Question E4E08:
What current flows equally on all conductors of an unshielded multiconductor cable?  

Choices:  
A: Differential-mode current  
B: Common-mode current  
C: Reactive current only  
D: Magnetically-coupled current only    

Correct Answer is  B: Common-mode current  

To answer E4E08:

Correct: B — Common-mode current

1) Why it’s correct
- Common-mode current is the component that flows in the same direction and with equal magnitude on all conductors of a cable, returning via an external path (chassis, ground, or space).  
- On an unshielded multiconductor cable, the equal-in-all-wires component is, by definition, the common-mode current.

2) Why the others are incorrect
- A: Differential-mode current  
  - Differential-mode flows in opposite directions on different conductors (equal and opposite pairs), not equally and in the same direction on all conductors.
- C: Reactive current only  
  - “Reactive” describes the phase relationship (due to reactance), not the distribution across conductors. It doesn’t describe “equal on all conductors.”
- D: Magnetically-coupled current only  
  - Magnetic coupling is a mechanism of induction, not a current mode. It doesn’t specify the equal-on-all-conductors property.

3) Useful formulas and rules of thumb
- For a 2-conductor line with currents $I_A$ and $I_B$:
  $$I_{CM} = \frac{I_A + I_B}{2}, \quad I_{DM} = \frac{I_A - I_B}{2}$$
  - Common-mode: same direction, tends to radiate; choked by ferrites.  
  - Differential-mode: equal and opposite, fields largely cancel; twisting helps reduce radiation.
- For $n$ conductors, the common-mode component is the equal part on each conductor; the net sum for common-mode is nonzero:
  $$\sum I \neq 0 \quad (\text{common-mode}), \qquad \sum I = 0 \quad (\text{differential-mode})$$

Rule of thumb: If current is equal and in the same direction on all conductors, it’s common-mode.