The second category, Momentary Interruptions , refers to a loss of voltage ranging from less than a cycle to several cycles. Though they do not last long enough to be called outages, in reality that's what they are. Voltage Sags And Swells are the third category. These are variations in the RMS voltage from about one half cycle to several seconds. They are characterized by changes in the RMS voltage value.
Sags refer to a reduction in the voltage, while swells deal with a voltage increase.
Should a sag or swell last for longer than several seconds, it is referred to as an Undervoltage Or Overvoltage . This is the fourth category. These disturbances may last indefinitely. The fifth category is Harmonic Distortion . If the voltage or current waveshape is not sinusoidal, it is considered distorted.