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Common-mode impedance coupling

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Two or more devices are interconnected by the power supply and communication cables (see Fig. R30). 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.
Z1\, I1=\left ( Zsign\, + Z2 \right )I2\Rightarrow \frac{I2}{I1}=\frac{Z1}{\left ( Zsign\, + Z2 \right )}
Current I2, present on the signal line, disturbs device 2.

Fig. R30Definition of common-mode impedance coupling


(see Fig. R31)

  • 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. R31Example of common-mode impedance coupling


(see Fig. R32)

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. R32Counter-measures of common-mode impedance coupling