Any change in energy storage in a system involves a transient. During the transient potentially large amounts of energy are in transit along circuits as stored energy re-distributes. It is normally during these transients that the highest interference takes place.

Consider the simple example of the opening transient on an inductor.

If the switch opens and clears the circuit, the current is I₀ then the energy stored in the inductor is :

Since the current in the main circuit is forced to zero the only path for this current is through the stray capacitance, which will charge to a peak value :

which may be obtained by energy conservation :

This peak voltage can be far higher than the normal operating voltage V of the circuit.