Overvoltage is caused due to surges such as lightning or due to switching loads on and off. In generators, it can be caused due to the failure of the field controller.
Heavy winds which can cause lines to snap or trees to fall on them. This can cause open circuits, earth faults and short circuits.
Ageing which can result in weakening of insulation and failure. This can result in short circuits and earth faults.
Chemical pollution and deposition on the insulators that can result in flash overs
Faults due to wildlife, such as birds, snakes, mice, etc which can cause short circuits and earth faults.
Collision of vehicles on towers and transmission poles can cause the towers to fall.
The table below will give an idea of the percentage of the types of faults in the different equipment in a power system
Overhead lines 50%
Transformers 12%
Switch gear 15%
Cables 10%
Miscellaneous 8%
Instrument Transformers 2%
Control Equipment 3%