brownout and blackout

brownout

 is an intentional or unintentional drop in voltage in an electrical power supply system. Intentional brownouts are used for load reduction in an emergency. The reduction lasts for minutes or hours, as opposed to short-term voltage sag (or dip). The term brownout comes from the dimming experienced by lighting when the voltage sags. A voltage reduction may be an effect of disruption of an electrical grid, or may occasionally be imposed in an effort to reduce load and prevent a power outage known as a blackout.

blackout

 is a complete interruption of power in a given service area. Rolling blackouts are controlled and usually preplanned interruptions of service. A brownout is a partial, temporary reduction in system voltage or total system capacity. Blackouts come without warning, last for indeterminate periods, and are typically caused by catastrophic equipment failure or severe weather. The nature and cause of the blackout determines who is affected. Rolling blackouts typically occur with at least some advance warning, normally last for a fixed length of time, and are deliberately produced by utility companies. They can be used as a means of coping with peak power demands that cannot be met from existing supply.