An electric power system, a fault or fault current is any abnormal electric current. For example, a short circuit is a fault in which current bypasses the normal load. An open-circuit fault occurs if a circuit is interrupted by some failure. In three-phase systems, a fault may involve one or more phases and ground, or may occur only between phases. In a "ground fault" or "earth fault", current flows into the earth. The prospective short circuit current of a predictable fault can be calculated for most situations. In power systems, protective devices can detect fault conditions and operate circuit breakers and other devices to limit the loss of service due to a failure.
There are mainly two types electrical faults.
Symmetric fault
A symmetric or balanced fault affects each of the three phases equally. In transmission line faults, roughly 5% are symmetric. This is in contrast to an asymmetrical fault, where the three phases are not affected equally.
Asymmetric fault
An asymmetric or unbalanced fault does not affect each of the three phases equally. Common types of asymmetric faults, and their causes:
Line-to-Line
A short circuit between lines, caused by ionization of air, or when lines come into physical contact, for example due to a broken insulator. In transmission line faults, roughly 5% - 10% are asymmetric line-to-line faults.
Line-to-Ground
A short circuit between one line and ground, very often caused by physical contact, for example due to lightning or other storm damage. In transmission line faults, roughly 65% - 70% are asymmetric line-to-ground faults.
Reason of faults.
Insulation failure or break down.
Lighting surge.
Unbalance current or voltage.
Voltage drop.
Stability fall.
Mechanical fault in transmission line.
Over voltage.
Under Frequency.
Reversal of power.
Temperature Rise.