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A power surge is a fast yet short duration of electrical transients in current, voltage or transferred energy that happens in an electrical circuit.Typically a power surge is caused by an oversupply in voltage either from the power company or from various external sources, although an oversupply of current is also possible; either way it affects the overall power moving through the electrical lines, hence the term power surge.
A power surge is a phenomenon that results when something causes a boost in the overall electrical charge at some point in power lines. This event increases the electrical potential energy in the lines and causes more power to be discharged from the wall socket and into appliances or a load. The most well-known cause of power surges is electrical storms. When lightning strikes near power lines, even if there is no direct contact, the rapid ionization of the air and the sheer amount of energy in the lightning are enough to induce additional potential energy in the lines, causing a power surge. The best way to prevent appliances from being damaged by power surges caused by lightning is to unplug them during lightning storms.
A power surge can also be caused by rapidly changing load within a circuit, that of a home or an office building, such as in appliances and machinery, which have large motors that turn inductive and capacitive loads on and off. The inductive or capacitive loads found in appliances like air conditioners and refrigerators draw in lots of power, causing the overall voltage going to other appliances to drop. The sudden switching off of such appliances can cause the overall voltage to spike, causing a slight surge, one that is usually not harmful since it can be within tolerances for most appliances, but if the inductive or capacitive load is big enough, then the surge can also be big, which is why in situations where such loads are present, surge protectors are also installed. This can happen in a widespread manner when the whole portion of an electric grid goes down, causing power surges in other portions of the grid since the overall load has just come down, but the generated power remains the same until the generated or supplied power is lowered automatically or manually by the power company.