In this paper, a rigorous and independent validation of two different approaches for calculating the ground-return impedance and admittance of multiconductor underground cable systems using the transmission line theory is carried out. Furthermore, analyses are performed to evaluate the accuracy of a closed-form approximation for the calculation of the ground-return admittance of underground cable systems. The validations are based on the full-wave finite-difference time-domain (FDTD) method and consider the calculation of transients on flat and trefoil underground cable arrangements for different excitation types. Short cable lengths of 50 m and 100 m and soil resistivities of up to 1000 Ωm are considered. The results demonstrate the validity of the transmission line theory for the calculation of fast transients (with risetimes as low as 0.2 µs) on underground cables provided the ground-return parameters are rigorously determined, with the advantage of presenting much greater efficiency and easiness to implement in electromagnetic transient simulators compared to the full-wave FDTD method. Lastly, it is shown that the ground-return admittance approximation, despite its simplicity, leads to results comparable to those obtained through more complete formulations for the calculation of transients in underground cables, but more efficiently and without significant loss of accuracy.