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A transient event is a short-lived burst of energy in a system caused by a sudden change of state.
The source of the transient energy may be an internal event or a nearby event. The energy then couples to other parts of the system, typically appearing as a short burst of oscillation.
Electrical Engineering 
In electrical engineering, oscillation is an effect caused by a transient response of a circuit or system. It is a momentary event preceding the steady state (electronics) during a sudden change of a circuit.
Mathematically, it can be modeled as a damped harmonic oscillator.
An example of transient oscillation can be found in digital (pulse) signals in computer networks. Each pulse produces two transients, an oscillation resulting from the sudden rise in voltage and another oscillation from the sudden drop in voltage. This is generally considered an undesirable effect as it introduces variations in the high and low voltages of a signal, causing instability.
In electrical and electronic engineering such electromagnetic pulses (EMP) occur internally as the result of the operation of switching devices. Engineers use voltage regulators and surge protectors to prevent transients in electricity from affecting delicate equipment. External sources include lightning (LEMP), electrostatic discharge (ESD) and nuclear EMP (NEMP).
Within Electromagnetic compatibility testing, transients are deliberately administered to electronic equipment for testing their performance and resilience to transient interference. Many such tests administer the induced fast transient oscillation directly, in the form of a damped sine wave, rather than attempt to reproduce the original source. International standards define the magnitude and methods used to apply them.
The European standard for Electrical Fast Transient (EFT) testing is EN-61000-4-4. The U.S. equivalent is IEEE C37.90. Both of these standards are similar. Which standard you choose is based on the intended market.
See also 
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