Use of screen conductors to protect plastic- sheathed underground telephone cables against damage from lightning

Under severe lightning conditions and high ground resistivity, the telephone cables in South Africa were extensively and frequently damaged by lightning unless the cable was continuous metal pipe or equipped with a low- impedance metal sheath. During the past 10 yr, however it has been found that in the same areas polyethylene sheathed and insulated cables without either a metal sheath or pipe but with ground wires buried in the same trench, have been completely free of lightning damage. An attempt is made to explain in mathematical terms why plastic- sheathed cables with ground wires give such favorable results in practice.