Assuming its correct installation, factors such as temperature, cycling, fatigue, vibration, and corrosion all work to cause the gradual degradation and increase in resistance of an electrical device. These influences build up over some time until a level is reached at which the device no longer operates correctly. The application will determine the critical degrading factor.
Environmental and chemical attacks are relentless. Even air will oxidise organic materials, while the ingress of moisture, oil, and salt will degrade connections even more rapidly. Chemical corrosion can attack the cross-sectional area of an element, reducing its effective size while increasing the component’s resistance. Electrical stresses, particularly sustained overvoltages or impulses, can cause welds to loosen. Mechanical stress from vibration during operation can also degrade connections, causing resistance to increase. These conditions result in excessive heating at the location where the component is carrying the rated current, based on the formula W=I²R. For example:
- 6000 A across a 1 μΩ bus = 36 Watts
- 6000 A across a 100 mΩ bus = 3600 kW
If left unattended, these problems can lead to failure in the electrical system containing the affected components. Excessive heating will ultimately cause failure due to burnout, which can open an energised circuit. Backup battery power supplies provide a good practical example of how degradation can occur under normal operating conditions. Changes in current flow cause the terminal connections to expand and contract, causing them to loosen or corrode. Additionally, connections are exposed to acid vapours, causing further degradation. These conditions cause a decrease in the surface-to-surface contact area with an associated increase in surface-to-surface contact resistance, ultimately resulting in excessive heating at the junction.