Modern equipment can be sensitive to brief disturbances on the ac power mains. Electrical systems are subject to a wide variety of power quality problems which can interrupt production processes, affect sensitive equipment, and cause downtime, scrap, and capacity losses. The most common disturbance, by far, is a brief reduction in voltage, lasting for a few hundred milliseconds. These voltage sags, known as voltage dips in the IEC literature, are the most common power problem encountered. Besides fuse or breaker operation, motor starting, or capacitor switching that trigger voltage sags, they are also caused by short circuits on the power distribution system.