Failure Statistics for Overhead Lines and Underground Cables

The best way to compare underground cables with overhead lines is through the availability of the system to transmit power. The UK National Grid published statistics that show non-availability of 0.126 hours per year per kilometer of 400 kV overhead line, compared with 6.4 hours per year for its 400 kV underground cables, some of which are old oil-filled lines. The international average for 400 kV cables appears to be around 3.4 hours per year. This reflects the fact that although cables suffer interruptions much less frequently than overhead lines, they do take longer to put back into service. However, despite difficulties claimed for repairing underground cables, cables in service are still available for 99.96% of the time.

Underground power cables are up to 90 percent cheaper to operate than overhead lines as they are out of reach of many of the accidents that can befall overhead lines. However, underground cables have much higher costs when a fault does appear.

Hydro Quebec estimated that a minor fault for an underground cable takes about five days to repair, compared with one day for an overhead line, whilst a major cable repair will take 20 days compared to 7 days for an overhead line.