Voltage sag

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A voltage sag (U.S. English) or voltage dip (British English) is a short duration reduction in rms voltage which can be caused by a short circuit, overload or starting of electric motors.[1] A voltage sag happens when the rms voltage decreases between 10 and 90 percent of nominal voltage for one-half cycle to one minute.[1][2] Some references defines the duration of a sag for a period of 0.5 cycle to a few seconds,[3][4] and longer duration of low voltage would be called a "sustained sag".[3]


The term "sag" should not be confused with brownout which is the reduction of voltage for minutes or hours,[5] or transient which is the reduction of voltage for a very short duration (less than a half cycle).


Voltage swell is the opposite of voltage sag. Voltage swell, which is a momentary increase in voltage, happens when a heavy load turns off in a power system.[6]


There are several factors which cause a voltage sag to happen:

  • Since the electric motors draw more current when they are starting than when they are running at their rated speed, starting an electric motor can be a reason of a voltage sag.[1][6]
  • When a line-to-ground fault occurs, there will be a voltage sag until the protective switch gear operates.[1][6]
  • Some accidents in power lines such as lightning or falling an object can be a cause of line-to-ground fault and a voltage sag as a result.[6]
  • Sudden load changes or excessive loads can cause a voltage sag.[6]
  • Depending on the transformer connections, transformers energizing could be another reason for happening voltage sags.[4]
  • Voltage sags can arrive from the utility but most are caused by in-building equipment. In residental homes, we usually see voltage sags when the refrigerator, air-conditioner or furnace fan start up.


  1. ^ a b c d Bollen, Math H.J. (1999). Solving power quality problems : voltage sags and interruptions. New York: IEEE Press. p. 139. ISBN 978-0-7803-4713-7. 
  2. ^ "Industrial Voltage Regulator Power Conditioner". Utility Systems Technologies. Retrieved 25 September 2013. 
  3. ^ a b Vijayaraghavan, G, Mark Brown and Malcolm Barnes (2004). Practical grounding, bonding, shielding and surge protection. Oxford: Newnes. p. 134. ISBN 978-0-08-048018-3. 
  4. ^ a b Remus Teodorescu, Marco Liserre, Pedro Rodríguez (2011). Grid Converters for Photovoltaic and Wind Power Systems. Wiley-IEEE Press. ISBN 978-1-119-95720-1. 
  5. ^ Standler, Ronald B. (1989). Protection of electronic circuits from overvoltages. New York: Wiley. p. 40. ISBN 9780471611219. 
  6. ^ a b c d e Kazibwe, Wilson E.; Sendaula, Musoke H. (1993). Electric power quality control techniques. New York: Van Nostrand Reinhold. p. 11. ISBN 978-0-442-01093-5.