Common-mode impedance coupling
From Electrical Installation Guide
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.
(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
(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