Everywhere 60 cycle power is used, a local ground is attached to
the power grid to return all unbalanced current flow to ground. 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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