Unfortunately, this is not always the case. Voltage irregularities are common due to a number of reasons.
Brownouts occur when the power grid is overtaxed.
Extended and extensive brownouts are usually caused by heavy regional demand on power due to unusually hot or cold weather. Utility brownouts can last from a few minutes to several hours. Sags are much shorter in duration, lasting from a fraction of a second up to several seconds. Sags usually are caused by local conditions such as sudden current inrush when equipment is switched on. Common culprits are air conditioners, heaters, copy machines and laser printers, as well as large power amplification and lighting systems.