For a three-phase three-wire line consisting of three solid cylindrical conductors, each with radius \(r\) and with equal phase spacing \(\mathrm{D}\) between any two conductors, the inductance in \(\mathrm{H} / \mathrm{m}\) per phase is given by

(a) \(2 \times 10^{-7} \ln \left(\frac{\mathrm{D}}{r^{\prime}}ight)\)

(b) \(4 \times 10^{-7} \ln \left(\frac{\mathrm{D}}{r^{\prime}}ight)\)

(c) \(6 \times 10^{-7} \ln \left(\frac{\mathrm{D}}{r^{\prime}}ight)\)

where \(r^{\prime}=e^{-\frac{1}{4}} r=0.778 r\)