Voltage spike
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Voltage spikes are fast, short duration surges (overvoltages) in the electric potential in a given circuit. These are typically caused by lightning strikes, although power outages, tripped circuit breakers, short circuits, and power transitions in other large equipment on the same power line, and malfunctions caused by the power company can also cause them. While technically, the pulse produced by a nuclear explosion detonation produces a voltage spike, these rare events are commonly referred to as an electromagnetic pulse (EMP).
The effect of a voltage spike is to produce a temporary increase in current. For sensitive electronics this can cause excessive current. In semiconductor junctions, the voltage may exceed the reverse breakdown potential, thus destroying or severely weakening the device. A transient voltage suppression diode, transil, varistor, or a range of other overvoltage protective devices can be used to minimize this damage.
While generally referred to as a voltage spike, the phenomenon in question is actually an energy spike, in that it is measured not in volts but in joules. The root cause is the rapid buildup and decay of a magnetic field, which will induce whatever voltage on the lines as is necessary to pass the given quantity of energy.
Most equipment damage from surges and spikes can be prevented by the use of surge protection equipment.
