A voltage sag1 is an event where the line rms voltage decreases from
the nominal line-voltage for a short period of time. Figure 3.1 shows an
80 percent sag with a duration of a few 60-Hz cycles. This type of variation
can occur if a large load on the line experiences a line-to-ground
fault, such as a short in a three-phase motor or a fault in a utility or
plant feeder