voltage backfeed to cable line

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  #1  
Old 01-10-05, 01:46 PM
joelsraymond
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voltage backfeed to cable line

The cable company says 18-25 volts is backfeeding from the tv out to the cable line and burning out the connector in the box outside. They blame the tv's chasis ground. After they left, I got out my multimeter and performed some tests because the tv is only 3 months old.

With the tv plugged into the wall, turned on and *nothing* else connected I verified 25 volts from the coax jack on the tv using the cable line shield (this goes directly through the wall to the pole) as the ground. According to the tech the box is grounded outside. This is basically the test that they did but with a non-contact voltmeter, which I assume uses them as the ground.

I carried another tv into the room and got the same result. I also tried the same thing with the cable box and got the same result. Thus making me think the tv is not the problem.

I bought an outlet tester and it shows the outlet is wired properly.
I then tested the neutral and ground on the outlet using the cable coax shield again as the ground. Here I only get about 0.5-1.0 volts which seems within tolerance. If I would have seen voltage here that would have meant a floating ground or neutral? I didn't so...

Now I'm pretty much stumped. The outlet seems fine. All devices give the same 25 volt backfeed. Another outlet gives the same result.

The only thing left I can find on the internet that may be pertinent is if the ground and neutral are tied together. Could this cause the problems I'm seeing? Would this only be at the outlet or could this be in the breaker box?

Any ideas would be appreciated.

Thanks

joel
 
  #2  
Old 01-10-05, 01:55 PM
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Are you using an analog or digital voltmeter?
 
  #3  
Old 01-10-05, 02:00 PM
joelsraymond
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i'm using a $30 radio shack digital multimeter
 
  #4  
Old 01-10-05, 02:22 PM
joelsraymond
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i can take an analog multimeter home from work tonight and try it, if that would make a difference.
 
  #5  
Old 01-10-05, 03:48 PM
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Try an analog meter and tell us what it measures.
 
  #6  
Old 01-10-05, 05:12 PM
joelsraymond
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I just got home and tried an analog meter on the outlet and the tv test from above. I now get a reading of only <1 volt. I just read online about the fake voltage readings with digital meters when testing AC. I wish I would have known that before buying the digital one. Oh well.

I took the faceplate off and checked the wiring. Looks like it's only two-wire. The ground is a copper wire tied to the metal outlet box. I would think this isn't good but I'm only getting <1 volt when the tv is on.

So now I even more confused. What could be burning out the connector outside? How can I convince the cable guy that the 25 volts doesn't exist. The first thing he will do when he shows up is put that non-contact meter with the LED's up against the coax and get a reading. I guess I could show him the analog reading.

Are there any other tests I should be doing?
I rent but the landlord is open to improvements.

Thanks for the help so far!

joel
 
  #7  
Old 01-10-05, 05:19 PM
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So if you had never talked to the cable company, and if you didn't own a voltmeter, would you have a problem? If so, what would that problem be? You mention something about "burning out the connector outside"? What exactly is happening? Did the cable guy refuse to connect the cable?
 
  #8  
Old 01-10-05, 05:50 PM
joelsraymond
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I'm sorry I should have been more clear. We have had cable for awhile. We lost service and the cable guy came out. He said there was voltage on the line and the connector outside the house at the box was burned. A cable goes from our living room right through the wall directly to the pole about 5 feet away. He proceeded to split the cable from another source at the outside box. About a week later we lost cable again. The tech came back a second time and said there was voltage on the line and said an engineer would have to come out. The engineer came out, held his voltmeter up to the cable line and said there was around 25 volts backfeeding and that's what was causing the burned connectors and loss of service. He said we had to get the TV or the electric in the house looked at before they would repair and reconnect the line.

I was performing these tests in hopes of verifying if it was the tv or the house wiring. Since both tv's and the cable box give the same results I'm running on the assumption that it is the house wiring. Although now I'm not really sure what is going on because the readings look like there is nothing wrong.

Thanks,

joel
 
  #9  
Old 01-10-05, 06:32 PM
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Does your TV have a two wire plug or a three wire plug? And, is the receptacle properly wired? If two wire, is it polarized?

Is the receptacle a two hole receptacle, or is it three holes? If two hole is it polarized? Is it properly wired? Is the hot on the smaller of the two plug in connectors?
 
  #10  
Old 01-10-05, 06:46 PM
joelsraymond
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The tv has a two prong plug.

The outlet is a three prong outlet, but it looks like I have an old two wire electrical system. The ground is a copper wire tied to the back of the metal outlet box. An outlet tester shows that it is wired correctly. The voltmeter shows the 120 on the proper narrow plug.

