Significant amounts of energy can be transferred to the pipeline during a short circuit or fault condition on an overhead transmission system resulting in a line-to-ground short circuit. While electrical faults are abnormal, given the life span of pipelines, the likelihood that a pipeline will be exposed to a line-to-ground fault is very high and the risks should be mitigated to prevent damage to the infrastructure. Electrical faults on the power lines can occur as a result of lightning, high winds, structure or insulator failure, or even accidental contact between the power line and another structure or piece of construction equipment.
During fault conditions the energy being transmitted by the power line will return to its source using all available paths including static lines, shield wires, the earth, and even pipelines. The path of the fault current is predicated upon the relative impedances of all parallel paths available to the fault. Separation distance between the faulted system and the pipeline as well as the impedance between the pipeline and the surrounding soils are critical factors in determining the expected current flowing through the pipeline. Pipeline coatings contribute significantly to the impedance between the earth and the pipeline. The higher the coating quality (fewer holidays) and the higher the coating’s rated breakdown voltage, the lower the amount of current carried by the steel. Unfortunately, the magnitude of a fault current can be many times greater than the steady state current normally present in the power lines and can result in high pipeline voltages. Fortunately, these fault currents are only present for very short periods of time as the power line protective system can react in milliseconds to remove the fault, but even that short period of time can result in coating damage, melting, or cracking of the pipeline to say nothing of the safety hazards presented to pipeline personnel and even the general public where accessible.