A blackout is a complete interruption of energy in a given service area. Continuous blackouts are controlled and are usually pre-planned service interruptions. A voltage drop is a partial and temporary reduction of the system voltage or the total capacity of the system.
The blackouts come without warning, last for indeterminate periods and are usually caused by catastrophic equipment failures or adverse weather conditions. The nature and cause of the blackout determines who is affected.
Rotary blackouts typically occur with at least some advance warning, usually last a fixed period of time and are deliberately produced by utilities. They can be used as a means to cope with peak power demands that can not be met by the existing supply. Rolling blackouts typically affect only a specific service area, and the power supplier will typically distribute these blackouts between various service areas to ensure that no specific area suffers substantially more than any other. Planned interruptions and blackouts differ slightly in that planned interruptions are generally announced well in advance and are more commonly needed to allow routine maintenance, while rotational blackouts can occur with relatively little warning and are intended to reduce stress of the system's energy load.
In most cases, blackouts are deliberately produced by energy providers as an emergency measure to prevent the system from completely failing (fainting). In general, a utility will decrease the system voltage by 10-25%, usually for a short period of time. This reduction generally has a minimal effect on heating and lighting systems, most of which can operate reliably for short periods with suboptimal voltage, but sensitive electronic equipment that requires reasonably accurate voltages may not work and blackouts Long term can cause premature wear on electronic devices. Computer drives often suffer write failures when supplied with suboptimal voltage, and electric motors tend to run hotter when needed to produce the same horsepower during a power failure.
Normal voltage fluctuations do not qualify as voltage drops. The system voltage in many service areas can vary up to five percent above or below the "rated" line voltage. The manufacturers of electrical and electronic products know this. Most commercial and consumer products in North America are designed to operate normally and safely for long periods at voltages ranging from 115 to 125 volts.