I don't know what you mean by polarized.

joel
 
  #11  
Old 01-10-05, 06:57 PM
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Some 2 wire plugs are polarized. Polarized means that one terminal of the plug is larger than the other. If looking from the side, it is wider than the other one. The larger terminal is the wire that needs to connect to the neutral.

Polarized plugs are used in most newer lamps to make certain that the connector at the bottom of the inside of the lamp socket is the hot side of the circuit, and that the outside of the socket is the negative side, to avoid accidental shocks.

Polarized plugs are also used in many electronic devices (TVs VCR, DVD Players, etc.) to make sure that the neutral and the hot wire feed the device in the way intended by the design engineers.

Is the TVs plug polarized or can it be inserted either way into the receptacle?

It sounds like you do have proper wiring, but that whoever installed the new receptacle missed one step. The copper wire that you say is tied to the back of the box also needs to attach to the ground screw on the receptacle. This makes certain that receptacle is properly grounded.

However, if your receptacle tester is showing that the ground hole on the receptacle is in fact grounded, then there is a connection between the receptacle ground and the metal box. It may or may not be a proper connection, but it is a connection.
 
  #12  
Old 01-10-05, 07:02 PM
joelsraymond
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Ok I see.

The tv plug is polarized.
The copper wire goes from the receptacle to the back of the box and that is all. It looks to be only a couple inches long. I don't think it goes anywhere else unless it is connected on the back of the box.
 
  #13  
Old 01-10-05, 07:26 PM
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I misunderstood what you said. I thought you said that the copper wire from the cable went to the box but nowhere else.

You have a misleading (and potentially dangerous) setup. Whoever installed those three prong receptacles made a big mistake. You should very soon either install two prong receptacles, or make the receptacles grounded.

I would look elsewhere in your house for a problem. Where is the incoming cable wire connected? There should be a connection somewhere, usually to a cold water pipe.

One final question, you aren't trying to run the cable through a surge protector are you?
 
  #14  
Old 01-10-05, 07:29 PM
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Bob, the receptacle is already grounded. That's not the problem.
 
  #15  
Old 01-10-05, 09:19 PM
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If the metal electrical box is grounded, perhaps through the use of AC cable (the metal armored stuff), or the use of conduit as a wiring method, then the receptacle can be grounded by using a short jumper that goes to the back of the box. In fact, if the receptacle is properly designed, then the metal screws that connect it to the metal box is sufficient.

A short jumper might also indicate a 'bootleg ground', a dangerous situation where the ground terminal of the receptacle is connected to the neutral supply wire somewhere in the receptacle box.

You would have to look more closely and describe the receptacle, the box, and the wires, to determine which is which; I don't think that you provided sufficient information.

Regarding your problem with the cable, I'd suggest looking at 'ground bonding'. In particular, look at your electrical panel and find the grounding electrode conductor. This is a thick wire that goes to ground electrodes, either ground rods, your cold water supply pipe, building steel, or some combination of these. Then look at the cable box that is on the outside of your house, and find its grounding electrode conductor. Determine if it goes to its own ground rod, or if it is somehow tied to your building electrical system.

The ground conductor for the box on the outside of your house is supposed to be grounded, but this ground must attach to your house electrical system ground. Using different ground rods can cause serious problems if there is any current flow through the earth in the vicinity of your home.

-Jon
 
  #16  
Old 01-11-05, 05:19 AM
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I service TVs at a hospital, and the vast majority of them use the coax as signal and power source. They have over 30 volts running through standard, el-cheapo coax connectors, and they only burn out if theres a short somewhere.
 
  #17  
Old 01-11-05, 07:53 AM
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I still don't see the connection between the 25V on the line (AC or DC?) and the burning of the connector. I say switch to DirecTV.
If the cable is grounded at the entry point then it shouldn't cause any problems.
 
  #18  
Old 01-11-05, 07:54 AM
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I just remembered that the cable company does feed voltage onto the line to power up their inline amplifiers. I'm not a cable tech guy so I don't know how much voltage, but 25V sounds like a good voltage.
 
  #19  
Old 01-11-05, 08:04 AM
joelsraymond
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@Winnie - I can't really tell if the metal receptacle box is grounded to anything. I tried to take the box out of the wall but it wouldn't seem to come out. It may have some metal conduit attached. Both hot and neutral are wired directly to the correct places on the receptacle. These are older wires and are not black and white. I've verifed that they are wired correctly with both the outlet tester and a voltmeter. The copper wire (no plastic shielding around it) goes from a screw on the back of the receptcle to a screw on the back of the metal box the receptacle is contained in. There are no other connections/wires in the receptacle box. I know I need to be able to say if the box is somehow connected to anything else (grounded) but that doesn't seem possible, even with a bit of yanking on it.

