Video Security Basics

Solving Ground Loop Problems

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 zero volts. When a ground loop is present, this level fluctuates above and below zero volts. The greater the difference, the more severe the distortion or tearing. If the potential is too great it can destroy the equipment.

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 switch, or one of many other possibilities.

Pelco's GIT100 ground isolation transformer is the most effective and easy solution to any ground loop problem.

If, in rare instances, some ground loops are still present after installation, it means that there is more than one source and further investigation is required.

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. (Underwriters Laboratories) laws. When the earth ground is removed, a voltage could 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.

So, the important thing to remember is that while ground loops are perplexing, they are not unexplainable. Knowing that loops do exist and understanding how to correct them will make the task of removing loops much easier – increasing the performance of your CCTV system.