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| Copper Tube |
Copper tube possesses many physical characteristics which make it desirable as an electrical conductor. These may be listed in order of diminishing importance as follows:
Copper tube, however, has several disadvantages. Its rigidity limits its being bent in sharp changes of direction, which necessitates the use of additional connectors. Its expansion and contraction, due to changes in temperature, must be absorbed by flexible connections or by additional tube, so shaped to allow for expansion and contraction. For substation construction copper tube, for its many advantageous physical properties, is the most widely used conductor. Its few handicaps are overcome by the ever increasing number of various types of connectors designed to handle difficult connector problems. |
| ALUMINUM TUBE |
The aluminum tubular conductor possesses almost all of the physical characteristics of the copper tubular conductor which make it a desirable conductor. By nature of its shape it possesses a high rigidity, a low impedance to the flow of alternating current and low corona discharge at high voltages. Volume for volume aluminum has a lower conductivity than copper, but on a weight basis aluminum has approximately twice the conductivity of copper. By virtue of its rigidity and low unit weight, aluminum tube enables the use of long spans with minimum deflections. Its principal disadvantages are low tensile strength and conductivity both of which necessitate larger conductors to develop equivalent strength and conductivity. |
| STEEL TUBE |
| Galvanized steel tubular conductors, because of their high strength and low cost, are employed to a limited extent for substation buses. They are rarely employed in humid climates in vicinities of industrial sections or the seashore, for in such areas the galvanized coating affords little protection to the steel bus which soon deteriorates after the galvanized coating has been corroded away. Also making it limited in its use is its low conductivity and magnetic nature which set up high power loss through the I²R losses and hysteresis. |
| CURRENT CARRYING CAPACITY |
| The current carrying capacity of any bus most generally is determined by
the amount of current the conductor will accommodate for a given temperature rise.
Although from an economical consideration the current carrying capacity may be determined
by its energy or power loss. In determining the current carrying capacity by
temperature rise, a 30° C rise over a maximum ambient temperature of 40° C is usually
taken as a basis. Current carrying capacity may therefore be considered a function
of temperature rise. From this it can be seen that the greater the ability to dissipate
heat the greater the current carrying capacity. For indoor installation where there is no
natural movement of air, the ability of the conductor to dissipate heat is solely
dependent upon radiation and convection currents of the air set up by the bus.
However, for outdoor installations where there are natural air currents and wind, the
ability of the conductor to dissipate its heat to the surroundings is considerably
greater. For this effect, outdoor conductors depending on their size, are rated from 15 to
25% higher in capacity. When an increased carrying capacity is desired with tubular conductors, a gain in current carrying capacity is frequently obtained by dissipating the heat generated through power losses by setting up forced ventilation through the inside of the bus. |