These disturbances are high frequency voltage spikes, typically in the micro-second range. The next major power problem is a fluctuating voltage situation. Fluctuating voltages can also be termed voltage sags or surges. These type of disturbances are short-term under or over voltage conditions that can last from one cycle to several cycles. This occurs when motors start-up suddenly drawing current from the line, lightning strikes, fault clearing, and from power factor switching.
Brownouts are another of the major power problems.
Brownouts happen less frequently than transients or fluctuating voltage, but are much more noticeable when they occur. Brownouts seem to be popular on those hot and muggy days in the dead of summer when everybody has their air conditioner turned as high as it will go. Brownouts are defined as longer term under voltage conditions lasting from several seconds to several hours, depending on the cause. This type of disturbance can typically be measured with a standard digital or analog voltmeter. If your load is attached to the utility and distorts the input line currents as all non-linear loads do, high frequency or harmonic currents are generated.