A sag is a short reduction in RMS voltage, most often caused by a fault elsewhere on the network or by a large motor starting, lasting from half a cycle to about a minute. It is the most common power quality event and the one that drops out contactors, resets PLCs and trips drives on undervoltage. Events are described by retained voltage and duration and plotted against equipment tolerance curves such as ITIC or SEMI F47. Line crews also use the word for the vertical dip of an overhead conductor between supports, which grows as the wire heats.
Every time the crusher starts we see a sag to 82 percent on the plant bus and the packaging line drops out.
IEC documents and most of Europe say dip; North Americans say sag. Same event.
Also heard as
- voltage sag
- voltage dip
- dip
Related terms
Swell
Short-lived rise in RMS voltage above normal, often seen on the healthy phases during an earth fault.
Brownout
Sustained drop in supply voltage, caused by overload or applied on purpose to cut demand during a shortage.
Fault current
Abnormally high current that flows when insulation fails and a short circuit or earth fault forms.
Stiff
Describes a supply with low source impedance, so its voltage barely moves when load or fault current changes.