Current waveform parameters of CN tower lightning return strokes

Lightning strikes to the Toronto Canadian National (CN) Tower have been observed since 1978. In this paper, a brief description of the lightning measurement systems is provided, with emphasis on the current measurement system. Current and field measurement systems with broad bandwidths and high resolutions have been operational since 1991. Typical current derivative and current waveforms of a CN Tower lightning return stroke are also presented. Extensive cumulative statistics of waveform parameters (maximum steepness, risetime to maximum steepness, wavefront peak, risetime to wavefront peak, and absolute peak) of lightning return-stroke current signals measured at the CN Tower during a 10-year period have been derived. The statistical analysis shows that in 50% of the current records: the current wavefront maximum steepness exceeds 18.8 kA/μs, the risetime to maximum steepness exceeds 138 ns, the current wavefront peak exceeds 5 kA, the risetime to wavefront peak exceeds 636 ns and the current absolute peak exceeds 7.2 kA. The statistical analysis also shows that first return strokes feature lower current steepness and higher current peak when compared with subsequent return strokes. Recommendations concerning instrumentation and techniques for the proper measurement of lightning return-stroke currents are discussed. The presented statistical results will be of assistance in the establishment of more sophisticated protective measures against lightning hazards, especially at elevated objects.