A blackout is a total power outage on a portion of the electric grid. In some cases, the term is used synonymously with “power outage.” In other cases, “power outage” refers to local issues that cause a few customers to lose power while “blackout” refers to a wider-area outage.
A blackout is differentiated from a brownout in that a blackout is a total loss of power whereas a brownout is a condition where lower-than-normal voltage is delivered by the grid, but power is still provided.
Local power outages are usually caused by failure or overload of a specific component such as a transformer or substation, or a fault that results in protective equipment isolating a circuit or portion of a circuit to avoid damage or unsafe conditions. Wider area blackouts are typically caused by sudden failure of a major system component such as a transmission line or large generator. Such a failure can cause system frequency or voltage to drop below acceptable tolerances. If the system is unable to respond promptly, protective equipment will begin to shut down additional generators or lines, potentially resulting in a cascading outage that spreads through the interconnected system. After a blackout, power is slowly restored to customers in a controlled process called system restoration.
While a cascading outage is an unplanned and uncontrolled process, a rolling blackout is a planned process. Rolling blackouts are implemented purposefully by utilities when sufficient supply is not available to meet demand. In this case, utilities will disconnect specific circuits to reduce overall system load for a limited amount of time. If supply is still not available to meet demand, the initial circuits will be reconnected, and different circuits are then disconnected. This allows outages to be spread across the system without requiring any specific customer to be out of power for too long. Rolling blackouts are often announced to the public ahead of time, though they may at times occur on short notice.
Both blackouts and rolling blackouts are different from local planned outages, which are small-scale intentional outages scheduled and announced in advance to allow for equipment maintenance or upgrades.