Ground loops in DCS analog I/O circuits occur when there are multiple ground paths between devices at different potentials , creating unwanted circulating currents that corrupt 4–20 mA signals. Root Conditions for Ground Loops A ground loop requires two things simultaneously : At least two different ground points at different electrical potentials A galvanic (conductive) path connecting those grounds Common Causes in DCS Analog I/O Cause How It Happens Typical Symptom Multiple ground connections in single loop Transmitter and DCS card both grounded; signal wire creates third path Fluctuating 4–20 mA readings; actual current differs from commanded (e.g., send 16 mA, receive 12 mA) Different ground potentials across locations Field device grounded at plant, DCS grounded at control room; ground resistance differs Intermittent signal noise; offset errors; digital multimeter shows mV difference between grounds Cable shield grounded at both ends Drain wire ...