1.8 Lightning current measurements of upward and downward flashes

It is generally assumed that downward flashes (cloud-to-earth flashes) place greater stress on objects struck by lightning than upward flashes (earth-to-cloud flashes). This applies in particular for short strokes. On flat terrain and in structures with low heights, downward flashes are predominantly to be expected. If, however, structures are situated in an exposed location and / or have a greater height, upward flashes typically occur. The parameters defined in the lightning protection standards generally apply to upward and downward flashes. In the case of upward flashes, especially the first long stroke with or without superimposed impulse currents must be considered. The determination of lightning current parameters and their dependence on upward and downward flashes is currently still the subject of fundamental scientific research at various lightning measurement stations around the world. Figure 1.8.1 shows the lightning measuring station operated by the Austrian research group ALDIS on the Gaisberg mountain near Salzburg / Austria. In parallel with this, DEHN has been performing lightning current measurements at this measuring station by means of a mobile lightning current detection unit since 2007. The results of these comparison measurements basically confirm the lightning current parameters as described in lightning protection standard DIN EN IEC 62305-1. The high number of superimposed impulse currents in case of upward flashes is particularly remarkable. With an average of 8 short strokes (either superimposed on the long stroke or subsequent to the long stroke), considerably more impulse currents were recorded than the 3 to 4 subsequent strokes which typically occur in case of downward flashes. Thus, the 3 to 4 impulse discharges per flash stated in the lightning protection standards only apply to downward flashes. For 10 years (2000 to 2009), ALDIS has been recording 10 flashes with total charges exceeding the maximum charge value of 300 As depending on the lightning protection level (LPL). These high charge values were recorded only during winter thunderstorms. In the first measuring period, the mobile system also correctly recorded long strokes during winter thunderstorms with higher charges than the charges specified for the hazard class LPL I. Figure 1.8.2 shows a long stroke with a charge of 405 As recorded in January 2007. These extreme loads, which exceed the charge value of 300 As of LPL I, may have to be taken into account when taking lightning protection measures for high structures and exposed locations such as wind turbines and transmitters.