Current carrying capacity

Aluminum is an excehent conductor of electricity, having a volume conductivity 62% of that of copper. Because of the difference in densities of the two metals, an aluminum conductor weighs only half as much as a copper conductor of equal current carrying capacity. Because of its lightness, aluminum... 

Aluminium and copper conductors start oxidizing at about 90°C. The oxides of aluminium (AI2O) and copper (CuO) are poor conductors of electricity. They may adversely affect bus conductors, particularly at joints, and reduce their current-carrying capacity over time, and lead to their overheating, even to an eventual failure. Universal practice therefore is to restrict the operating temperature... 

Since the skin effect results in an increase in the effective resistance of the busbar system it directly influences the heating and the voltage drop of the conductor and indirectly reduces its current-carrying capacity. If is the resistance as a result of this effect then the heat generated... 

Current-carrying capacity of copper and aluminium conductors 30/916... 

A metal being used for the purpose of current carrying must be checked for its conductivity. This is proportional to its current-carrying capacity. This will ascertain the correctness of size and grade of the metal chosen for a particular duty. It is necessary to avoid overheating of the conductor during continuous operation beyond the limits in Table 28.2. The electrical conductivity of a metal is reciprocal to its resistivity. The resistivity may be expressed in terms of the following units ... 

The resistivity and conductivity of standard annealed copper and a few recommended aluminium grades being used widely for electrical applications are given in Table 30.1. Their corresponding current-carrying capacities in percent, with respect to a standard reference (say, 100% lACS) are also provided in the table. 

We can derive the same inference from Tables 30.2, 30.4 and 30.5, specifying current ratings for different cross-sections. The current-carrying capacity varies with the cross-section not in a linear but in an inconsistent way depending upon the cross-section and the number of conductors used in parallel. It is not possible to define accurately the current rating of a conductor through a mathematical expression. This can be established only by laboratory tests. 

Grounding plates or lattices made of pure copper, while displaying good current-carrying capacities, do not provide a particularly low resistance due to the depth at which they can be buried. The third alternative is to bury lengths of copper tape around the installation. The use of reinforced concrete foundations for grounding electrodes has also recently been considered. 

Current-carrying capacity (A) Weight (kg/1000 m) Overall diameter (mm) Current-carrying capacity (A) Weight (kg/1000 m) Overall diameter (mm)... 

Electrical power distribution within an industrial installation is most often at a voltage up to and including 33 kV. This section describes the types of cable suitable for power circuits for use up to 33 kV and considers the factors, which will influence the current-carrying capacity of such cables. 

Material used for conductors comprise copper or aluminum in either stranded or solid form. Copper is the most common type of conductor due to its good conductivity and ease of working. Despite having a conductivity of only 61 per cent of that copper, aluminum can be used as the conductor material. The lower density of aluminum results in the weight of an aluminum cable offsetting, to a certain extent, that of the additional material necessary to achieve the required current-carrying capacity. 

The current-carrying capacity under full-load conditions ... 

If one of the structures to be bonded is the sheath or metallic armouring of an electric supply cable, special precautions will be necessary to ensure that the voltage rise at the bond in the event of an instantaneous earth fault on the power-supply system does not endanger personnel or equipment associated with other buried structures. The bond and any associated current-limiting device should be suitably insulated and of adequate current-carrying capacity. 

The current-carrying capacity of the wire is not directly related to the dielectric. This is determined by the conductor resistance and the heating effect that it produces in the wire. The required current-carrying capacity determines the size of the wire and thus the size of the insulator. The temperature rise caused by the current flow determines the type of insulation to be used. If the wire is limited to 140°F (60°C) service, the insulation can be one of those discussed above. If the wire is to operate at 300°F (150° C), another specification for plastic wire with better heat resistance such as TP polyester or PTFE is used. 

The electrical conductance of a solution is a measure of its current-carrying capacity and is therefore determined by the total ionic strength. It is a nonspecific property and for this reason direct conductance measurements are of little use unless the solution contains only the electrolyte to be determined or the concentrations of other ionic species in the solution are known. Conductometric titrations, in which the species of interest are converted to non-ionic forms by neutralization, precipitation, etc. are of more value. The equivalence point may be located graphically by plotting the change in conductance as a function of the volume of titrant added. 

A variety of buffers is used in electrophoresis. The selected buffer must contain ions to carry the current. Other than current-carrying capacity, the most critical criterion for buffer selection is the stability of the sample to be analyzed. Many proteins are unstable in acidic pHs, so alkaline buffers are frequently employed. Tris-(hydroxymethyl)amino methane (TRIS or THAM), sodium acetate, and ethylenedi-aminetetraacetate (EDTA) are common solutes in buffers, with pHs between 7.9 and 8.9 typical. (Refer to Chapter 5 for a discussion of buffers.) These buffers also work well with nucleic acid fragments. In addition, phosphate buffers, e.g., 10 mMK3P04, are often used with nucleic acid fragments (1.0 mM = 0.0010 M). 

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