Application note from Littelfuse on sources of transient voltages that can induced in electronic circuits. Link here (PDF)
This Note is an overview of the sources and nature of various transient overvoltages, and the problems that may result.
Transients in electrical circuits result from the sudden release of previously stored energy. This energy can be stored within the circuit and released by a voluntary or controlled switching action or it can be outside the circuit and injected or coupled into the circuit of interest.
Transients may occur either in repeatable fashion or as random impulses. Repeatable transients, such as commutation voltage spikes, inductive load switching, etc., are more easily observed, defined and suppressed. Random transients occur at unpredictable times, at remote locations, and may require installation of monitoring instruments to detect their occurrence. Experience has been accumulated to provide reasonable guidelines of the transient environments in low voltage AC power circuits, telecommunications equipment and automotive electrical systems.
Effective transient overvoltage protection from a clamping device requires that the impulse energy be dissipated in the suppressor and the voltage held low enough to ensure the survival of circuit components. The following sections will discuss in detail the two categories of transients, how they occur, their effects and their detection.