Direct measurement of lightning currents on a telecommunication tower

The current of a lightning discharge rises to a peak amplitude of more than 200,000 A within a few microseconds. For estimation of the physical effects of the lightning discharge on electrical and electronic devices and other objects, characteristic parameters of the lightning current are usually used. These parameters vary in a wide range for individual lightning discharges. The knowledge of the probability distribution of these parameters is required for an appropriate dimensioning of lightning and overvoltage protection measures. The standards and methods currently in use for the design of lightning and surge protective measures are based on the results of measuring series of lightning current carried out in Switzerland 30 - 40 years ago. The experience of the last years reveals very often that the data of those measurements are limited in many details, especially in the application for lightning and surge protection of sensitive electronic devices. The reason is, that the available instruments for recording of several details of lightning current wave shapes were very limited ad that time. The aim of this research project was the systematic record of lightning current waveforms using up-to-date measuring techniques, over several years. A preliminary investigation revealed the telecommunication tower on mount Gaisberg near Salzburg an especially suitable measuring location. A very high annual lightning strike frequency of 40 - 50 direct discharges was expected. The measuring equipment to record the wave shape of the lightning current of direct strikes to the tower was installed in summer 1998. Until now 207 events have been recorded. In the following years the measuring equipment was improved and extended. In 1999 a working group, with four involved institutions and different research projects, was established. Therewith an optimal utilization of the available infrastructure and instrumentation is guaranteed. Apart the lightning current now also the parameters of the electromagnetic field are record and the lightning strike to the tower is recorded with a highspeed digital videocamera. The first evaluation of the measuring results shows that a considerable part of the lightning strikes were upward- discharges with current amplitudes of a few 100 A, but electrical charge transfer of up to 200 As. Peak current amplitudes of more than 35 kA were not registered in the last three years. Records with the highspeed videocamera were successful only in five cases up to now. Unfortunately most time the direct view to the tower was impaired trough fog. It is intended to continue the measurements at mount Gaisberg over several years to improve the statistical power of the collected data. In this regard, several international research institutions also expressed their interest for cooperation. The measuring results in longer terms should form the basis for the elaboration as well as adaptation of national and international standards and methods of lightning and surge protection.