Specifications

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 laying in a cable tray or conduit box
can accidentally contact ground, don't let this happen. Use tape 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 monitor equipment.
At the monitor station you may have many pieces of equipment connected
together, like a (Quad, Tape Recorder, Monitor) all of which plug into the main 60
cycle power. This will not present a problem if you plug all of the equipment into the
same power line at the monitor point. Making sure that all the equipment share the
same ground point at the monitor station. Also try to keep the video cables between
equipment, (the service loops) as short as possible.
If you already have an installation that has 60 cycle 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.
CCTV TRAINING MANUAL PAGE 4 OF 28