Voltage in coaxial cable

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Old 08-02-16, 05:12 PM
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Voltage in coaxial cable

Recently, i had comcast run lines for their TV and internet in my home. the technician told me that i had too much voltage in the coaxial cables which causes the cable and internet modem to reset over and over again. he said that there is a problem with the wiring in the house and that needed to be addressed before he could continue with the installation. so after he left, i tested every outlet in the house with an outlet tester, and each outlet was wired up fine. (no open ground, no open neutral, no open positive). then I checked the ground on the outside of the house where the comcast line comes in from the pole, and the ground was fine. then i used my multimeter to measure the voltage on the coaxial cable that comes into the house which read 97 volts. note that this is the coaxial cable that comes from the distribution box on the outside of the house through the wall and to a spliter that goes to the internet modem and tv. now since this coaxial cable is not connected to anything, how is the wiring in the house affecting the coaxial cable?
 
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Old 08-02-16, 05:49 PM
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Hi David and welcome to the forum. It sounds like what we refer to as a phantom reading. The cause is a high impedance digital meter, it that is what you are using. Most digital meters have a high resistance to ground so the coax cables are acting like an antenna. Some of the pros here are better versed on this than I am, but terminating that cable into a device should provide the load needed to eliminate that reading.

Is your meter a digital and did you notice what the installer was using?

Bud
 
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Old 08-02-16, 05:58 PM
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The multimeter i have is a Klein Tools digital meter. and i didnt get a chance to see what the installer was using.

Whenever i plug the coaxial cable into the TV or the modem, it causes the cable and internet to reset over and over again. the installer said this is because there is too much voltage in the coaxial cables.
 
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Old 08-02-16, 06:41 PM
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From where to where did you measure the 97 volts?

Have you bonded to the home grounding electrode system the frame of the first piece of equipment into which you plugged the coax cable coming from the utility pole?
 
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Old 08-02-16, 07:17 PM
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When you measured the coax cable, were you measuring DC volts or AC volts?
Were you measuring between the center contact of the coax and the outer shell?
When you measured, the cable wasn't connected to anything but the cable company's box correct?

If you connect a short coax cable to your splitter where the cable feed would normally connect, what do you measure there?
 
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Old 08-02-16, 07:35 PM
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AllanJ: i measured the voltage on the coaxial cable that comes directly from the comcast box installed on the side of the house. im not exactly sure what you meant by the second question.

CarbideTipped: i measured for AC volts between the center contact of the coax and the outer shell. the cable was connected to only the cable companys box, nothing else. i can't measure the line again with the splitter until tomorrow, or are you talking about the connection point on the wall?
 
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Old 08-02-16, 07:40 PM
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i measured the voltage on the coaxial cable that comes directly from the comcast box installed on the side of the house.
That sounds like a Comcast problem to me. I suspect the technician didn't know what to do so he blamed it on your house wiring and left, hoping that someone else would come back on the next trip.
 
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Old 08-02-16, 07:53 PM
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thats what i was thinking. if i measured the voltage on the coaxial cable coming directly from the box, then i dont see how the wiring in the house could affect the cables.
 
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Old 08-02-16, 11:56 PM
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I checked the ground on the outside of the house where the comcast line comes in from the pole, and the ground was fine
How did you check it to determine it was fine ?
The only was to confirm the cable ground was actually ground is to check it against a known ground like an outside faucet if connected in metal pipe.

i measured for AC volts between the center contact of the coax and the outer shell.
Highly unlikely to measure 97vac from center wire to shield. Not to question your troubleshooting methods but look closely at the meter. Make sure it didn't say mv on the screen.

If there is in fact 97vac there... that is 100% the cable companies problem. There would be no ghost reading from center to shield. Do not connect anything to it as it runs the risk of frying.
 
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Old 08-03-16, 08:57 AM
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i used an extension cord and plugged it into a outlet inside the house that is known to be a good outlet and plugged my multimeter into the positive and touched the other multimeter prong onto the ground on the outside of the house and the meter read a good 120V.

and another thing i noticed too, is that i would plug my multimeter prong into a known good neutral and touch the center wire on the coaxial and get 97vac, then with the prong still in the neutral i would touch the outer shield and get 120vac.
 
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Old 08-03-16, 09:37 AM
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So then you did not measure 97v from center conductor to shield ?

There are too many inconsistencies here for us to follow you.

The cable service should be grounded at the pole and where it enters your house. I believe you told us that the ground is confirmed good. That places the shield of the cable at ground. It can't be grounded outside of the house and then have 120vac on it inside the house.
 
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Old 08-03-16, 09:47 AM
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im sorry, im trying my best to explain the situation for you. i would plug my multimeter prong into a known good neutral and touch the center wire on the coaxial and get 97vac, that is how i got the 97vac. the companys cable comes from the pole, is grounded on the outside of the house, then ran to the companys box, and then into the house. and yes, the ground on the outside of the house is good.
 
