Induced voltages on buried coaxial cable by direct lightning strokes

Abstract

AbstractLarge numbers of failures occur in coaxial line repeaters which are located in areas subject to high thunderstorm activity. J. Kemp [3] has calculated the voltage induced in coaxial cables which undergo direct lightning strokes, but it has been found that the failure rate that occurs in actual installations may, in some cases, be as great as 100 times the failure rate calculated by Kemp's method. Transient phenomena are generally calculated by means of transforms using Heaviside's operational method, but in the case of transmission lines, because of the need to employ approximations, very significant errors can be introduced. In particular, many of the buried cables installed in recent years have metallic sheaths which are insulated from the earth and it has become necessary to take this fact into account to arrive at accurate results.Using line constants which take into account the skin effect, the induced voltage is here determined by numerical computation of the inverse Laplace transform and from this the failure rate is computed. The results are in almost perfect agreement with actual conditions. In addition, comparing the results obtained by this method with those obtained by the conventional method, we see that when the metallic sheath is insulated from earth the induced voltage becomes large because the lightning wave flowing into the metallic sheath is not attenuated very much and a high excitation voltage is therefore distributed over a long distance. Since this method permits quite complicated computations, it is expected that its application will facilitate development of suitable lightning protection methods.