Electrical fault
A defect causing abnormal electric current in power systems.
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.
- field
- Electric power systems
- known_for
- Classification into transient, semi-persistent, persistent, symmetric, asymmetric, internal, external, bolted, ground, and arcing faults
Lore & Background
Faults are classified by persistence into transient, semi-persistent, and persistent faults. A transient fault clears when power is briefly disconnected, such as from momentary tree contact or lightning. A semi-persistent fault, like tree contact, may clear itself if allowed to burn briefly. A persistent fault requires repair, as it remains regardless of power. By symmetry, faults are asymmetric (unbalanced) or symmetric (balanced). Asymmetric faults include line-to-line, single line-to-ground, and double line-to-ground faults, with single line-to-ground faults being most common in transmission lines. Symmetric faults affect all phases equally and are rarer but can cause severe damage. Internal faults develop inside equipment, while external faults occur outside. A bolted fault notionally has zero impedance, giving maximum prospective short-circuit current. A ground fault allows unintended connection to earth, and an arcing fault involves an electric arc that can be difficult to detect.
Reader's Guide
Electrical faults are central to power system design and protection. The prospective short-circuit current of a predictable fault can be calculated, and protective devices detect faults to operate circuit breakers and limit service loss. Analysis often uses symmetrical components for asymmetric faults, and symmetric faults can be analyzed via standard power system methods. Realistically, fault resistance varies from near zero to high, and arcs are highly non-linear. Calculations may be performed for subtransient, transient, and steady-state time ranges. Understanding fault types—transient, persistent, asymmetric, symmetric, bolted, ground, arcing—enables engineers to design systems that minimize damage and restore power efficiently. The article notes that roughly 65–70% of transmission line faults are single line-to-ground, while symmetric faults account for only 2–5% but can cause very severe damage.
Did You Know?
- In transmission line faults, roughly 65%–70% are asymmetric single line-to-ground faults.
- Symmetric faults account for 2 to 5% of all system faults but can cause very severe damage.
- A bolted fault notionally has zero impedance, giving the maximum prospective short-circuit current.
- Transient faults may still cause damage at the site of the original fault or elsewhere in the network.
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