In the EMC World, there are two common terms that will be found in many EMI discussions or considered as fundamental concepts, those are “differential mode” and “common mode”. For conducted coupling topic, those two terms need to be understood that the two separate circuit paths can coexist in the same set of conductors.
Differential Mode: Where the signal or power flow through a conductor and return using the intended path by the designer or flowing “differentially” in opposition to each other.
Common Mode: Where the parasitic circuit (unwanted) is formed between the desired circuit (conductor and return path) and the structure within which it is located. The signal or power flows in the same direction in the circuit.
For the simplest explanation, power supply can be one of the examples to explain those 2 modes. In the Differential Mode conducted coupling, Interference between the phases appears such as in L and N or between P1/P2/P3 of the power supply and is carried into or out of the equipment by the phase conductors only. Meanwhile for Common Mode conducted coupling, the interference does not appear between the conductors. It appears on each conductor with reference to a third point and the interference loop includes this third point. In the case of power supply, there are two possibilities for the third points: a) safety earth wire and b) external structure (shown in the previous figure).
The procedure for measuring common mode currents is simple. A current clamp is put around both line and neutral wires and then, measured. For coaxial cables, the shielding is the return path. The same procedure can be applied for power transmission in direct-current (DC) and, printed circuit boards, for example. Common mode currents flow out both conductors in the same direction, usually radiating as well as conducting back into the power line.