singlelinetoground

Single line-to-ground, often abbreviated as SLG or SLG fault, is a common electrical fault in power systems where exactly one phase conductor makes an unintended conductive connection to the earth while the other phases remain intact. This type of fault occurs in both overhead transmission and distribution lines and in underground cables, and it is one of the most frequent fault types in many networks.

Causes and characteristics. SLG faults typically arise from insulation breakdown due to aging, moisture, contamination, mechanical damage, vegetation contact, or lightning. The fault creates an unbalanced condition and introduces a path from the faulted phase through the earth back to the source. The fault current magnitude depends on the impedance of the fault path, the system grounding arrangement, and soil resistivity. In solidly grounded systems, high fault currents can occur; in impedance-grounded or ungrounded systems, the fault current may be limited, producing smaller currents but potentially significant voltages on healthy phases.

Grounding and protection. The behavior of SLG faults is influenced by the system’s grounding scheme, such as solidly grounded, resistance-grounded, or impedance-grounded configurations. Protection typically relies on ground-fault relays or residual-current/overcurrent protection to detect the unbalanced current and isolate the faulted circuit. Transformers, feeders, and protection schemes are designed to clear SLG faults promptly to minimize equipment damage and voltage disruption.

Impact and analysis. SLG faults cause voltage sags and large unbalances, transient overvoltages, and potential equipment insulation stress. They are frequently analyzed using positive-, negative-, and zero-sequence network models to determine fault currents and relay settings. Prevention focuses on proper insulation coordination, robust grounding, insulation maintenance, and vegetation management around lines.