Faults in overhead lines happen due to insulators degradation and imperfection, corona , aerosols and moisture. Although the insulators are cheap to replace troubleshooting and finding the broken or damaged insulator in overhead lines can be tricky in some situations where fault is not indicated by really, fault is not visible , flashover happens at only high voltage and load can, not accessible terrain etc. The reason for cables faults can be many such as pour craftmanship, sheath physical damage during installation, overloading, digging and other mechanical activities, material imperfection stress etc.
Over headlines insulators fault causes power supply downtime which sometimes results with penalties, production losses and additional network problems.