Depending on the type of the electrical network, the source of the voltage sag (or dip), and the settings of the protective devices, the magnitude, duration, and phase of the voltage sag (or dip) during an event may behave differently. Because of the nonstationary characteristics of the voltage sag, the conventional fast Fourier transform (FFT) and Hilbert Huang transform (HHT) methods show ambiguities at the starting and at the recovery points of the voltage sag waveform. This can lead to errors in the calculation of the single-event characteristics (SEC) of the voltage sag. This article investigates the application of a novel automatic segmented HHT (SHHT) method for evaluating the SEC of the voltage sag caused by different types of symmetrical and unsymmetrical faults in both transmission and distribution networks. It also investigates the influence of several factors on the three important parameters of the voltage sag - the magnitude, the phase angle, and the point-on-wave (POW) of the voltage sag waveform. These factors include the types of fault, the location of the fault, and the X/R ratio of the lines before and after the fault, the transformer and load connections, and the POW of the sag initiation. The simulation results and tests with real data validate the effectiveness of the SHHT method in the evaluation of the SEC of the voltage sag when compared with those obtained using the traditional FFT and the analytical method.