A sag or an undervoltage is a temporary decrease in power lasting up to over a minute. A sag or an undervoltage typically happens whenever heavy machinery is turned on. A great amount of power is used by the heavy machinery during startup, leaving a small amount or power available for other equipment to use.
Voltage sags also happen when the main source of power is affected by natural events like lightning strikes, strong winds and power lines getting hit by falling tree branches. A sag or an undervoltage may affect equipment within 100 miles of the main power grid of a utility company.
Due to this, overheating can occur for some equipment while others may malfunction. Lights may also dim, and batteries may not be charged properly due to an undervoltage situation. An undervoltage or sag may cause a computer to lose data and systems to lock up.
In the past, the term brownout was used to describe this situation. However, its use has been discouraged by an international standards organization, the Institute of Electrical and Electronics Engineers. The term brownout is typically used for situations where the lack of power lasts for a couple of hours.
To handle undervoltages and voltage sags, you can provide heavy machinery their own circuit and avoid connecting them to the circuits used by other equipment. Automatic voltage regulators can also reduce the effects of undervoltage and voltage sags. These devices regulate the level of voltage used by specific equipment and ensure they always receive a suitable amount of power.
An uninterruptible power supply or UPS can also protect equipment from voltage sags. The batteries of these devices provide power for a specific period of time as long as the batteries are not dead. The batteries of a UPS normally last up to around 20 minutes.
References:
http://www.on365.co.uk/Power__and__Cooling/Most_Common_Power_Problems.aspx
https://www.tripplite.com/support/articles/articleid/common-power-problems
http://www.powerqualityworld.com/2011/03/power-quality-basics-undervoltage.html