Lightning-Induced Fault Current Arcs to Communication Cables Buried in Sand Near Overhead Electric Power Lines.

A lightning stroke to an aerial electric power line may cause severe damage to a communications cable buried beneath it by initiating a fault current arc through the earth from the pole's ground electrode to the cable shield. The damage caused through this mechanism is much more significant than that caused by either a direct stroke to the cable or through a ground electrode that is not associated with an electric power line. This is due to the fact that a large amount of energy is available from the power line when a fault current arc is initiated to the cable. The existence of the phenomenon was confirmed in initial tests performed on cable samples buried in a sand-filled cubical steel vessel 2 m on a side. A single soil electrode was subjected simultaneously to lightning impulses from a 3 MV Marx generator and 60 Hz power from a 1500 kVA, 15 kV transformer. The ratio of fault current arcs to applied impulses as a function of power voltage and cable-to-soil electrode clearance was then examined more closely in a smaller soil container in which conditions could be more precisely controlled. The apparatus was designed to allow rapid data acquisition. Sufficient data was accumulated to write an empirical formula for the damage radius around a power line ground electrode and to propose a model of the power arc inception mechanism in sand.