Accurate Formula by Schelkunoff

Mutual impedance between the inner and the outer surfaces Z  

K x) is the modified Bessel function of kinds 1 and 2, respectively, with order n 

As is clear from the earlier equation, there exist three component impedances for a cylindrical conductor. In the case of a circular solid conductor, z,=z =0 for r =0, and Zq becomes 

When we say a conductor internal impedance z), it means Zq, the outer surface impedance given in Equation 1.9 or in Equation 1.10 as far as an overhead line is concerned. However, in the case of a cable, z, is composed of a number of component impedances as in Equation 1.9 and also of an insulator impedance between metallic conductors, because the cable is, in general, composed of a core conductor carrying a current and a metallic sheath (shield or screen) for a current return path [4,5]. 

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