A brownout is a prolonged undervoltage lasting minutes to hours, as opposed to a brief sag. It happens when a network is overloaded, or when an operator deliberately lowers voltage by 3 to 5 percent through tap changers to trim resistive demand, a practice utilities call voltage reduction, or conservation voltage reduction when it runs routinely to save energy. Lamps dim, motors draw more current and run hotter, and some electronic loads drop out.
We went to 5 percent voltage reduction across the metro, so expect brownout complaints from the usual industrial customers.
Also heard as
- sustained undervoltage
Do not mix it up with
- Sag: A sag lasts from half a cycle to about a minute; a brownout is a sustained low voltage.
Related terms
Sag
Brief drop in RMS supply voltage, usually to between 10 and 90 percent of nominal, lasting up to about a minute.
Load shedding
Deliberate disconnection of blocks of customer load to stop frequency or voltage from collapsing.
OLTC
On-load tap changer: a transformer switch that changes the turns ratio under load to regulate voltage.