Transients or spikes, also referred to as transient spikes, are brief periods of extreme voltage in an electrical system resulting from adverse system conditions ranging from crossed transmission lines to lightning strikes to switching faults originating at a distribution facility. A strong or prolonged spike can instantly destroy a wide range of electronic equipment, and even frequent spikes of less extreme voltage can cause accelerated deterioration of electronic components. Harmful transients can be short enough that they are measured in millionths of a second, although they could last as long as several seconds depending on the cause and severity. A spike from a lightning strike could last for up to a second and elevate system voltage from several hundred to several thousand volts. A spike from a switching fault could produce tens of thousands of volts of pressure for only a few millionths of a second.

Line surge is the term usually applied to longer transient spikes, although correctly applies to transients of any length that are carried to a device by electrical power lines.

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