) but use a gas other than air such as SF6, (sulphur hexafluoride).
g) Totally enclosed busbars using compound or oil as the insulation medium.
The type of busbar system selected for a specific duty is determined by requirements of voltage, current, frequency, electrical safety, reliability, short-circuit currents and environmental considerations. Table 1 outlines how these factors apply to the design of busbars in electricity generation and industrial processes.
Table 1 Comparison of typical design requirements for power generation and industrial process systems
| Feature | Generation | Industrial Processes | |
| 1 | Voltage drop | Normally not important | Important |
| 2 | Temperature rise | Usually near to maximum allowable. Capitalisation becoming important. | In many cases low due to optimisation of first cost and running costs. |
| 3 | Current range | Zero to 40 k A a .c . with frequencies of zero to 400 Hz. | Zero to 200 kA a.c. and d.c. |
| 4 | Jointing and connections | Usually bolted but high current applications are often fully welded. Joint preparation very important | Usually bolted. Joint preparation very important. |
| 5 | Cross-sectional area | Usually minimum. Somewhat larger if optimisation is required. | Usually larger than minimum required due to optimisation and voltage drop considerations. |
| 6 | Kelvin's Law | Not applied. Other forms of optimisation are often used. | Applies. Also other forms of optimisation and capitalisation used |
| 7 | Construction | Up to 36 k V. Individually engineered using basic designs and concepts. | Usually low voltage. Individually engineered. Standard products for low current/voltage applications. |
| 8 | Enclosures | Totally enclosed with or without ventilation. | Usually open. Enclosed or protected by screens when using standard products. |
| 9 | Fault capacity | Usually large. Designed to meet system requirement. | Usually similar to running current. Standard products to suit system short circuit. |
| 10 | Phase arrangement | Normally 3 phase flat though sometimes trefoil. | Normally flat but transposition used to improve current distribution on large systems |
| 11 | Load factor | Usually high. Normally 1.0. | Usually high but many have widely varying loads. |
| 12 | Cost | Low when compared with associated plant. | Major consideration in many cases. Particularly when optimisation/capitalisation is used. |
| 13 | Effects of failure | Very serious. High energies dissipated into fault. | Limited by low voltage and busbar size. |
| 14 | Copper type | High conductivity. | High conductivity. |
15 Copper shape Usually rectangular. Tubular used for high current force-cooled. Usually large cross section rectangular. Tubular used for some low current high voltage applications and high current force-cooled.
At the present time the only two commercially available materials suitable for conductor purposes are copper and aluminium. The table below gives a comparison of some of their properties. It can be seen that for conductivity and strength, high conductivity copper is superior to aluminium. The only disadvantage of copper is its density; for a given current and temperature rise, an aluminium conductor would be lighter, even though its cross-section would be larger. In enclosed systems however, space considerations are of greater importance than weight. Even in open-air systems the weight of the busbars, which are supported at intervals, is not necessarily the decisive factor.
Table 2 Typical relative properties of copper and aluminium
| Copper(CW004A) | Aluminium (1350) | Units | |
| Electrical conductivity (annealed) | 101 | 61 | % IACS |
| Electrical resistivity (annealed) | 1.72 | 2.83 | mW cm |
| Temperature coefficient of resistance(annealed) | 0.0039 | 0.004 | /° C |
| Thermal conductivity at 20°C | 397 | 230 | W/mK |
| Coefficient of expansion | 17 x 10–6 | 23 x 10–6 | /° C |
| Tensile strength (annealed) | 200 – 250 | 50 – 60 | N/mm2 |
| Tensile strength (half–hard) | 260 – 300 | 85 – 100 | N/mm2 |
| 0.2% proof stress (annealed) | 50 – 55 | 20 – 30 | N/mm2 |
| 0.2% proof stress (half–hard) | 170 – 200 | 60 – 65 | N/mm2 |
| Elastic modulus | 116 – 130 | 70 | kN/mm2 |
| Specific heat | 385 | 900 | J/kg K |
| Density | 8.91 | 2.70 | g/cm3 |
| Melting point | 1083 | 660 | °C |
The electromagnetic stresses set up in the bar are usually more severe than the stress introduced by its weight. In particular, heavy current-carrying equipment necessitates the use of large size conductors, and space considerations may be important. It should be realised that the use of copper at higher operating temperatures than would be permissible for aluminium allows smaller and lighter copper sections to be used than would be required at lower temperatures.
