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

Abstract

A lightning strike to an overhead structure will cause a brief arc through the soil from its lightning ground to any nearby grounded metal utility line such as a gas pipe, sewer line, or communications cable. A limited amount of damage to the buried line may result from such a stroke. However, if the overhead structure happens to be an energized conductor of an electric power line, the situation becomes dangerous: the lightning impulse will establish 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 cases, this conductive path will allow the establishment of a large, long-duration power fault current from the lightning struck power conductor to the buried utility line. This power arc will terminate on the grounded pipe or cable shield, causing rupture and failure. The existence of this damage mechanism was confirmed in the laboratory with a full scale mock-up of a utility right-of-way. The phenomenon of lightning-triggered arc establishment through 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 MV to 2.8 MV and 60 Hz power line voltages from 6.24 kV to 15.71 kV were used. Soil condition, electrode spacing, power line voltage, lightning impulse voltage and geometry were found to govern the probability of a lightning-initiated fault current arc through the soil in a predictable manner. For soil of 500000 /spl Omega/-cm resistivity, the distance between a simulated power system lightning ground and a buried cable at which a fault current arc is not initiated was found to be about 40 cm. This safe distance was proportional to the geometric mean of the power line voltage and the peak lightning impulse voltage.