Variable frequency drives generate switching harmonics that couple into nearby 4-20mA instrument loops and corrupt analog readings. This article identifies the four primary coupling mechanisms, presents a six-step field diagnosis procedure, and details the shielding, filtering, and grounding techniques that reduce loop noise below the 0.05 mA threshold required for reliable SIS and DCS operation in ABB ACS880 and Schneider Altivar 630 installations.
