Common-mode impedance coupling

Definition

Two or more devices are interconnected by the power supply and communication cables (see Fig. R28). When external currents (lightning, fault currents, disturbances) flow via these common-mode impedances, an undesirable voltage appears between points A and B which are supposed to be equipotential. This stray voltage can disturb low-level or fast electronic circuits.

All cables, including the protective conductors, have an impedance, particularly at high frequencies.

The exposed conductive parts (ECP) of devices 1 and 2 are connected to a common earthing terminal via connections with impedances Z1 and Z2.
The stray overvoltage flows to the earth via Z1. The potential of device 1 increases to Z1 I1. The difference in potential with device 2 (initial potential = 0) results in the appearance of current I2.
[math]\displaystyle{ Z1\, I1=\left ( Zsign\, + Z2 \right )I2\Rightarrow \frac{I2}{I1}=\frac{Z1}{\left ( Zsign\, + Z2 \right )} }[/math]
Current I2, present on the signal line, disturbs device 2.

Fig. R28 – Definition of common-mode impedance coupling

Examples

(see Fig. R29)

• Devices linked by a common reference conductor (e.g. PEN, PE) affected by fast or intense (di/dt) current variations (fault current, lightning strike, short-circuit, load changes, chopping circuits, harmonic currents, power factor correction capacitor banks, etc.)
• A common return path for a number of electrical sources

Fig. R29 – Example of common-mode impedance coupling

Counter-measures

(see Fig. R34)

If they cannot be eliminated, common-mode impedances must at least be as low as possible. To reduce the effects of common-mode impedances, it is necessary to:

• Reduce impedances:
  • Mesh the common references,
  • Use short cables or flat braids which, for equal sizes, have a lower impedance than round cables,
  • Install functional equipotential bonding between devices.
• Reduce the level of the disturbing currents by adding common-mode filtering and differential-mode inductors

If the impedance of the parallel earthing conductor PEC (Z sup) is very low compared to Z sign, most of the disturbing current flows via the PEC, i.e. not via the signal line as in the previous case.
The difference in potential between devices 1 and 2 becomes very low and the disturbance acceptable.

Fig. R30 – Counter-measures of common-mode impedance coupling