Electrical fault

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 three-phase systems, a fault may involve one or more phases and ground, or may occur only between phases. 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.

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.