CCTV INSTALLATION TIPS (cont)
As an example, you will notice that every main power breaker
box will have a ground wire or conduit going to a ground rod or
similar device connected to an earth ground. Every correctly
installed power outlet will have a connection to this ground.
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 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.
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