In power engineering, specifically three phase power, an asymmetric or unbalanced fault is a fault which does not affect each of the three phases equally. This is in contrast to a symmetric fault, where each of the phases is affected equally. In practice, most faults in power systems are unbalanced; however, as asymmetric faults are difficult to analyze, analysis of asymmetric faults is built up from a thorough understanding of symmetric faults.

Common types of asymmetric faults, and their causes:

• line-to-line - a short circuit between lines, caused by ionization of air, or when lines come into physical contact, for example due to a broken insulator.
• line-to-ground - a short circuit between one line and ground, very often caused by physical contact, for example due to lightning or other storm damage
• double line-to-ground - two lines come into contact with the ground (and each other), also commonly due to storm damage

Analysis

An asymmetric fault breaks the underlying assumptions used in three phase power, namely that the load is balanced on all three phases. Consequently, it is impossible to directly use tools such as the one-line diagram, where only one phase is considered. However, due to the linearity of power systems, it is usual to consider the resulting voltages and currents as a superposition of symmetric components, to which three phase analysis can be applied.

The method of symmetric components is perhaps somewhat unintuitive, but can be verified to give correct results. The power system is seen as a superposition of three components:

• a positive-phase component, in which the phase is in the same order as the original system
• a negative-phase component, in which the phase is in the opposite order as the original system
• a zero-phase component, which is not truly a three phase system, but instead all three phases are in phase which each other