Replacement Cycles

An overhead line is exposed to the elements and depends on the air to remove heat from resistive losses. It is subject to damage from:

• Natural exposure and corrosion
• Fatigue from frequent cycles in temperature as current loads and air temperatures change during the day
• External influences such as excessive wind or ice loads, trees falling or hitting lines, or from accidental human interference

There is nothing that can be done about the problems suffered by overhead lines and the problems are built into a maintenance programme for a line over time. Typically an overhead line cable will be replaced every 15 years, while the towers will have a lifetime of around 40-50 years.

Underground cables are buried within engineered trenches or ducts. They experience no weather exposure and very stable operating temperatures. They are less prone to degradation. However, they are vulnerable to being disturbed by:

• Humans during excavations for buildings or drainage systems
• Ingress of tree roots
• Changes in soil moisture levels leading to overheating

The problems faced by cables can be dealt with through well-developed precautionary measures to minimise the chance of their occurrence. An underground cable is designed to last 40 years, but will probably last significantly longer, making a considerable difference to the life cycle economics of the cable compared with overhead line solutions.