Page 965 - Industrial Power Engineering and Applications Handbook

Properties and ratings of current-carrying conductors 30/915
30.1 Properties and current ratings
for aluminium and copper Ultimate tensile
strength
conductors
I Yield point /
In Table 30.1 we provide the general properties of
aluminium and copper conductors. The table also makes
a general comparison between the two widely used metals
for the purpose of carrying current.
Elastic limit
Proportional limit
30.1.1 Important definitions of properties M
of a metal
For ease of application of the above table we give below
important definitions of the mechanical and electrical Strain -
properties of a metal.
* Tensile proof strength = 0.6 to 0.8
30.1.2 Physical and mechanical properties
Figure 30.1 Stress-strain curve
1 Specific heat (This is a physical property)
The specific heat of a substance is the ratio of the the stress-strain curve corresponding to a definite
heat required to raise the temperature of a certain amount of permanent set (elongation) of, say,
weight by 1 "C to that required to raise the temperature
of the same weight of water by 1°C. 0.1% or 0.2% of the test specimen.
2 Stresses
This is the force per unit area expressed in kgf/mm' 30.1.3 Electrical properties
and is represented in a number of ways, depending
upon the type of force applied, e.g. Resistivity of metal of a current-carrying
Tensile stress: the force that will stretch or lengthen conductor
the material and act at right angles to the area
subjected to such a force. A metal being used for the purpose of current carrying
Ultimate tensile strength: the maximum stress value must be checked for its conductivity. This is proportional
as obtained on a stress-strain curve (Figure 30.1). to its current-carrying capacity. This will ascertain the
Compressive stress: the force that will compress correctness of size and grade of the metal chosen for a
or shorten the material and act at right angles to the particular duty. It is necessary to avoid overheating of
area subjected to such a force. the conductor during continuous operation beyond the
Shearing stress: the force that will shear the material limits in Table 28.2. The electrical conductivity of a metal
and act in the plane of the area and at right angles is reciprocal to its resistivity. The resistivity may be
to the tensile or compressive stress. expressed in terms of the following units:
Modulus of elasticity (E): the ratio of the unit
stress to the unit strain within the proportional limits Volume resistivity or specific resistance: this is the
of a material in tension or compression. Refer to resistance of a conductor of unit length and unit cross-
Figure 30.1. sectional area, i.e.
- Proportional limit: the point on the stress-strain
curve at which will commence the deviation in
the stress-strain relationship from a straight line
to a parabolic curve (Figure 30.1). and lpuR.cm = 10' ~ R.mm'
Elastic limit: the maximum stress a test specimen m
may be subjected to and which may return to its Mass resistivity : this is the resistance of a conductor
original length when the stress is released. of unit length and unit mass;
- Yield point: a point on the stress-strain curve i.e. R . gm/m'
that defines the mechanical strength of a material which is also equal to the volume resistivity multiplied
under different stress conditions at which a sudden by the density:
increase in strain occurs without a corresponding i.e. (R m) x (gm/rn3) = R. gm/m'
increase in the stress (Figure 30.1).
- Yield strength or tensile proof stress: the Length resistivity: this is the resistance of a conductor
maximum stress that can be applied without per unit length, i.e. R/m.
permanent deformation of the test specimen. For Conductivity
the materials that have an elastic limit (some
materials may not have an elastic region) this Therefore, the electrical conductivity with reference to
may be expressed as the value of the stress on say, volume conductivity, can be expressed by