Probability of insulation failures in isolated LPS according to the used insulation technologies

The calculation of separation distances as described in the IEC-Standard for lightning protection is the basis for correct dimensioning of isolated or partly isolated lightning protection systems. In the new IEC 62305 series the negative first stroke became part of the standardization process the first time. Up to now the calculation of separation distances in the IEC-Standard is determined only by the parameters of negative subsequent strokes. The paper shows, that the parameters of the negative first stroke have a big impact on this calculation. By means of three different examples the influence of first and subsequent strokes on the probability of insulation failure per flash was investigated. These examples represent insulated standoffs and insulated down conductors which are the main technologies currently used in isolated LPS. The calculations are done using the logarithmic normal distributions of I and di/dt. As a result it is shown that the probability of insulation failure of an insulated LPS depends strongly on the technology used. It seems to be that the rod-rod spark gap which is basis for the calculation of separation distances according to the IEC standard as well as the insulated stand-off show a similar performance. On the other hand the insulated down conductor shows a completely different behaviour and seems to be oversized.