EMC Question of the Week: May 8, 2023

Common-mode current probe around a shielded cable

A common-mode current probe measures 10 μA of current at 50 MHz on a shielded twisted pair cable. The twisted wire pair itself carries 30 μA of common-mode current at 50 MHz. How much 50-MHz current flows on the outer surface of the shield?

  1. 10 μA
  2. 20 μA
  3. 30 μA
  4. 40 μA

Answer

The best answer is “a.” All of the high-frequency common-mode current flows on the outer surface of a cable shield. The current on the inner surface of a cable shield is equal in magnitude and 180o out of phase with the common-mode current on the wires inside the shield. This will be true no matter how the cable shield is terminated, because field lines emanating from the shielded wires cannot penetrate the shield at high frequencies. The current induced on the inner surface of the shield must be equal and opposite to the common-mode current on the shielded wires.

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