Five-Year Field Investigation of Lightning Effects on Transmission Lines III-Lightning-Stroke Characteristics and Their Effects in Transmission Systems

Seventeen negative strokes to transmission towers and the Cathedral of Learning (CL) in Pittsburgh, Pa., have been recorded. The crest currents ranged from 11 to 50 kA. Only one positive stroke of 80 kA appeared; it was at the CL. The greatest rate of change of stroke current recorded was 230 kA/μs during an interval of 0.05 μs. All stroke currents had approximately the same rate of rise during given time intervals. Measurement of ground wire current enabled determination of tower-top potentials which resulted almost entirely from the tower surge impedance, since tower footing resistances were low. There is evidence that induction from passing leaders and strokes to earth nearby will produce high voltage on exposed conductors. Records of field gradients accompanying nearby strokes showed as much as 120 kV/m.