2.Copper for Busbar Purposes

 

Types of High Conductivity Copper available

Properties of Wrought HC Copper

 

In most countries, coppers of different types for specific applications have been given separate identities. In the United Kingdom this takes the form of an alloy designation number which is used in all British Standards relevant to copper and its alloys. Copper for electrical purposes is covered by the following British Standards:

BS 1432 : 1987 (strip with drawn or rolled edges)

BS 1433 : 1970 (Rod and bar)

BS 1434 : 1985 (Commutator bars)

BS 1977 : 1976 (High conductivity tubes)

BS 4109 : 1970 (wire for general electrical purposes and for insulated and flexible cords)

BS 4608 : 1970 (Rolled sheet, strip and foil)

(Copies of these are obtainable from the BSI Sales Office. 398 Chiswick High Road, London WS4 4AL.)

To bring the UK in line with current European requirements BS EN standards are being introduced.  The European Standards relevant to electrical applications are expected to supersede the British Standards in due course.

The current standards most relevant to busbar applications are BS 1432, BS 1433 and BS 1977 which specify that the end products shall be manufactured from copper complying with the following requirements:

Cu-ETP     Electrolytic tough pitch high conductivity copper CW004A (formerly C101)
Cu-FRHC
     Fire-refined tough pitch high conductivity copper CW005A (formerly C102)
Cu-OF
        Oxygen-free high conductivity copper CW008A (formerly C103)

European Standards EN1976 and EN1978 have replaced BS 6017:1981.  Table 5 shows the European material designations along with International Standards Organisation (ISO) and old British Standard designations.

Table 5 EN, BS and ISO designations for refinery shapes and wrought coppers

  Designation
Description ISO cast and wrought European Designation Former UK Designations
Electrolytic tough pitch high-conductivity copper Cu-ETP CW004A C101
Fire- refined tough pitch high-conductivity copper Cu-FRHC CW005A C102
Oxygen-free high-conductivity copper Cu-OF CW008A C103

Copper to be used for electrical purposes is of high purity because impurities in copper, together with the changes in micro-structure produced by working, materially affect the mechanical and electrical properties. The degree to which the electrical conductivity is affected by an impurity depends largely on the element present. For example, the presence of only 0.04% phosphorus reduces the conductivity of high conductivity copper to around 80% IACS. (The approximate effect on conductivity of various impurity elements is shown in Figure 1). The level of total impurities, including oxygen, should therefore be less than 0.1% and copper of this type is known as high conductivity (HC) copper.

Microscopic and analytical controls are applied to ensure a consistent product and in the annealed condition conductivities over 100% IACS are usual. This figure corresponds to the standard resistivity of 0.017241 mWm set some years ago by the International Electrotechnical Commission.

Figure 1 - Approximate effect of impurity elements on the electrical resistivity of copper

Types of High Conductivity Copper available

Tough pitch copper,CW004A and CW005A (C101 and C102 )

Coppers of this type, produced by fire-refining or remelting of electrolytic cathode, contain a small, deliberate addition of oxygen which scavenges impurities from the metal. It is present in the form of fine, well-distributed cuprous oxide particles only visible by microscopic examination of a polished section of the metal. Typical oxygen contents of these coppers fall in the range 0.02-0.05%. Between these limits the presence of the oxygen in this form has only a slight effect on the mechanical and electrical properties of the copper. It can, however, give rise to porosity and intergranular cracks or fissures if the copper is heated in a reducing atmosphere, as can happen during welding or brazing. This is a result of the reaction of the cuprous oxide particles with hydrogen and is known as 'hydrogen embrittlement'. Provided a reducing atmosphere is avoided, good welds and brazes can be readily achieved. (See Jointing of Copper Busbars.)

Oxygen-free high-conductivity copper, CW008A (C103)

In view of the above remarks, if welding and brazing operations under reducing conditions are unavoidable, it is necessary to use a different (and more expensive) grade of high conductivity copper which is specially produced for this purpose. This type of copper, known as 'oxygen-free high conductivity copper', is normally produced by melting and casting under a protective atmosphere. To obtain the high conductivity required it is necessary to select the best raw materials. The result is a high purity product containing 99.95% copper. This enables a conductivity of 100% IACS to be specified even in the absence of the scavenging oxygen.

Effects of hot and cold working on structures

In the as-cast form, HC copper is available in wirebar and billet form, although the advancement of modern casting technology is leading to a decline in wirebar production. The cast shape is hot-worked by rolling or extrusion to produce a form suitable for further processing by cold work into its final wrought form, either by rolling or drawing through dies.

In the case of tough-pitch HC copper, the as-cast structure is coarse-grained with oxygen present as copper-cuprous oxide eutectic in the grain boundaries. The hot working operation breaks up the coarse grains and disperses the cuprous oxide to give a uniform distribution of oxide particles throughout a new network of fine grains. In the case of oxygen-free HC copper, the hot working operation breaks up the coarse grains into a new network of fine grains.