What is a ground loop?

• A. A condition where a signal cable is grounded at multiple points causing unwanted current flow
• B. A loop of copper cable used for shielding
• C. A cable that is grounded only at the power source
• D. A loop used for physical routing of wires

Answer: (A)

A ground loop occurs when the signal reference path is connected to ground at more than one point, creating an unwanted return path for current. In instrumentation and 4–20 mA loops, the measurement relies on a stable reference between transmitter and receiver. If the signal cable or shield is grounded at several locations, different ground potentials exist along the loop. This causes stray current to flow through the ground path, and the loop’s physical area can pick up electromagnetic interference from nearby power lines and equipment. The result is noise, offset, or instability in the measured signal, which is exactly what a ground loop introduces.

This matches the description of grounding a signal cable at multiple points, leading to unwanted current flow. The other descriptions describe shielding, grounding at a single point, or merely routing wires, none of which describes the problematic multiple-ground-path condition.