The ability of copper to absorb the heavy electromagnetic and thermal stresses generated by overload conditions also gives a considerable factor of safety. Other factors, such as the cost of frequent supports for the relatively limp aluminium, and the greater cost of insulation of the larger surface area, must be considered when evaluating the materials.
From published creep data, it can be seen that high conductivity aluminium exhibits evidence of significant creep at ambient temperature if heavily stressed. At the same stress, a similar rate of creep is only shown by high conductivity copper at a temperature of 150°C, which is above the usual operating temperature of busbars.
Table 3 Comparison of creep and fatigue properties of high conductivity copper and aluminium
a) Creep properties
| Material | Testing Temp. °C | Min. Creep Rate % per 1000 h | Stress N/mm2 |
| Al (1080) annealed | 20 | 0.022 | 26 * |
| HC Cu annealed | 150 | 0.022 | 26 * |
| Cu-0.086% Ag 50% c.w. | 130 | 0.004 | 138 |
| Cu-0.086% Ag 50% c.w. | 225 | 0.029 | 96. |
5
* Interpolated from fig.3
b) Fatigue properties
| Material | Fatigue strength N/mm2 | No. of cycles x 106 | |
| HC Al | annealed | 20 | 50 |
| half-hard (H8) | 45 | 50 | |
| HC Copper | annealed | 62 | 300 |
| half-hard | 115 | 300 |
If much higher stresses or temperatures are to be allowed for, copper containing small amounts (about 0.1%) of silver can be used successfully. The creep resistance and softening resistance of copper-silver alloys increase with increasing silver content.
In the conditions in which high conductivity aluminium and copper are used, either annealed (or as-welded) or half-hard, the fatigue strength of copper is approximately double that of aluminium. This gives a useful reserve of strength against failure initiated by mechanical or thermal cycling.
The greater hardness of copper compared with aluminium gives it better resistance to mechanical damage both during erection and in service. It is also less likely to develop problems in clamped joints due to cold metal flow under the prolonged application of a high contact pressure. Its higher modulus of elasticity 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.
Because copper is less prone to the formation of high resistance surface oxide films than aluminium, good quality mechanical joints are easier to produce in copper conductors. Welded joints are also readily made. Switch contacts and similar parts are nearly always produced from copper or a copper alloy. The use of copper for the busbars to which these parts are connected therefore avoids contacts between dissimilar metals and the inherent jointing and corrosion problems associated with them.
The higher melting point and thermal conductivity of copper reduce the possibility of damage resulting from hot spots or accidental flashovers in service. If arcing occurs, copper busbars are less likely to support the arc than aluminium. Table 4 shows that copper can self-extinguish arcs across smaller separations, and at higher busbar currents. This self-extinguishing behaviour is related to the much larger heat input required to vaporise copper than aluminium.
Table 4 Self-extinguishing arcs in copper and aluminium busbars
| Copper | Aluminium | |
| Minimum busbar spacing, mm | 50 | 100 |
| Maximum current per busbar, A | 4500 | 3220 |
Copper liberates considerably less heat during oxidation than aluminium and is therefore much less likely to sustain combustion in the case of accidental ignition by an arc. The large amounts of heat liberated by the oxidation of aluminium in this event are sufficient to vaporise more metal than was originally oxidised. This vaporised aluminium can itself rapidly oxidise, thus sustaining the reaction. The excess heat generated in this way heats nearby materials, including the busbar itself, the air and any supporting fixtures. As the busbar and air temperatures rise, the rates of the vaporisation and oxidation increase, so accelerating the whole process. As the air temperature is increased, the air expands and propels hot oxide particles. The busbar may reach its melting point, further increasing the rate of oxidation and providing hot liquid to be propelled, while other materials such as wood panels may be raised to their ignition temperatures. These dangers are obviated by the use of copper busbars.
Finally, copper is an economical conductor material. It gives long and reliable service at minimum maintenance costs, and when an installation is eventually replaced the copper will have a high recovery value. Because of its many advantages, copper is still used worldwide as an electrical conductor material despite attempts at substitution.
