The second category, momentary interruptions , refers to a loss of voltage for periods from less than a cycle to several cycles. They do not last long enough to be called outages, but in effect, are. Voltage sags and swells are the third category. These are variations in the RMS voltage from about one half cycle up to several seconds. They are characterized by changes in the RMS value of the voltage.
Sags refer to a reduction in the voltage, while swells refer to an increase.
Should a sag or swell last for a longer period of time than several seconds, it becomes an undervoltage or overvoltage. This is the fourth category, and these disturbances may last indefinitely. The fifth category of power anomalies is harmonic distortion. If the voltage or current waveshape is not sinusoidal, it is described as distorted. Since our entire power system was developed with sine waves in mind, harmonic distortion challenges many of our basic concepts about how to transmit, distribute, use and maintain electric power.