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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.controleng.com/single-article/voltage-sags-and-what-to-do-about-them/0499ada0dfbb1d6747ee2bf3adacd2ee.html

http://www.electricalsolutions.net.au/content/electrical-distribution/article/understanding-the-most-common-power-quality-problems-897329076

http://www.powerqualityworld.com/2011/03/power-quality-basics-undervoltage.html

http://www.hersheyenergy.com/voltage_irregularities.html

UPS is the acronym for uninterruptible power supply. It is a system that ensures sensitive electronic devices, such as servers and PCs, continue to operate if and when the main power source goes out. A UPS normally provides enough power for a few minutes to allow you to save your work before the computer shuts down.

A UPS system gives instant protection from power failures through the power stored in its batteries. While it may be possible for a standby generator to automatically start up when a power failure occurs, there is a short delay before the generator provides the power needed to keep your PC running. This is where a UPS system comes in since it provides you enough power while your standby generator is starting up.

It is also important to take note that a UPS system should not be considered as a replacement for a standby generator since it only provides enough power for a few minutes. A UPS system is normally used together with a standby generator in data centers since it fills in the gap in between the interruption of the main power supply and the replacement power provided by the standby generator.

Many UPS systems feature a software component facilitating the automatic backup and shut down of your PC whenever the main power source is interrupted. This feature is useful if you are not close to your computer when a power failure occurs.

The most common type of UPS system used in the market is an online UPS system. It is made up of a charger or rectifier, power inverter, and batteries. The power that passes through the charger and inverter is in the form of alternating current (AC) before it is stored in the batteries as direct current (DC). Whenever the main power source is interrupted, the DC power is converted into AC before it is used to power the devices connected to the UPS system.

References:

http://www.webopedia.com/TERM/U/UPS.html

http://www.businessdictionary.com/definition/uninterruptible-power-supply-UPS.html

http://www.amps.org.uk/what-is-ups-

http://www.wikiforu.com/2012/07/classification-types-of-uninterrupted-power-supply-ups.html