Copper wires resist mechanical creeping

Copper wires resist the mechanical deformation (creep) all metals experience when subjected to stress, and are therefore safer and more reliable

Creep is the plastic deformation of metal conductors that occurs when these are subjected to an external stress (pulling force). Creep depends on the stress level, its duration and the temperature, and is different for each metal. Creep is irreversible, unlike elongation which is elastic and reverses as soon as the external force is removed.

Why is creep an issue?

Creep is an issue for insulated electrical wires and cables as these are subjected to external stresses through e.g. a) pulling during cable laying b) thermal cycles and c) electromagnetic forces.

Creep is also an issue at terminations, where a screw can squeeze and deform the conductor, reducing the contact pressure and leading to increased joint resistance, a loose connection, oxidation, arcing and overheating.

Similarly creep is an issue for enameled wires, for example those used in distribution transformers. Frequent energization of distribution transformers is unavoidable in weak power networks and power deficient areas, due to various reasons such as extension and maintenance works, scheduled and unscheduled load shedding, faults, and the power demand exceeding generation. Distribution transformers experience large magnetic inrush currents when switched on, which can be several times the rated operating current. These currents produce excessive mechanical stresses and intense localized heating (hot spots). Each time this occurs, the winding conductors creep by a small amount. Over a period of years, this repeated creeping causes the winding conductors to bunch closer to each other than the allowable minimum safety clearances specified by standards. If the elongation in the wire is more than the allowable gap between two layers of the winding then an insulation rupture occurs leading to an inter-turn short-circuit and thus to a failure of the transformer.

Creep rate comparison – copper vs. aluminium

From published creep data, it is seen that a high-conductivity aluminium wire (Al 1030, annealed) exhibits a significant creep rate of 0.022 % per 1000 hours under a stress of 26 N/mm2 at an ambient temperature of 200C. By comparison, a high conductivity copper wire has a negligible creep rate under identical conditions. For copper to exhibit a creep of 0.022 %, the temperature would have to be as high as 150°C, which is well above normal operating conditions. Oxygen free copper creeps at an even lower rate. Comparative testing carried out by Powertech Laboratories Inc. has determined that while the 8000 series aluminium alloys have an improved creep performance as compared to the 1000 series of aluminium, these still lag the performance of copper. Further, there are wide variations among the 8000 series alloys procured from different sources and some of these were found to be even worse than the 1000 series.

Therefore, due to its superior creep resistance, conductors made of copper are safer and more reliable.

References

  1. High Conductivity Copper for Electrical Engineering, ECI publication no. Cu0232, Feb 2016 
  2. CONNECTABILITY TESTING OF COPPER AND ALUMINUM WIRING, PHASE 2:
  3. Current Cycling Tests – Mechanical Connectors - POWERTECH LABS INC. Report #: PL-00236-REP1 , 27 June 2014.   https://www.youtube.com/watch?v=ninqsZihz7g
  4. Reliability of power connections, BRAUNOVIC Milenko, Journal of Zhejiang University SCIENCE A,ISSN 1009-3095 (Print); ISSN 1862-1775 (Online), January 2007
  5. Creep Life Assessment of Distribution Transformers, N.S. Beniwal, D.K. Dwivedi, H.O. Gupta, Engineering Failure Analysis, volume 17, Issue 5, July 2010, pages 1077-1085.
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