If a blackout occurs while your computer is on, current work in RAM or cache is lost. A spike, also referred to as an impulse, is an instantaneous, dramatic increase in voltage. Like the force of a tidal wave, a spike can enter electronic equipment through AC, network, serial, or phone lines and damage or completely destroy components. Spikes are typically caused by a nearby lightning strike but can also occur when utility power comes back on line after having been knocked out in a storm or as the result of a car accident. Catastrophic damage can occur to hardware and data may be lost.
Surges are short-term increases in voltage, typically lasting at least a fraction of a second.
When this equipment is switched off, the extra voltage is dissipated through the power line. Computers and similar sensitive electronic devices are designed to receive power within a certain voltage range. Anything outside of expected peak and average voltage levels will stress delicate components and cause premature failure. Electrical noise is caused by many factors and phenomena, including lightning, load switching, generators, radio transmitters, and industrial equipment. It may be intermittent or chronic.