Similar to the case of the inductive coupling, the capacitive coupling becomes large if:
x the two circuits are close together
x the voltage difference of the two circuits is large
x the signals in the external circuit are rapidly varying in time and therefore possess a large high frequency content.
As an example, one may consider the cables of a circuit of a power supply and of a circuit of a local area network, which may lay close to and parallel with each other over a distance of 10 m in a cable tray. If the current in the power cable has a pure sine form at 50 Hz at 230 V, the disturbing signal in the data cable reaches an amplitude of 10 V, which may be acceptable. However, if the current in the power cable possesses high frequency components generated by non-linear loads, the disturbing signal in the data cable may reach an amplitude of more than 90 V, which may lead to poor performance or malfunctions of LANs.
If the cabling and shielding requirements are planned properly, and the installation is carried out carefully, these types of disturbances can be avoided or at least minimised to a tolerable level.