A fault in a wire or in a cable could be the partial or total failure in the installation.

                   Majority of fault on 15 kv or higher transmission line are caused by the lighting which can be the cause of flash over of in insulator and the high potential difference between the conductor and ground supporting tower causes ionization,

 

and this ionization provides a path to ground for the charge produced by the lightning stroke. When ionized way is formed then cause of low impedance factor of conductor to ground, power current starts flowing from conductor to ground and from ground to the grounded neutral of transformer or generator located in the generating stations.

                                                          When the power current is flowing from conductor to ground then power becomes very much high from its safe limit. So that the opening of circuit breaker isolate the faulty area from the rest transmission line. This opening of circuit breaker interrupts the flow of current in ionized path and by this deionization takes place in the faulty area. After the interval of almost 20 cycles deionization completely done with its work and breakers are usually reclosed without the producing the arc.

                                                                   Sporn and miller submitted a report by their nine year experience in field of electrical transmission lines with 1634 circuits of miles. The have observed that most of system which are being operated at 132kv have shown 635 problems of flashover , from which 570 were completely reclosed by ultra-high speed of latest breakers, and of which 8 were proved to be permanent faults where the reclosing of breaker would have been impossible to take place .

                   The permanent faults caused by the breakage of conductor of transmission line and being on the ground. 

                                                                             The study has shown that in between 70-80% faults in transmission line are usually single phase to ground faults

 

which comes from the flash over of only on line to the tower and ground. The percentage of three phase fault is only 5%. And there are some other faults called as line to line faults

 

which do not involve ground in them. Another fault named by double line to ground faults. All above faults are unsymmetrical and cause an unbalance between phases.