voltage on coax cable


Old 05-18-06, 07:44 PM
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voltage on coax cable

Hello I have a cable modem and have been having connection issues so I called road runner, they sent a tech guy out who thus checked the line. He said there was about 12volts or so and that it could be affecting the cable modem (slowly burning it out or something).

The coax cable is grounded through a cable that is connected to the breaker box which is in turn connected to a ground rod.
I went to see if the cable really did have voltage and I couldn't confirm this. I'm guessing I'm not using the multimeter properly, one point should be on the coax cable shielding, obviously, but where should i be placing the other point? It can't be the ground rod, because that'd be like putting both points on the same cable since its all connected.
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Old 05-19-06, 06:13 AM
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The coax has _two_ conductors. The center conductor and the outer shield. You always expect _some_ voltage on the center conductor; it carries the signal after all. The outer shield should be grounded and at 0V relative to any other grounded components in your home.

Checking for voltage between shield and another ground is useful; you are correct that they _should_ be connected and thus should be at zero voltage; so if you measure a voltage this indicates a bad connection.

Checking for voltage between the center conductor and the shield or ground is only useful if you know what the voltage should be in your system. It could reasonably be anything from 0V (AC signal only) on up. Sometimes components (TVs, VCR, etc) can fail and place voltage on the center conductor that shouldn't be there.


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