Decrease in induced voltage on buried coaxial cable due to direct lightning stroke by earthing of metallic sheath

Abstract

AbstractWhen a direct lightning stroke hits a cable in which the metallic sheath is insulated from ground a high voltage is induced between the metallic sheath and the cable core and this may result in multiple insulation breakdowns in the cable and associated equipment. Compared to the case in which the sheath is in direct contact with the ground, attenuation of the lightning current in the cable is relatively small and it has been found that this is the reason for the low attenuation of the distributed electromotive force in the circuit formed by the core and the metallic sheath. If the metallic sheath is grounded to cause a distributed flow of lightning current into the earth, attenuation of the sheath current can be greatly increased and it can be expected that the induced voltage will be lowered, reducing the failure rate. Using the inverse Laplace transformation, the sheath current and induced voltage are computed for the case where the coaxial cable is grounded at equal intervals in a repeater section. It is found that grounding at 450‐m intervals results in induced voltage between the inner and outer conductors of the coaxial pair which is about 60% of the voltage for grounding at 3600 m intervals and the failure rate is reduced to about one‐fourth. This method permits accurate determination of the crest value of the inflowing current which will not cause insulation breakdown failure in the cable and permits us to devise lightning countermeasures which will hold the failure occurrence rate to allowable limits.