A methodology for characterizing the phase-angle jump associated with voltage sags in radial systems using incremental voltage quantities is presented in this paper. The incremental voltage phasor is defined as the difference between the pre-fault and the during-fault voltage phasors at the point of common coupling. The characteristics of this phasor are stated for different fault conditions as a function of system impedances. Both fault impedance and source neutral grounding impedance are considered in the analysis. Furthermore, the vector representation of phasors and impedance diagrams are employed as a graphical aid for understanding the incremental quantities behavior as a function of the system parameters. An algorithm for fault classification is also presented. Finally, the theoretical analysis is confirmed using real data provided by the Department of Energy and the Electric Power Research Institute. The results of this paper can be useful in future works for fault classification and fault location.