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Line-to-line-to-ground, often abbreviated as L-L-G or written as line-to-line-to-ground fault, is a fault condition in electric power systems in which two phase conductors and the earth form a conductive path. It is a type of multiple-line fault, typically more severe than single-line-to-ground faults due to higher fault currents and greater insulation stress on equipment.

Line-to-line-to-ground faults usually occur during switching transients, insulation failures, or when two energized conductors contact a grounded object simultaneously. They can also arise from lightning events or equipment faults that bring two lines into contact with a grounded structure or object, creating a three-conductor fault path.

Electrically, the fault involves currents from two energized lines flowing toward ground, with a portion returning through the grounding system. Analyzing an L-L-G fault often requires symmetric components or network equivalents, since the interaction of zero-sequence, positive-sequence, and negative-sequence networks depends on how the system is grounded and on transformer connections. Because two lines are involved, the fault current magnitudes can be larger and the voltage distribution more complex than for single-line-to-ground faults.

Protection against L-L-G faults typically relies on a combination of overcurrent protection, differential protection, and distance or ground-fault relays. Fast detection and clearance are important to limit equipment damage, arcing, and safety hazards. The consequences of an L-L-G fault can include insulation damage, overheating of conductors and transformers, mechanical stress, and possible outages if protective devices operate to isolate the faulted section.

See also: line-to-ground fault, line-to-line fault, three-phase fault.