Copper conductors withstand short-circuits better

Copper conductors retain adequate mechanical strength to be able to withstand the large electromagnetic forces during short-circuits in spite of the intense heating

When a short-circuit occurs, electrical conductors carry a current which is a large multiple of the normal operating current (usually in tens of kilo-amperes) until the operation of the protective device (usually within tens of milliseconds). Because the time duration is so small, there is no significant cooling effect and all the heat generated by the current flow is retained in the conductor, i.e. the heating is adiabatic. This leads to a rapid rise in the temperature, which although within the permissible design limit for the material (2000C for copper), results in a temporary but reversible deterioration of the mechanical properties of the conductor, such as yield strength, creep resistance, etc due to softening of the material.

Concurrent with this intense heating, conductors carrying these large short-circuit currents are subjected to mechanical stresses caused by the equally huge magnetic forces produced by the currents. The magnetic forces can be of repulsion between conductors in adjacent phases or of attraction if the conductors have been electrically paralleled in the same phase for increased ampacity. These forces result in bending stresses on rigid conductors, tension stresses and deflection in flexible conductors and bending, compression or tension loads on the insulators, support structures, terminations and apparatus. When alternating currents are flowing, the forces have a steady component, but also a vibrational component at twice the frequency of the alternating current. In the special case of busbars mounted on supports, the unidirectional component of the forces, exacerbated by the vibrational component, can lead to permanent bending and distortion of the bars or damage to, and even breakage of supports. Further, each busbar section has a resonant frequency. If this frequency is close to twice the supply current frequency (or any significant harmonic current), then resonant vibration of these beams may occur, leading to even higher vibrational displacements and possibly to metal fatigue or loosening of joints and connections.

In order to be able to withstand these extreme stresses during short-circuits without any lasting damage to itself or to other equipment, the conductor material must resist softening and retain its mechanical properties such as yield strength, creep resistance, etc at a sufficiently high level in spite of the elevated temperature. High conductivity copper fulfils these requirements and does not soften significantly or lose its mechanical properties even if the temperature rises to as high as 250°C for a few seconds due to its high melting point.  If the operating conditions are more severe, the addition of 0.06% silver can raise the softening temperature of copper conductors by approximately 100 K without any significant effect on its conductivity, at the same time appreciably increasing its creep resistance even further.

References

  1. Copper for Busbars, ECI publication no Cu0201, May 2014
  2. High Conductivity Copper for Electrical Engineering, ECI publication no. Cu0232, Feb 2016 
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