Which option BEST describes how ground loops are prevented?

• A. Improve shield thickness with longer shields.
• B. Install multiple shield connections to ground at several points.
• C. Ground shields at one point, use isolators, avoid tying commons together unnecessarily.
• D. Remove shielding altogether.

Answer: (C)

Ground loops happen when there are multiple paths to ground for the signal and shield conductors, so current can flow through the shield or return paths and introduce noise into the measurement. Shielding helps protect the signal, but grounding the shield in more than one place creates loop currents that negate that protection by forming unwanted current paths. Grounding the shield at a single point keeps it as a quiet boundary and avoids those loop currents. Adding galvanic isolation in the signal path further breaks any potential ground-path differences between devices, so noise cannot travel along the loop. Avoid tying instrument grounds together unnecessarily, because that reintroduces alternate ground paths and sustains the loop. This combination effectively minimizes loop currents and keeps the 4–20 mA/HART signal clean. Options that increase shield grounding points don’t stop loops and can worsen them, and removing shielding removes protective noise reduction, so they’re not as effective.