A damage mechanism: Lightning-initiated fault-current arcs to communication cables buried beneath overhead electric power lines

A lightning strike to an overhead structure will cause a brief are through the soil from its lightning ground to any nearby grounded metal utility lin e, such as a gas pipe, sewer line, or communications cable. A limited amoun t of damage to the buried line may result from such a stroke. However, if t he overhead structure happens to be an energized conductor of an electric p ower line, the situation becomes dangerous; the lightning impulse will esta blish a conductive path across the power-line insulator, down the pole, and through the soil to the buried utility line. In a significant number of ca ses, this conductive path will allow the establishment of a large, long-dur ation power fault current from the lightning-struck power conductor to the buried utility line. This power are will terminate on the grounded pipe or cable shield, causing rupture and failure. The existence of this damage mec hanism was confirmed in the laboratory with a full-scale mock-up of a utili ty right-of-way. The phenomenon of lightning-triggered are establishment th rough soil was then examined more closely with a high-resolution apparatus in which most parameters could be tightly controlled. Artificial lightning impulses from 0.3 to 2.8 MV and 60-Hz power-line voltages from 6.24 to 15.7 1 kV were used. Soil condition, electrode spacing, power-line voltage, ligh tning impulse voltage, and geometry were found to govern the probability of a lightning-initiated fault current are through the soil in a predictable manner. For soil of 500 000-Ohm.cm resistivity, the distance between a simu lated power system lightning ground and a buried cable at which a fault cur rent are is not initiated was found to be about 40 cm, This safe distance w as proportional to the geometric mean of the power-line voltage and the pea k lightning impulse voltage.