Copper has high yield strength

Copper conductors can withstand higher pulling forces than aluminium conductors without necking or breaking

When a pulling force (tensile stress) is applied to an electrical conductor, it becomes marginally longer. In the elastic range of deformation, the conductor returns to its original length and shape as soon as the tensile stress is removed. However, as the tensile stress becomes higher, there comes a point beyond which some fraction of the deformation is permanent, and does not reverse. This is the elastic limit called the yield point and the stress at which it occurs is defined as the yield stress or yield strength of the material. In effect, the yield strength is the maximum design stress a conductor can be subjected to. Since yield points are difficult to determine exactly, a practical engineering measure for comparing conductors is 0.2% proof stress which is the stress required to produce a permanent 0.2% plastic lengthening of the conductor.

To complete the picture, if the tensile stress is raised still further, the cross-sectional area of the conductor reduces at its weakest point and it begins to neck and finally break when the ultimate tensile strength (UTS) also known simply as tensile strength is reached.  Stress is measured as force per unit cross sectional area and the metric unit is Newtons per square mm.

Referring to the table below, high conductivity copper withstands between 3 to 6 times higher 0.2% proof stress than low alloyed aluminium in the annealed condition ( annealing is the process of heat treatment to soften a metal after cold working), thus enabling copper conductors to withstand higher pulling stresses without necking or breaking. These values would vary with the extent of cold working and alloying, but copper’s performance is superior under all comparable conditions.

Characteristic

 

Copper

(ETP Grade)

Aluminium

( 99.5%)

Tensile Strength Annealed (N/mm2)

200

50 - 60

0.2% Proof Stress Annealed (N/mm2)

≤ 120

20 – 30

It is this physical property and copper’s inherent flexibility that allows for copper cables to be easily drawn through conduits and trunking with minimal risk of necking, stretching or breaking when mechanical cable pulling is employed.  Conversely, while aluminium cabling exhibits a marginal weight advantage over copper for equal ampacity and the mechanical forces required for pulling in may be lower to this extent for straight runs with low friction, these are not low enough to mitigate the large differences in proof stress. Therefore, when long runs of aluminium conductor cables are pulled through containment systems, and subjected to high pulling forces, these can stretch and “neck-down”, reducing the current carrying capacity of the cables which may result in dangerous overheating. In extreme cases, mechanical drawing in of aluminium conductor cables over long or multidirectional routes can even result in irreparable physical damage. Thankfully, copper cables do not share this risk.

References

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