Occurrence of Dangerous Touch Voltages During Lightning Strikes

If people are in the vicinity of buildings during a thunderstorm, there is always a risk that they may be injured by dangerous touch voltages. In particular, the immediate vicinity of lightning conductors is a hazardous area, since in addition to induced voltages, there are potential differences in the ground that overlap in their effect. When touching metal parts of buildings, such as metal facades or rainwater downpipes that are part of the equipotential bonding system, humans get a stationary voltage stress from the earth potential rise and additionally form a closed conductor loop in which dangerous induced voltages can occur. These voltages lead to currents that flow through the human body and the heart, where they may cause ventricular fibrillation. The paper aims to investigate the influencing factors on touch voltages when not the down conductor itself is touched. Influencing parameters are the size and the type of the earth-termination system, the number of down conductors, surface insulation layers on ground, potential grading, or even incoming lines. For this purpose, specific parameters are varied, combined and evaluated. The investigation shows that the voltage stress on the human body consists of an induced voltage stress and a stationary voltage stress caused by the potential gradient area on ground surface. Both stresses are independent from each other. Induced voltages can have high peak values and diminish after short periods of time and on contrary stationary voltages are lower in amplitude but last for long durations. An insulation on ground might help (in the reduction of the dangerous hazardous voltages over the human body) but it has always the risk to electrically break down. The best configuration one can has is to use at least three well separated down conductors, a necessary potential equalization system in the potential gradient area in direct vicinity to all metal parts combined with an insulation layer. Using this configuration, touch voltages even in the case of lightning protection level I (LPL I) do not exceed hazardous body energy thresholds, regardless which low frequency earth resistance is considered.