What is a Voltage Sag?
A sag or dip, as defined by IEEE Standard 1159-1995, IEEE Recommended Practice for Monitoring Electric Power Quality, is a decrease in rms voltage or current at the power frequency for durations from 0.5 cycles to 1 minute, reported as the remaining voltage. Typical values are between 0.1 pu and 0.9 pu. Terminology used to describe the magnitude of a voltage sag is often confusing. The recommended usage is “a sag to 20%”, which means that the line voltage is reduced down to 20% of the normal value, not reduced by 20%. Using the preposition “of” (as in “a sag of 20%”, or “a 20% sag”) is discouraged. This preference is consistent with IEC practice, and with most disturbance analyzers which also report remaining voltage. Just as an unspecified voltage designation is accepted to mean line-to-line potential, so also will an unspecified sag magnitude refers to the remaining voltage. Where possible, specify the nominal, or base, voltage and the remaining voltage. Voltage sags are usually asso
