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Last edited 17 Oct 2024

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Voltage sag

BSRIA Power quality guide (AG 2/2000) was written by C C Pearson and V Uthayanan and published by BSRIA in July 2000. It states:

A sag is a decrease to between 10% and 90% of nominal rms voltage or current at the fundamental frequency for durations from 0.5 cycles to one minute. Sags of longer than about 10 to 20 milliseconds may cause computer equipment to malfunction.

The power quality community has used the term sag for many years to describe a short-duration voltage decrease. Although the term has not been formally defined, it has been increasingly accepted and used by utilities, manufacturers, and end users. The IEC definition of this phenomenon is dip. The two terms are considered interchangeable.

Terminology used to describe the magnitude of a voltage sag is often confusing. A "20% sag" can refer to a sag which results in a voltage of 80% or 20%. The preferred terminology would be one that leaves no doubt as to the resulting voltage level: "a sag to 80%" or "a sag whose magnitude was 20%." When not specified otherwise, a 20% sag will be considered an event during which the rms voltage decreased by 20% to 80%. The nominal, or base, voltage level should also be specified.

Voltage sags are usually associated with system faults but can also be caused by energization of heavy loads or starting of large motors. They can be caused by an air conditioning (or other) motor starting or utility problems hundreds of miles away and may not be noticed by the naked eye. Some sensitive machines in a facility are affected (shut down) by voltage sags or the more severe interruptions. Voltage sags are usually far less costly to solve than interruptions, so it is important to monitor power circuits with equipment capable of capturing waveform information.