


1. Transients


2. Interruption

3. Sag or Undervoltage
Usually caused by system faults or the switching on of loads with heavy startup currents, a sag is a reduction of AC voltage at a given frequency lasting for .5 cycles to a minute. Think of the drop in water pressure when multiple faucets in a single home are turned on. Sags can cause significant equipment damage over time.

Often referred to as a “brownout” (albeit incorrectly so)undervoltages are the results of long-term power consistency problems that create sags in the short term. Undervoltages can lead to the failure of non-linear loads such as computer power supplies.

4. Swell or Overvoltage
The opposite of a sag is a swellcaused by an increase in AC voltage lasting .5 cycles to one minute. Common sources include high-impedance neutral connections, sudden reductions in large loads or single-phase faults on a three-phase system. Like sags, the damaging effects of swells – degradation of electrical contacts and semiconductor damage – usually go unnoticed for extended periods of time. However, immediate, more noticeable results are the flickering of lights and data errors.

Similar to undervoltages, overvoltages are the result of long term problems that create temporary power swells. Overvoltage conditions can create high current draw and cause equipment to overheat and become stressed.

5. Waveform distortion

6. Voltage fluctuations

Of course there are ways to protect against each of these types of power disturbances, often by using UPS systems, backup generators, surge protectors, power line conditioners conditioning, proper grounding and more. To learn more about each of these types of power disturbances, and how to protect against them, read APC by Schneider Electric white paper number 18, “The Seven Types of Power Problems.”