A single core lead covered cable is to be designed for 68 kV(peak value) to earth. Its conductor diameter is 2 cm and its three insulating materials have permittivities of 5, 4 and 3 respectively with the corresponding maximum safe working stress of 38 kV/cm (r.m.s), 26 kV/cm (r.m.s) and 20 kV/cm (r.m.s) respectively. Find the minimum diameter of lead sheath.
