A total loss of utility power, possibly resulting from a lightning strike, downed power lines, grid overdemands, accidents, etc.

Short-term low voltage condition that can be triggered by the start-up of large loads, such as an air conditioner or power tools, or by utility switching, equipment failure, etc. Power sags can result in system crashes and hardware damage.

A short-term high voltage condition commonly called a "spike". A spike is the opposite of a power sag, and can result from a sudden change in load (such as the air conditioner unit switching off), utility switching, lightning strikes, etc. Power surges can cause serious hardware damage.

Commonly called a "brownout", an undervoltage is like a prolonged sag, lasting from minutes to sometimes days. These are commonly the result of utility changes or extreme loading on the grid (for example, in the summer when many businesses and homes are running air conditioning units).

Overvoltage is a long-term "spike" lasting for minutes or more. These conditions can result from lightning strikes or from improper loading on the grid, equipment failures at transformer stations, and similar external events. These frequently result in equipment damage and potential loss of data.

This is high frequency waveform interference, similar to the type of "radio interference" that used to be experienced on televisions. It can result from radio transmissions, electromagnetic sources such as welding machines and SCR-driven printers and lightning storms. "Noisy" power can interfere with some components, especially those operating in high fault-tolerance environments like medical facilities.

Prime utility power is usually maintained at a specific number of cycles per second (60Hz in the US, 50 Hz in Europe and other areas). Standby generators may not deliver the full 60Hz on startup, or as loads are moved onto the generator. These variations in frequency can cause data loss and damage to sensitive components.

A switching transient is also called a "notch" or instantaneous undervoltage. It is an immediate drop in voltage that usually lasts for a few microseconds, often less than a full wavelength. However, for very sensitive components this can cause damage or data loss.

Harmonic distortion results in an enviroment where there are "non-linear" loads. Common office machinery like copiers, laser printers and fax machines have a nominal power draw while they are in stand-by, but increase load significantly when they need to heat up their fuser assemblies prior to output. This shift in the load causes a "feedback" into the office power grid, that may result in disruptions of sensitive components, overheating and possibly even hardware damage.