Ground loop

Patent Granted In an electrical system, a ground loop is an unwanted current in a conductor connecting two points that are supposed to be at the same potential, often ground, but are actually at different potentials. Ground loops are a major cause of noise, hum, and interference in audio, video, and computer systems. They can also create an electric shock hazard, since ostensibly "grounded" parts of the equipment, which are often accessible to users, are not at ground potential.