Why should shielding not be grounded at both ends?

• A. It can create ground loops
• B. It always improves shielding
• C. It prevents EMI
• D. It increases signal strength

Answer: (A)

Shielding around a signal cable works by giving external noise a low-impedance path to ground, so that interference is diverted away from the conductors. When you ground the shield at both ends, you create a complete electrical loop that connects the two ground points through the shield. If there’s any voltage difference between those grounds, a current will flow around that loop. The shield then carries this circulating current, and because the shield has finite resistance and inductance, a voltage drop and noise can appear along its length. That noise can couple into the signal conductors or into the equipment at either end, degrading the measurement rather than improving it. So grounding at both ends can invite ground loops and EMI in the signal path, which is why it’s usually avoided. In practice, shielding is typically grounded at a single point to drain noise without forming a loop.