IEEE 1159-2009, IEEE Recommended Practice for Monitoring Electric Power defines undervoltage as “a drop in AC RMS voltage in an electrical power system, typically 80% to 90% of its nominal value or lower, at the normal line frequency lasting for at least a minute.”
This is not a voltage sag, which is a much shorter duration voltage event, and not an interruption that occurs when the voltage goes to zero. Undervoltages can be an intentional or unintentional drop in voltage.
Brownout, a slang term, is sometimes used to describe an undervoltage condition. The brownout term is associated with the dimming of incandescent lighting in the past from an undervoltage condition.
This is likely where the term came from due to the change in the intensity and the brownish color of the light. IEEE discourages the use of the term “brownout”, and the more precise “undervoltage” should be used.
This whitepaper defines undervoltage, examines some typical causes of undervoltages, and explains the effects of undervoltage on customer loads.