Terms and definitions

Here are the standard types of power quality disturbances. For information on how to reduce these disturbances, see power quality buffering equipment applications.

Distortion (harmonics)


Distortion occurs when harmonic frequencies are added to the 60 Hertz (60Hz) voltage or current waveform, making the usually smooth wave appear jagged or distorted. Distortion can be caused by solid state devices such as rectifiers, adjustable speed controls, fluorescent lights, and even computers themselves.

At high levels, distortion can cause computers to malfunctions and cause motors, transformers, and wires to heat up excessively. Distortion is probably the most complicated and least understood of all power disturbances.

Frequency deviations


Normal utility power in the United States is supplied at a frequency of 60 cycles per second, or 60 Hertz (60Hz). On large interconnected utility systems such as SRP's, frequency is very stable and deviations are rarely a problem.

However, on smaller power systems, especially those supplied by on-site generators, frequency deviations can cause electronic equipment to malfunction and affect the speed of motor driven clocks.



Transients are sudden but significant deviations from normal voltage or current levels. Transients typically last from 200 millionths of a second to half a second. Transients are typically caused by lightning, electrostatic discharges, load switching or faulty wiring.

Transients can erase or alter computer data, resulting in difficult-to-detect computational errors. In extreme cases, transients can destroy electronic circuitry and damage electrical equipment.



Noise, or more specifically electrical noise, is a rapid succession of transients tracking up and down along the voltage waveform. The magnitude of these rapid transients is usually much less than that of an isolated transient.

Noise often originates in electrical motors and motor control devices, electric arc furnaces, electric welders, relays, and remote atmospheric discharges such as lightning.

Although less destructive than a large rapid transient, electrical noise can cause computers to malfunction and can interfere with the operation of communications equipment or other sensitive electronic equipment.

Voltage sags


A voltage sag is a short duration decrease in voltage values. Voltage sags longer than two minutes are classified as undervoltages. Common causes of voltage sags and undervoltages are short circuits (faults) on the electric power system, motor starting, customer load additions, and large load additions in the utility service area.

Sags can cause computers and other sensitive equipment to malfunction or simply shut off. Undervoltage conditions can damage certain types of electrical equipment.



Interruptions occur when voltage levels drop to zero. Interruptions are classified as momentary, temporary, or long-term. Momentary interruptions occur when service is interrupted, but then is automatically restored in less than two seconds.

Temporary interruptions occur when service is interrupted for more than two seconds, but is automatically restored in less than 2 minutes. Long-term interruptions last longer than two minutes and may require field work to restore service.

In some cases, momentary outages may go unnoticed or cause no apparent problems. However, even momentary outages can last long enough to shut down computers and disrupt the operation of sensitive electrical equipment.

Voltage swells


A voltage swell is a short duration increase in voltage values. Voltage swells lasting longer than two minutes are classified as overvoltages. Voltage swells and overvoltages are commonly caused by large load changes and power line switching.

If swells reach too high a peak, they can damage electrical equipment. The utility's voltage regulating equipment may not react quickly enough to prevent all swells or sags.


Flicker can be defined as small amplitude changes in voltage levels occurring at frequencies less then 25 Hertz (25Hz). Flicker is caused by large, rapidly fluctuating loads such as arc furnaces and electric welders.

Flicker is rarely harmful to electronic equipment, but is more of a nuisance because it causes annoying, noticeable changes in lighting levels.