Swissgrid differentiates between planned and unplanned maintenance work. Planned maintenance work includes cutting back plants close to the line, protection against corrosion, replacing insulators and refurbishing masts and their sockets. Work to be carried out is determined according to annual visual inspections. Planning is complex and starts approximately one year in advance. That is because despite the maintenance work, the stability of the grid must be guaranteed at all times. This work must be coordinated in advance with partners in Switzerland and transmission system operators in neighbouring countries.
Unplanned maintenance work is necessary if systems are damaged or disturbed by external influences. The most common causes include environmental influences, such as lightning strikes, wind, heat, avalanches or mudslides. If a line is damaged, it usually turns off automatically. If it cannot be re-connected, it will be earthed and taken out of service.
The on-call service has to search the site to locate the damage, and a helicopter may also be used on rough terrain. The most common causes of malfunction include storm damage and lightning strikes, which can usually be rectified quickly. Events like the ones in the Albula Pass in October 2018, when an intense low-pressure weather system knocked over four masts, are extremely rare.
