Shielded data cables: which option describes correct grounding practice for shielding in EMI-sensitive or long runs?

• A. Use shielded cables only for power conductors; shielding is not necessary for data lines
• B. Ground shielding at both ends to prevent circulating currents
• C. Shielding adds no EMI benefit and increases cost
• D. Use shielded data cables for EMI-sensitive or long runs; ground shielding at one end to avoid circulating currents

Answer: (D)

Shielding reduces electromagnetic interference on data cables, and the way you ground that shield matters to avoid loops. For EMI-sensitive or long runs, use shielded data cables and ground the shield at a single point. This ties the shield to the system ground where it’s most stable, and it prevents the shield from carrying circulating currents along the length of the run. Grounding the shield at both ends can create a ground loop if the end grounds are at different potentials, which can introduce noise instead of suppressing it. By grounding at one end, you still gain the protective benefit of the shield while avoiding those loop currents, keeping EMI under control without adding unwanted noise.