Abstract: A method of use and system for determining the longitudinal active resistance of a neutral conductor of an underground electrical cable, while the electrical cable remains in service. The method is conducted by applying a selective frequency test current signal to the neutral conductor of the underground cable connected between a pair of grounded structures, e.g., power transformers, from a test signal generator that is connected across the neutral conductor using first and second bifilar winding signal-voltage cables. The method utilizes indirect voltage determination to obtain the voltage drop across the neutral conductor while mitigating induced voltage effects which occur when direct voltage measurement is used. A plurality of selective frequency test current signals are used to obtain a plurality of longitudinal active resistance values. Any conventional extrapolation method is then used to obtain a longitudinal active resistance value at 0 Hz.