The positive or negative phase sequence impedance of a pipe-type cable may be calculated by conventional formulas, suitably modified to include the effect of magnetic properties of the pipe. Calculation of the zero phase sequence impedance, however, is considerably more complicated, involving not only the permeability of the steel pipe, which depends upon the zero phase sequence current, but also the resistance of the shield skid wire assemblies. Formulas and curves are presented by which the zero phase sequence impedance may be determined for any particular case.
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Jack Neher (1964) studied this question.