Page 200 - Handbook of Electrical Engineering

184 HANDBOOK OF ELECTRICAL ENGINEERING

9.1.1 Copper and Aluminium

Copper and aluminium are used in their highly refined form for the power conductors of cables. The
total impurities contained in high conductivity copper should be less than 0.1% and for aluminium
less than 0.5%. The measured conductivity of these metals will have its highest value when they
are annealed. Hard drawn conductors will have conductivity that is several percentage points lower
than the annealed value. Note that castings made of these materials will generally have conductivity
slightly lower than their rolled and drawn forms.
The presence of oxygen in the form of oxides is the most common impurity. It slightly reduces
the conductivity, malleability and ductility of the metal.
Table 9.1 shows some of the electrical and physical properties of these two metals. For use in
most power cable applications these metals are formed into annealed conductors.
Copper is generally the preferred material for cable conductors used in the oil industry. Alu-
minium is seldom chosen for conductors. It is sometimes used for the armouring of single-core
cables that carry AC, or DC, if a substantial AC ripple is present e.g., DC, motors fed from a
thyristor controlled power source.

9.1.2 Tin

Tin metal is occasionally specified to provide a thin layer on the outer surface of copper conductors.
Historically this tin layer gave protection against corrosion of the copper surface from rubber insu-
lation, which contained substances such as sulphur. Large proportions of sulphur were added during
the vulcanising process to increase the hardness and tensile strength of the rubber. The modern use
of ‘plastic’ insulation instead of rubber compounds means that the layer of tin is no longer required
in most applications. Tin is beneficial in situations where soldered lugs are used to terminate the
copper conductors, although modern methods of crimping lugs onto their conductors has tended to
make the use of tin unnecessary.

Table 9.1. Electrical and physical properties of cable conductors
Property Copper Aluminum
Annealed Hard drawn Annealed Hard drawn
◦
Resistivity at 20 C 1.72 1.78 to 1.80 2.80 2.83
−8
(ohm-m × 10 )
Temperature coefficient of 0.00393 0.00393 0.00403 0.00403
◦
electrical resistance at 20 C, in
per unit of constant mass
Coefficient of linear expansion per 17.0 × 10 −6 17.0 × 10 −6 23.0 × 10 −6 23.0 × 10 −6
◦
unit per C
Approx. 0.1% proof stress, tons/sq 4.0 20.0 2.0 10.0
inch
Thermal conductivity W/mK 384.0 384.0 209.4 209.4
Density kg/m 3 8.89 × 10 −3 8.89 × 10 −3 2.71 × 10 −3 2.71 × 10 −3
Specific heat kJ/kg K 0.394 0.394 0.904 0.904
Modulus of elasticity lbs/sq inch – 18.0 9.6 10.0