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Old 08-03-16, 10:05 AM
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Ok...so confirm again what you get when you measure to the shield.
 
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Old 08-03-16, 10:44 AM
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remeasured from center conductor to shield and got 0.04-0.05 vac
 
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Old 08-03-16, 11:58 AM
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I would stop doing your own testing. Without the right equipment, it's really hard to figure out what the actual problem is. (and the right equipment is in the thousands of dollars).

Call Comcast and have them come back out and figure out what's wrong. If they still think it's your house wiring, have them hook up the modem directly to the incoming wire and disconnect your internal wiring completely to prove that it all works without your inside wiring.

They'll either determine it is actually their problem, or narrow down the problem to your wiring - in which case may need to be repaired or replaced. You're paying them a LOT of money, let them do the troubleshooting work!
 
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Old 08-03-16, 01:09 PM
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Yeah, let's move off of the coax.

For piece of mind here, if you measure hot to neutral, and neutral to ground at a wall receptacle, what does the meter read?
 
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Old 08-04-16, 12:49 PM
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What is the voltage measured between the cable company coax shield (or plug shell) hanging loose and your house ground? Use for the latter the terminal strip inside your service panel where all the bare ground wires are attached, or any wire (such as going to a ground rod) also attached to that terminal strip.

Something to try next: Using a long wire connect the house ground to the body of your splitter. How is your reception off of the cable system?

What does the ground from the cable box on your outside wall consist of? If it includes a ground rod, connect a wire to the wire attached to that ground rod and to the fat ground wire coming out of your service panel and going to another ground rod or to a cold water pipe. (Any kind of wire can be used temporarily or experimentally; use #6 gauge copper to make a permanent connection.) Does this connection improve the reception any?
 
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Old 08-04-16, 05:12 PM
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At this point.... I don't think we should be offering furter help in this situation. I certainly don't recommend connecting grounds to something that potentially has high voltage on it.

This is pretty much an issue with the cable company and they need to sort it out.
 
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Old 08-05-16, 06:17 AM
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I would strongly suggest that at least all of your own equipment be properly grounded. For any piece of equipment in doubt, run a 14 gauge wire from a screw penetrating to the chassis or from the shell of one of the input or output jacks. Daisy chain to other pieces of equipment. Connect the far end to a known ground for example the terminal strip in the service panel where all the ground wires are attached.

Do the above while the cable company coax is not connected.

For now do not plug the cable company coax directly into any of your own equipment (without a splitter in between). You don't want to take a chance of frying your equipment in case there is a spurious high voltage on the cable line.

If the incoming signal voltage is too high, connecting a second splitter in series will attenuate the signal and should improve things. If a second splitter makes things worse then the signal itself was not too strong but this does not rule out a spurious overriding harmful voltage.

Since the cable technician actually said something about problems in your electrical system, it is even more important that you have proper grounding of your equipment. It is okay to be redundant to be doubly sure the issue is not yours. This may include running the long wire I suggested. Historically there have been a few cases where cable company equipment got fried when the final connection was made and a lot of finger pointing and blaming followed.

A reminder: When you say you have voltage in something or on something, voltage is always relative to something else, often relative to the panel terminal strip with all the neutral wires when discussing power circuits. From time to time you will be asked to measure voltage relative to other things or places. One more test. Plug in and turn on a hair dryer. Measure voltage between hot and neutral in the other half of the duplex receptacle. A change of more than two or three volts means an issue in your power system, probably with your neutrals.
 

Last edited by AllanJ; 08-05-16 at 06:47 AM.
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Old 09-20-16, 10:18 AM
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I had a similar experience with voltage on a coaxial cable

I had a similar experience with voltage on a coaxial cable, but my voltage was a full 120 V. In my case it was due to a Samsung television that was backfeeding electricity to the path of least resistance which was the coaxial cable. I could go to any coaxial jack in the house and detect electrical current on it with a voltage tester.

I had an electrician out to the house and described the situation to him and his helper and they were skeptical about how much current was on the coaxial cable until they put a voltage meter on the outside splitter where the coaxial cables all connected. They measured a full 120 V and we then began the process of hunting down what the source of the current was.

We began unplugging electronics from the most recent change to the house's electrical which was a new outlet in the living room. As I unplugged the TV, the voltage meter reading on the outside coax cable went to zero.

So, in your specific case, the solution may vary as others have indicated where you could have a wiring problem inside one of your outlet boxes and this is creating a condition where the excess current is finding the path of least resistance on the coaxial cable. Or it could be that something you have a coaxial cable plugged into, like a TV or possibly something like a coaxial network adapter (to run ethernet signals over coaxial cabling), is backfeeding electricity.
 
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Old 09-20-16, 10:40 AM
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Welcome to the forums.

If you had 120v on the coax shield then not only did you have a faulty product..... you had an incorrectly installed cable system. The cable shield SHOULD be 100% grounded. Your TV would have blown an internal fuse immediately if the coax was properly grounded.
 
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