What is a ground loop and how does it cause problems?

A ground loop occurs when two or more ground points are at different potentials, creating current flow through signal cable shields or grounds. Effects: DC offset in measurements (different ground potentials add to signal), AC noise pickup (50/60 Hz hum from power systems), and intermittent readings (varying ground potential). Prevention: single-point grounding, signal isolation at field device or I/O card, proper shielding (ground shield at one end only), and differential inputs with high CMRR. Ground loops are common problems in industrial installations with long cable runs.