Overvoltage

When the voltage in a circuit or part of it is raised above its upper design limit, this is known as overvoltage. The conditions may be hazardous. Depending on its duration, the overvoltage event can be transient—a voltage spike—or permanent, leading to a power surge.

Electronic and electrical devices are designed to operate at a certain maximum supply voltage, and considerable damage can be caused by voltage that is higher than that for which the devices are rated.

For example, an electric light bulb has a wire in it that at the given rated voltage will carry a current just large enough for the wire to get very hot (giving off light and heat), but not hot enough for it to melt. The amount of current in a circuit depends on the voltage supplied: if the voltage is too high, then the wire may melt and the light bulb would have "burned out real time". Similarly other electrical devices may stop working, or may even burst into flames if an overvoltage is delivered to the circuit.

A typical natural source of transient overvoltage events is lightning. Bursts of solar wind following solar flares are also known to cause overvoltage in electrical circuits, especially onboard space satellites.

Man-made sources of spikes are usually caused by electromagnetic induction when switching on or off inductive loads (such as electric motors or electromagnets), or by switching heavy resistive AC loads when zero-crossing circuitry is not used - anywhere a large change of current takes place. One of the purposes of electromagnetic compatibility compliance is to eliminate such sources.

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