Capacitive Coupling

Capacitive Coupling

Mechanism Capacitive coupling is a coupling via the electric field. It occurs between neighboring circuits - such as between high power current and signal lines. A fluctuating potential difference between the two circuits allows electrical current to flow through the insulation medium, air for example, that lies between them. The two lines that are lying next to one another can be considered as electrodes of a capacitor which is indicated by coupling capacity CK.

Well known sources of interference are:  Switching off power lines  Inductivity switching  Lightning discharges  Electrostatic discharge The following circuit diagram shows the construction of capacitive coupling. Circuit 1 indicates a high power line for example and circuit 2 an analog measurement line. When the high power line is switched off, the potential difference between the two neighboring lines is changed. Interference current iK flows through the coupling capacity:

Character definitions:

Character Meaning 1 Circuit 1: Interference source (high power cable for example)

2 Circuit 2: Susceptible equipment with impedance Z2

CK

iK Coupling capacity

Interference current flowing through the coupling capacity

Size of the interference The amount of interference current ISt caused by capacitive coupling depends on coupling capacitance CK between the two conductors and the duration of the change in voltage du/dt on the power cable.

The interference voltage created in the susceptible equipment (circuit 2) depends on:

NOTE: The interference voltage created in the susceptible equipment is proportional to the value of impedance in the susceptible equipment. And the impedance increases with the frequency of the interference signal. This results in the interrelationships.  High impedance measurement transfer lines are more susceptible to interference than low impedance circuits.  The interference current increases with the frequency of the voltage that exists in the interference capacity of the "connecting clamps".  High coupling capacitances create a short circuit between the circuits that influence one another for HF interferences.

Coupling capacitance CK Coupling capacitance CK increases linearly with the distance that the two lines run in parallel and decreases according to a algorithm with the increased cable spacing:

l Distance that the lines run in parallel d Distance of the lines from one another D Line diameter

Realistic example values for coupling capacitance CK with a line diameter of d = 1 mm:  Tightly packed cable: CK up to 100 pF/m  Cable spacing 10 cm: CK = approx. 5 pF/m

NOTE: Starting with a distance of D = 20 cm, CK only decreases minimally.