Discussion
The surge impedance of an underground cable is around :
Answer: D
The surge impedance of an underground cable is around 50 Ω.
Surge impedance is \(Z_{0} = \sqrt{L/C}\). In a cable the conductors sit close together inside a sheath with a dielectric of high permittivity, so the inductance per kilometre is small and the capacitance large, and \(Z_{0}\) comes out low, typically 40 to 60 Ω.
About 400 Ω is the figure for an overhead line, where wide spacing in air gives high inductance and low capacitance. 100 Ω and 25 Ω are outside the usual range for power cables.
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