Common-mode impedance coupling: Difference between revisions
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== Definition == | == Definition == | ||
Two or more devices are interconnected by the power supply and communication cables (see | Two or more devices are interconnected by the power supply and communication cables (see {{FigRef|R32}}). 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.<br>All cables, including the protective conductors, have an impedance, particularly at high frequencies. | ||
[[File:Fig R22.jpg|none]] | [[File:Fig R22.jpg|none]] | ||
The exposed conductive parts (ECP) of devices 1 and 2 are connected to a common earthing terminal via connections with impedances Z1 and Z2.<br>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.<br><math>Z1\, I1=\left ( Zsign\, + Z2 \right )I2\Rightarrow \frac{I2}{I1}=\frac{Z1}{\left ( Zsign\, + Z2 \right )}</math> <br> | The exposed conductive parts (ECP) of devices 1 and 2 are connected to a common earthing terminal via connections with impedances Z1 and Z2.<br>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.<br><math>Z1\, I1=\left ( Zsign\, + Z2 \right )I2\Rightarrow \frac{I2}{I1}=\frac{Z1}{\left ( Zsign\, + Z2 \right )}</math> <br> | ||
Current I2, present on the signal line, disturbs device 2. <br><br>'''''Fig. R32:''' Definition of common-mode impedance coupling'' | Current I2, present on the signal line, disturbs device 2. <br><br>'''''Fig. R32:''' Definition of common-mode impedance coupling'' | ||
== Examples == | == Examples == | ||
(see | (see {{FigRef|R33}}) | ||
*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.) | *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 | *A common return path for a number of electrical sources | ||
[[File:Fig_R23.jpg|none]] | [[File:Fig_R23.jpg|none]] | ||
'''''Fig. R33:''' Example of common-mode impedance coupling'' | '''''Fig. R33:''' Example of common-mode impedance coupling'' | ||
== Counter-measures == | == Counter-measures == | ||
(see | (see {{FigRef|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: | 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: | ||
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*Reduce the level of the disturbing currents by adding common-mode filtering and differential-mode inductors | *Reduce the level of the disturbing currents by adding common-mode filtering and differential-mode inductors | ||
[[File:Fig R24.jpg|none]] | [[File:Fig R24.jpg|none]] | ||
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.<br>The difference in potential between devices 1 and 2 becomes very low and the disturbance acceptable. <br><br> | 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.<br>The difference in potential between devices 1 and 2 becomes very low and the disturbance acceptable. <br><br> | ||
'''''Fig. R34:''' Counter-measures of common-mode impedance coupling'' | '''''Fig. R34:''' Counter-measures of common-mode impedance coupling'' | ||
[[ru:Гальваническая (кондуктивная) связь]] | [[ru:Гальваническая (кондуктивная) связь]] | ||
[[zh:共模阻抗耦合]] | [[zh:共模阻抗耦合]] | ||
Revision as of 17:50, 13 December 2016
Definition
Two or more devices are interconnected by the power supply and communication cables (see Fig. R32). 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. R32: Definition of common-mode impedance coupling
Examples
(see Fig. R33)
- 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. R33: 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. R34: Counter-measures of common-mode impedance coupling
