14 July 2009 1 Comment

Ground Loop Issues with Security Camera Images

When you notice that your security camera pictures have either thin or thick horizontal bands or bars across the picture that slowly move up or down the screen, you may have a Ground Loop issue. Video Ground Loop problems, (sometimes referred to as 60 cycle Bars), are either faint lines or so wide that they obscure almost the entire picture.

The source of these bars are sometimes the result of the power company’s use of local grounds to balance the power grid. However, not all grounds are created equal. In fact the earth ground in one building is most likely to have a different voltage potential relative to any other building, even grounds inside the same building will have different voltages between them, based on the uneven current flow of the power load.

Here is how the 60 cycle bar gets into your video picture. If you connect a coaxial cable to a monitor or other equipment that plugs into the 60 cycle main power and the other end of the coaxial cable becomes grounded locally for any reason a Ground Loop is created. Any difference in the 60 cycle voltage between these two ground points will create a current flow in the shield of the coax that induces the 60 cycle AC voltage into your video signal.

It is easy to measure these differential voltages, simply disconnect the video cables at the monitor point and using your voltmeter on AC volts, measure between any two shields of the incoming video cables, you will be amazed at the difference.

The solution is to never connect both ends of a video cable to local grounds. Any cable can be grounded at one end without inducing the ground loop current. When you run coax cable from one building to another, it is acceptable to install through connection points, but do not allow the shields to come into contact with one another or the local ground. A coaxial connector touching a steel beam, laying in a cable tray or conduit box can accidentally contact ground, don’t let this happen. Use tape or heat shrink tubing on the connector to prevent accidental grounding. Also try not to attach the camera to any structure that is likely to be grounded. Remember that the camera is already grounded at the opposite end of the coaxial cable by the equipment. Make sure that all the equipment (DVR, Power Supply, UPS, etc.) share the same ground point at the DVR end. Also try to keep the video cables between equipment, (the service loops) as short as possible.

If you already have an installation that has Ground Loop bars, there are some steps you can take to solve the problem. If coaxial cable shields are connected together anywhere in the system, separate them if possible. Similarly remove all but one ground connection on each coaxial cable if possible. The ground is usually at the monitor end of the coaxial cable because the monitor equipment plugs into the 60 cycle main power supply which is grounded.

Sometimes a ground loop problem can be reduced by reversing the AC plug on the power transformer used to power the camera, or reverse the 24 VAC power connection to the camera. This technique will not work on DC powered cameras.

If the problem still persists, video isolation transformers can be installed at one end of the coaxial cable to block the shield current flow and eliminate the 60 cycle bars. The isolation transformers must be installed at the coaxial cable that is originating the Ground Loop problem.

One way to start isolating where the problem may be, is to use a Test or Field Monitor to find the origin of the ground loop problem. Start at the camera and move down the coaxial cable until you see the bars appear on the portable monitor. This then is the coaxial cable with the current in the shield. Clear the ground connection or install an isolation transformer at this point.

Ground Loop problems can make any or all of the equipment exhibit strange problems. For example, the DVR can start to “ghost” images on one or more channels, or even make the picture appear normal, except it shows up on a completely different channel than the one it is plugged into. I have seen ground loops make equipment do strange things. It may even damage the main DVR board if it is bad enough.

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One Response to “Ground Loop Issues with Security Camera Images”

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