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Quantifying Residential Voltage Sags

Category: PMI Products, Voltage Sag, Guardian

Read why it is important to quantify voltage sags at the service entrance, in order to determine if there is a sag issue on the customer or utility side of a meter.


Voltage sag complaints are one of the most common in residential applications. Despite complaints, many residential voltage sags are not actually troublesome, or caused by the utility.


Quantifying sag events and comparing them to industry-accepted thresholds helps separate true problems from situations where the customer is more sensitive than their equipment. Methods to quantify voltage sags and present this information in a clear manner are shown in this paper.


A voltage sag is defined by IEEE 1159 as a “decrease in RMS voltage to between 0.1 and 0.9 pu at the power frequency for durations of 0.5 cycle to 1 minute.” For a 120V nominal, this corresponds to 12V to 108V.


Most deep sags are caused by upstream faults that are quickly cleared by reclosers or distribution protection. Local loads, especially motor starts and other high inrush-current devices, can cause voltage sags, but these are often not as deep as utility faults.


Poor connections or wiring problems can introduce extra resistance in a circuit, causing excessive voltage drop, and thus excessive sag as seen by downstream devices. Voltage events on a faster time scale are oscillatory or impulsive transients. Slower voltage events are interruptions, or regulation issues such as under- or over-voltage...


This White Paper Includes:

  • Voltage Sag Overview
  • Spotting Voltage Sags
  • Measuring Voltage Sags
  • Determining Sag Acceptability

Download White Paper Here