In an electric power system, a fault is a defect that results in abnormality of electric current. A fault current is any abnormal electric current. For example, a short circuit in which a live wire touches a neutral or ground wire is a fault. An open-circuit fault occurs if a circuit is interrupted by a failure of a current-carrying wire (phase or neutral) or a blown fuse or circuit breaker. In a ground fault (or earth fault), current flows into the earth.

In a polyphase system, a fault may affect all phases equally, which is a "symmetric fault". If only some phases are affected, the resulting "asymmetric fault" becomes more complicated to analyse. The analysis of these types of faults is often simplified by using methods such as symmetrical components. In three-phase systems, a fault may involve one or more phases and ground, or may occur only between phases.

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. The design of systems to detect and interrupt power system faults is the main objective of power-system protection.

By persistence, faults are classified into transient, semi-persistent, and persistent faults. By symmetry, faults are classified into asymmetric and symmetric faults. By source, faults are classified into internal and external faults.

A transient fault is a fault that is no longer present if power is disconnected for a short time and then restored; or an insulation fault which only temporarily affects a device's dielectric properties which are restored after a short time. Many faults in overhead power lines are transient in nature. When a fault occurs, equipment used for power system protection operate to isolate the area of the fault. A transient fault will then clear and the power-line can be returned to service. Typical examples of transient faults include:

Transmission and distribution systems use an automatic re-close function which is commonly used on overhead lines to attempt to restore power in the event of a transient fault. This functionality is not as common on underground systems as faults there are typically of a persistent nature. Transient faults may still cause damage both at the site of the original fault or elsewhere in the network as fault current is generated.

A semipermanent fault, like tree contact, are faults that might clear themselves if allowed to burn for a short time (a tree branch might burn away). Semipermanent faults occur more frequently in the subtransmission lines and electric distribution.

A persistent fault, also known as permanent fault, is present regardless of power being applied and therefore has to be repaired. Faults in underground power cables are most often persistent due to mechanical damage to the cable, but are sometimes transient in nature due to lightning.