在电力系统里，所谓的故障是指任何不正常的电流（比如短路电流或断路）存在于电力系统中。在三相系统中，故障可能牵涉到单相、多相与地间或者相与相间之不正常电流，依其故障电流的相角变化，可分为对称性故障与非对称性故障。

对称性故障的成因是在三相间同时发生短路故障，此时三相的故障电流与相位差皆相等；非对称性故障三相之间电流的大小与相角皆不相等。三相平衡故障发生的机率并不高，但却是最严重的故障类型，因为其故障电流是最大的。

当故障发生之时，电力系统保护设备（如断路器）将被触发，阻绝故障之区域以减少造成的损害。

In 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.