Internal Voltages and Currents in Solid-Shielded Cables

Analytical techniques for determining the current induced in long conductors near the surface of the ground by surface fields have been developed by Sunde and Wait. These techniques have been verified for insulated and bare conductors in experiments conducted with a CW transmitter, and computer programs have been developed to calculate the currents using a version of Sunde's theory. Until recently, however, cable-current theory was limited to determining the total current induced in a cable. For many applications where shielded or multiply shielded cables are used, however, it is important to know what voltages and currents are induced in the signal-carrying core of the cable. The purpose of this note is to describe one technique that has been developed to compute these internal currents and voltages. The technique described in this paper combines the shielding theory of Kaden and the transmission-line theory of Sunde. Classical shielding theory is used to determine the axial electric-field strength at the inside surface of the shield from the current flowing in the shield. This axial electric field is then used in the equations for a transmission line with a distributed driving source to compute the current in the core and the core-to-shield voltage. (In the case of multiple shields, multiple applications of these two steps are required to progress from the outer shield through the inner shields to the core.