Every installation technician has faced it or soon will - the dreaded hum bars on the screen, a telltale sign of a ground loop. Ground loops are caused by voltage potential differences at the ground point on your equipment. Voltages in the range of microvolts up to millivolts or higher can cause ground loops. Ground loops occur when multiple audio/video components are powered from different outlets, most commonly, outlets that are tied to different circuitbreakers or even a different secondary from the power transformer.
Ground loops manifest themselves in the form of hum bars called herringbone (diagonal lines on the screen) and AC beat (a wide, barely visible bar that rolls from the bottom to the top of the screen). Often you can hear the ground loop as the noise on the video encroaches on the audio signal.