Capacitive coupling

From Electrical Installation Guide


The level of disturbance depends on the voltage variations (dv/dt) and the value of the coupling capacitance between the disturber and the victim.
Capacitive coupling increases with:

  • The frequency
  • The proximity of the disturber to the victim and the length of the parallel cables
  • The height of the cables with respect to a ground referencing plane
  • The input impedance of the victim circuit (circuits with a high input impedance are more vulnerable)
  • The insulation of the victim cable (εr of the cable insulation), particularly for tightly coupled pairs

Figure R33 shows the results of capacitive coupling (cross-talk) between two cables.

Fig. R33 – Typical result of capacitive coupling (capacitive cross-talk)


(see Fig. R34)

  • Nearby cables subjected to rapid voltage variations (dv/dt)
  • Start-up of fluorescent lamps
  • High-voltage switch-mode power supplies (photocopy machines, etc.)
  • Coupling capacitance between the primary and secondary windings of transformers
  • Cross-talk between cables
Fig. R34 – Example of capacitive coupling


(see Fig. R35)

  • Limit the length of parallel runs of disturbers and victims to the strict minimum
  • Increase the distance between the disturber and the victim
  • For two-wire connections, run the two wires as close together as possible
  • Position a PEC bonded at both ends and between the disturber and the victim
  • Use two or four-wire cables rather than individual conductors
  • Use symmetrical transmission systems on correctly implemented, symmetrical wiring systems
  • Shield the disturbing cables, the victim cables or both (the shielding must be bonded)
  • Reduce the dv/dt of the disturber by increasing the signal rise time where possible
Fig. R35 – Cable shielding with perforations reduces capacitive coupling