Voltage sag

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A voltage sag (U.S. English) or voltage dip[1] (British English) is a short duration reduction in rms voltage which can be caused by a short circuit, overload, or starting of electric motors.[2] A voltage sag happens when the rms voltage decreases between 10 and 90 percent of nominal voltage for one-half cycle to one minute.[2][3] Some references define the duration of a sag for a period of 0.5 cycle to a few seconds,[4][5] and a longer duration of low voltage would be called a "sustained sag."[4]

Related notions[edit]

The term "sag" should not be confused with a brownout, which is the reduction of voltage for minutes or hours.[6]

The term "transient," as used in power quality, is an umbrella term and can refer to sags, but also to swells, dropouts, etc.[7]


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.[8]


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

  • Since 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.[2][8]
  • When a line-to-ground fault occurs, there will be a voltage sag until the protective switch gear operates.[2][8]
  • Some accidents in power lines such as lightning or a falling object can be a cause of line-to-ground fault and a voltage sag as a result.[8]
  • Sudden load changes or excessive loads can cause a voltage sag.[8]
  • Depending on the transformer connections, transformers energizing could be another reason for voltage sags happening.[5]
  • Voltage sags can arrive from the utility but most are caused by in-building equipment. In residential homes, voltage sags are sometimes seen when refrigerators, air-conditioners, or furnace fans start up.

See also[edit]


  1. ^ "IEEE 493-2007 - IEEE Recommended Practice for the Design of Reliable Industrial and Commercial Power Systems". The IEEE Standards Association. 2007-02-12. Retrieved 2018-01-09.
  2. ^ 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.
  3. ^ "Industrial Voltage Regulator Power Conditioner". Utility Systems Technologies. Retrieved 25 September 2013.
  4. ^ 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.CS1 maint: multiple names: authors list (link)
  5. ^ 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.
  6. ^ Standler, Ronald B. (1989). Protection of electronic circuits from overvoltages. New York: Wiley. p. 40. ISBN 9780471611219.
  7. ^ R. Sastry Vedam; Mulukutla S. Sarma (2008). Power QVAR Compensation in Power Systems. CRC Press. pp. 4–5 and 23. ISBN 978-1-4200-6482-7.
  8. ^ 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.