Chapter # 1                      

CONDUCTOR MATERIAL                      

1.2 Copper

          Copper occurs in nature in a usable form and has been shaped and used for the past 10,000 years.  One of the jewelry pieces made of Copper, found in modern Iraq dates back to 8700 B.C! The metal has been extracted from sulphide ore since 5000 B.C.  Modern-day Copper is extracted predominantly from a sulphide ore or an oxide ore.  The process comprises:

Mining              Crushing              Concentration              Smelting              Refining

          After crushing, the concentration of ore is carried out by the froth flotation process.  The concentrate is subjected to smelting to form matte.  The matte is subsequently converted to blister Copper.  The purity of blister Copper is increased by fire refining.  It is further refined electrolytically to high purity Copper.  The refined Copper is 99.99 % pure and marketed in sheets of a thickness of 1 mm and a size of a 1 meter square.

          Most of the manufacturers of conductors use the wire bars, cathodes, or high purity scrap as the input raw material. These are melted in a controlled atmosphere and extruded in stages, to specified dimensions.  The extruded busbars are then heat-treated for the desired temper (hardness).

          

          Copper to be used for electrical purposes should be of high purity and good conductivity.  It is important to note that the conductivity of Copper is highly sensitive to the impurities.  A very small percentage of specific impurity will drastically alter the resistivity and consequently, the conductivity of Copper.  Therefore, the conductivity of Copper cannot have a direct correlation to its purity.

          It is observed that a very minuscule impurity of Titanium causes a very significant reduction in the conductivity while an impurity of Silver will marginally improve the conductivity.  The presence of Silicon and Iron that are abundant in nature, have a very adverse effect on the conductivity of Copper.

          In 1913 International Electro-Technical Commission defined International Annealed Copper Standard (IACS) to have 100% conductivity with the following properties at 20 ⁰C / 68 ⁰F.

          High Conductivity Copper is classified as under:

C101 or Cu – ETP               C102 or Cu – FRTP               C103 or Cu – OF

C 101 & C 102 – Tough Pitch HC Copper

          Tough pitch copper is produced by re-melting (or fire refining) electrolytic cathodes manufactured by the primary metal extraction plant.  A small amount of Oxygen is introduced to scavenge the impurities.  Oxygen manifests itself as uniformly dispersed cuprous oxide particles.  The Oxygen content is of the order of 0.02 to 0.05 %.  However, its presence can give rise to porosity if heated in a reducing atmosphere, as during welding.  This phenomenon is called Hydrogen embrittlement.  Tungsten inert gas or metal inert gas welding, in a reduction-free atmosphere, ensure welding, free of porosity, and fissures. 

C 103 – Oxygen-Free HC Copper

          Oxygen-free Copper is produced by melting and casting under a controlled atmosphere, a pure form of raw material.  It results in a purity of 99.95% Copper, with a conductivity of a minimum, 100% IACS.

Properties of Copper

          High conductivity Copper is available in wire-bar or cast billet form.  These are hot rolled or hot extruded to make them suitable for further processing by cold rolling or extrusions. Cold working has a significant effect on the mechanical and electrical properties.

          Hardness, tensile strength, and 0.2% Proof Strength increases and elongation decreases with cold working (expressed as a reduction of thickness by rolling / extrusion).  The increase in hardness will not be uniform across the cross-section of the conductor. Near the core, it is likely to remain soft.  Cold worked Copper will soften when subjected to elevated temperatures. Depending upon the cross-section of the conductor, preheating may be required, before taking up welding. Such an operation will significantly reduce hardness.  The heating of busbars may occur due to bad joint contacts.  It can also occur during a fault when bus cross-sections are close to the minimum requirements for a specified short time current rating. 

          Cold working on Copper can reduce the conductivity by up to 3%.

          It is not necessary to specify a high threshold value of conductivity (say 100%) for the conductors.  As has been mentioned, cold working can reduce the conductivity significantly.  The conductivity of the busbar must be guaranteed for the installed conditions.  It may, therefore, be prudent to specify an achievable threshold value and factor the same in the design.  While very high conductivity requirements may be necessary for certain critical applications, it is not a must for most of the applications of Busbar Systems.  Conductivity may be specified even marginally less than 100%.  The increase in cost may be disproportionate to the benefits of such requirements.  As will be seen later, better configuration and shape could bring out a good design and effective use of the material, than emphasizing on its conductivity.

Continued..........