I had a look at the fuse box. Actually it looks like the fuse box also has small breaker box split off from it. I can't find a ground that goes to anything. I looked at the incoming water pipes for a ground connector and didn't see anything. I also tried to follow wires from the fuse box. They are in metal conduit. I didn't see anything that looked like a ground.

The first time we had the cable loss of service the tech ran a ground for the cable outside. He ran a green wire from the splitter to a pipe on the side of the house. I'm not sure that any other ground is connected to this.

As soon as my landlord gets back from LA I'll be talking with her to get an electrician to come over and have a look. Hopefully some of the this information will help him.
 
  #20  
Old 01-11-05, 08:12 AM
joelsraymond
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@ trinitro The 25v was AC. Although with my ananlog voltmeter ,I'm not reading it. I get a <1volt. Is it possible that the non-contact voltmeters with the LED's that the cable company carries are similar to a digital multimeter? Thus giving them a false 25 volt reading like I originally got with my digital multimeter?

The tech thinks the voltage is coming from the house because he waved his non-contact volmeter up and down a piece of cable connected to the tv, but *not* connected to the outside cable line. He saw the voltage reading and basically said it wasn't coming from outside, so I have to fix it. He thinks that voltage is leaking outside when connected and burning the connector out.
 
  #21  
Old 01-11-05, 08:15 AM
joelsraymond
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@air biscuit - do you mean a short in the coax somewhere?
 
  #22  
Old 01-11-05, 08:31 AM
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I think that you can be pretty comfortable that your receptacle is correctly grounded. It is probably the case that this metal box is grounded through the armor of one of the cables feeding it.

If the 'pipe' on the side of your house is the metal conduit that feeds your meter, or goes from the meter to your main panel, then that is the correct place to bond the cable system ground wire. If it was any other pipe, then this was possibly not a correct install of the cable system ground.

If you have metal conduit between your meter and your main panel, then the grounding electrode conductors may be at your meter, and the metal conduit corresponds to the ground wire.

-Jon
 
  #23  
Old 01-11-05, 08:56 AM
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Originally Posted by joelsraymond
@air biscuit - do you mean a short in the coax somewhere?
Yeah.

My point is that it would seem to me to be unlikely that 25V would burn out a coax connector, as even cheap ones can handle more than 30V (AC or DC) without a problem.
 
  #24  
Old 01-11-05, 11:48 AM
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You have to measure between the inner conductor and the shielding to get a voltage reading. The shielding is grounded at the house, and at the pole in most instances, so any "leakage" from the TV to the shielding should be grounded. The shield is also grounded at the TV chassis.

You will measure a voltage between the conductor and the shielding. The cable company "injects" the voltage to power their inline amplifiers. This voltage is ignored by the actual TV appliance. I would think the voltage is DC and the cable signal is riding on top of it, but I could be wrong since most areas work with digital cable.

If you were to have such a large voltage when you get the two connectors close to eachother you should see a fairly large spark.

If the shielding will be punctured and for some reason 120V AC would come in contact with it you TV will get damage very quickly, the breaker will probably trip and you will measure 120V AC on the shielding (assuming it's not grounded).
 
  #25  
Old 01-11-05, 06:02 PM
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Joel, try something a bit different. Do you have a GFCI outlet in the house someplace? If you do, use an extension cord from the GFCI to the tv and see if you still the same readings. Make sure the extension cord has polarized plugs too. I am assuming the GFCI definately is wired right.
 
  #26  
Old 01-11-05, 06:19 PM
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you can purchase a grounded connector for your coax cable.. put it outside where the cable exits the house, drive a ground rod, or hook a ground cable to the cold water "pipe", (not plastic).... then connect it to the grd terminal.. put the coax from your house into one side and the coax to the cable company out the other side.. then you have grounded the shield on the cable.. I'm surprised the cable company didn't install one of these when they put the cable into your house..
 
  #27  
Old 01-11-05, 08:17 PM
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There can only be 1 ground system in any house. You can't just simply drive another ground rod. That can and will cause other problems.

The cable system should already be grounded at the point of entry into the house. It is the cable companies's resposibility to provide a good ground. In my area they accomplish this by grounding it the service entrance conduit. If your area uses SE cable they probably do it other ways.

Either way, all this does is ground the shielding of the cable. It does nothing to the core conductor.
 
 

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