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Power quality disturbances range from surges that last only microseconds to outages that continue for hours. Some common examples include the following: Descriptions of some common problems are: Flicker – Repetitious variations in electric service voltage that cause the intensity of a light bulb to change visibly. Undervoltages and overvoltages – Abnormally low or high voltage lasting more than a few seconds. Low voltage is more common and can result from overloaded circuits. 
Interruptions – Momentary gaps in power lasting up to two seconds when your power company’s automatic devices act to protect your local power supply. Each interruption is a signal that a longer outage has probably been prevented. 
Outages (blackouts) – Complete loss of power lasting from two seconds to several hours. The most common causes are lightning and wind during storms, animal contact with lines, auto accidents involving poles, and construction crews mistakenly digging up underground cables. 
Surges or spikes – Sharp, very brief increases in voltage lasting from microseconds to milliseconds-so fast you may not even see a light flicker. A nearby lightning flash can cause a very large spike. Energizing air conditioners, refrigerators, and other large appliances with motors can create voltage spikes. Despite their brevity, surges can disable a microprocessor. 
Voltage Sag
Voltage Swell Sags or swells – Momentary decreases or increases in voltage lasting less than a few seconds. Distant short circuits or the operation of large appliances, such as air conditioners, can cause sags. Sags that last more than a few seconds are often called brownouts. 
Harmonic Distortion – The ac sine wave can sometimes take on a different shape due to the presence of multiple frequencies. This is almost never a concern in homes. 
Electrical Noise – House wiring can pick up radio or TV signals, or similar high-frequency “noise”, from microwave ovens, fluorescent lights, laser printers, fax/copy machines or even faulty wiring or grounding. Although rarely damaging, noise can be annoying, appearing as snow or bands on a TV or audible stereo interference. | 
