Ground loop

A ground loop forms when two pieces of equipment are connected to ground at two different points, and those two 'grounds' are not actually at the same voltage. Now there are two paths back to earth, and current sneaks around the loop you accidentally created. The classic symptom is a 50/60 Hz hum in audio or a slow-rolling bar on a video screen: the hum you hear is mains current quietly circulating through your signal's ground wire.

Two things drive a ground loop. A voltage difference between the two ground points (often from mains current flowing in building wiring) pushes current around the loop; and any changing magnetic field passing through the loop's enclosed area induces extra current, exactly like a transformer with one turn. The fixes follow directly: break the loop (ground at a single point), shrink the loop area (route signal and return side by side), or insert isolation — an optical, magnetic or transformer-coupled break — so no DC ground current can flow yet the signal still passes.