Electrical fault

An electrical fault is a defect in an electric power system that results in an abnormality of electric current, known as fault current. Common examples include a short circuit, where a live wire contacts a neutral or ground wire; an open-circuit fault, caused by a broken wire or a blown fuse; and a ground fault, where current flows unintentionally into the earth. Faults are classified by persistence as transient, semi-persistent, or persistent. Transient faults, such as momentary tree contact, animal contact, lightning strikes, or conductor clashing, disappear if power is briefly removed and often occur on overhead lines.

Semi-persistent faults, like a tree branch contacting a line, may clear themselves after burning briefly. Persistent faults, common in underground cables due to mechanical damage, require repair regardless of power. By symmetry, faults are asymmetric or symmetric. Asymmetric faults, which affect phases unequally, include line-to-line faults (short circuits between lines, often from broken insulators), single line-to-ground faults (the most common type, caused by lightning or storm damage), and double line-to-ground faults.

Symmetric faults, which affect all phases equally, are rarer but can cause severe damage. Faults are also classified as internal or external to a device; for example, a transformer might develop an internal overpressure fault due to local overheating, while an overload condition is external. A bolted fault notionally has zero impedance, producing the maximum prospective short-circuit current. The prospective short-circuit current of a predictable fault can be calculated, and protective devices detect faults to operate circuit breakers, limiting service loss.