A ground loop is an electrical problem that happens when an AC current interferes with the ground reference level of the video signal. It occurs on the conductive path formed by the shield of the video cable and the chassis of the video equipment. A loop results from the difference between the voltage potential of the shield at one end of the cable, versus the voltage potential of the shield at the other end.
The electrical level of the shield is usually 0 volts. When a ground loop is present, this level fluctuates above and below 0 volts. The greater the difference, the more severe the distortion or tearing. If the potential is too great it can destroy the equipment.
What to look for:
Slight distortion of the picture
Hum bar or bars
Ground loops are an after-the-fact type of problem in which the end-user blames the installer, the installer blames the manufacturer, and actually nobody is at fault. Neither the manufacturer nor the installer can predict where a loop will occur. Only after the system is installed can it be determined if a problem will exist.
Ground loop problems can be corrected. It is important for both the dealer and the end user to be aware that this problem can occur. A ground loop problem may occur at several points in the system, and each occurrence of the problem must be corrected individually. Loops can occur between a camera and monitor, from a camera to a switcher or one of many other possibilities.
Not all ground loops show up as picture disturbances. Erratic or strange behavior of other devices can be traced to grounding problems. Remember, wires or connections labeled "common" or "neutral" are not mechanical ground connections even though they may be grounded within the equipment. Each should be treated as a separate wire and not allowed to touch ground or each other.
Many people will resort to shortcuts and try to remove the earth ground (which is the middle prong on a 3-prong electrical plug) from the camera or the associated equipment. Not only is this a very hazardous thing to do, it is also a violation of U.L. laws. When the earth ground is removed, a voltage can appear on the camera chassis. The camera will have what is called a "hot chassis" that can send 60 to 70 volts of electricity through anyone who touches it, which is enough to knock anyone off their feet.