According to the Electric Power Research Institute (EPRI) Power Electronics Applications Center, “A sag or dip, as defined by IEEE (Institute of Electrical and Electronics Engineers) Standard 1159-1995, ‘IEEE Recommended Practice for Monitoring Electric Power Quality,’ is a decrease in rms (root-mean-square) voltage or current at the power frequency for durations from 0.5 cycles to 1 minute, reported as the remaining voltage . . .
“Terminology used to describe the magnitude of a voltage sag (or short-duration voltage decrease) is confusing. The recommended usage is ‘a sag to 20 percent,’ which means that the line voltage is reduced down to 20 percent of the normal value, not reduced by 20 percent . . . The IEC (International Electrotechnical Commission) definition for this phenomenon is dip. The two terms are considered interchangeable, with sag being preferred in the United States . . .
“Typical sag durations defined in some publications range from two milliseconds (about one-eighth of a cycle) to a couple of minutes. Sag durations are subdivided here into three categories—instantaneous, momentary and temporary—coinciding with the three categories of interruptions and swells.”
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