Voltage present on local tv cable

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chattahoochee electrical

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I received a call from a customer stating that he was having intermittent trouble with his internet which is supplied by the local cable service. He called the cable service out to check the problem and they said that he had 47 volts feeding onto their equipment and he had 10 days to get the problem taken care of or they would cut his service. I went to the residence and started checking for the voltage and sure enough there was 47 volts present on the center pin of the cable. The house had 3 tv sets and the pc on the cable equipment. The voltage was only present on the cable supplying the tv in the family room. I first changed out the tv with one from the bedroom and another room and the problem was still there. I then disconnected all tvs and the computer and connected them one at a time ,the voltage did not reappear until I reconnected a tv to the cable in the family room, I also tried a new cable in the family room and I also brought a tv from my house to try and the voltage was still there. Has anyone come across this before?
 
chattahoochee electrical said:
I received a call from a customer stating that he was having intermittent trouble with his internet which is supplied by the local cable service. He called the cable service out to check the problem and they said that he had 47 volts feeding onto their equipment and he had 10 days to get the problem taken care of or they would cut his service. I went to the residence and started checking for the voltage and sure enough there was 47 volts present on the center pin of the cable. The house had 3 tv sets and the pc on the cable equipment. The voltage was only present on the cable supplying the tv in the family room. I first changed out the tv with one from the bedroom and another room and the problem was still there. I then disconnected all tvs and the computer and connected them one at a time ,the voltage did not reappear until I reconnected a tv to the cable in the family room, I also tried a new cable in the family room and I also brought a tv from my house to try and the voltage was still there. Has anyone come across this before?
Click to expand...
I'm not exactly sure of what you did, but I would check to make sure the voltages were good on the outlet feeding that TV you are having issues with. I would bring an extension cord to that outlet (from a different circuit) and check the voltages from that outlet to the cord, plus I would plug in the TV to the cord and check the cable to see if I still have the voltage on it. The results from this testing would determine what I would do next.
 
  1. are you sure you are not measuring phantom voltage?
  2. Is the receptacle a 2 wire system or 3 wire with proper EGC?

I have had similar issues in the past:

The system was 2 wire system with a cheater adaptor, the hot wire was pinched in the electrical box and the 3 prong cable box was connected to the cheater plug. Once they connecetd the cable to the cable box it shorted, because the cable is grounded but the electrical box was not.
 
I would check between the electric meter case and Earth with a DVM, just stick a screwdriver in the ground or even the probe is enough, if you find the voltage between these two points then call the POCO as there is a bad MGN connection down the road or at the transformer,

I have had this problem a few times and most times it was cause by the service neutral connection or primary MGN connection to the transformer or down the road.

It is most common when the TV or cable box has a 3-wire cord that grounds the chassis and the cable F connector on the back of the TV.

Also take an extension cord from a known properly wired receptacle that you can verify is wired correctly at the panel, in older houses with two-wire ungrounded circuits there are some who think its ok to trick the inspectors three light tester by bonding the ground terminal to the neutral terminal, this can set up a very dangerous shock hazard because if the neutral connection is ever compromised or the hot and neutral is reversed (which can happen when dealing with old cloth covered wire which can be hard to identify which is which) it can put 120 volts on everything grounded to this/these receptacle/'s in a few cases I found that this was the problem and while the neutral was still making contact there was enough voltage drop to cause a voltage between the TV and cable, although at 47 volts it would seem that it would be causing the tv some voltage problems.

Also what can be confusing is if another location such as a bedroom TV is on an ungrounded circuit, and the living room tv just happens to have a good ground then it can make it appear that its the living room TV causing the problem, try un hooking all the other TV's and cable box's and just try the living room by it self, if it still has the problem then use an extension cord as mentioned above to see if the circuit was not bootlegged by connecting the EGC to the neutral, it might not have been done at the receptacle for the TV, if you look in the breaker panel and you see ungrounded circuits but at the receptacles you seem to have a ground then suspect somewhere someone has used the circuit neutral for the EGC.

Let us know what you find or not then maybe I can help you with what to look for a little deeper.

The cable test set they use will display the message "Foreign Voltage" and how much when this problem happens.

Also remember the cable drop to the house is in parallel with the service neutral so a weak connection of the main service neutral can cause the problem but at 47 volts it wouldn't be my first choice as there would be other voltage problems in the house, a bad connection on the primary MGN/neutral could as 47 volts would be hardly missed from a 7200 volt primary so do the test from the service meter to Earth which will rule out this problem or confirm it, also if you find this much voltage between the meter and Earth then call the utility ASAP do not remove any grounding to water pipes or ground rods or the voltage could go very high depending upon where the bad connection is, and it is possible to go as high as the primary voltage so be very carefull.

Tell the lineman that comes out to put an amp meter on the transformers primary neutral, if there is no current then it shows that the transformer is returning the current through another path possible the grounding of the house.
 
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edward said:
  1. are you sure you are not measuring phantom voltage?
  2. Is the receptacle a 2 wire system or 3 wire with proper EGC?

I have had similar issues in the past:

The system was 2 wire system with a cheater adaptor, the hot wire was pinched in the electrical box and the 3 prong cable box was connected to the cheater plug. Once they connecetd the cable to the cable box it shorted, because the cable is grounded but the electrical box was not.
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Usually the TV input circuit attached to the F jack is differential between the center wire and the shield. But if the shield on that coax is not connected (at one end or the other) then it will allow the whole input circuit to float up to the "ground" in the TV power cord. That in turn will allow the TV chassis to go upward in potential from leakage or capacitive coupling (phantom voltage). That would then put 47 volts (but little current) on the center of the coax.

In addition to the possibility of a bad connector at any point along the cable, or simply a lack of ground at the cable box, it is also possible that a DC ground isolator was inserted into that run of cable somewhere. When there are legitimate off-ground potential problems (see the Mike Holt long video on Stray Voltages) somebody may have inserted a cable adapter that makes the ground connection only through a capacitor which is a small enough value to pass RF but blocks 60Hz.

The dead giveaway for this general type of problem will be that the shield of the F connector on the TV is at 47 volts whether it is connected to the cable or not.
You should also see 47 volts on the center pin of the F jack.

Other posters have addressed some of the ways that the TV ground could get to 47 volts.
 
Had this go round with the catv company before. Use a analog meter And see if you get the same results. With my digital multimeter I found that certain tv's plugged in but not powered up would show the magic number of 47ish volts sheild to ground. Cable company says it a electric problem and shows me with his test tv. Turned out that it was the only apartment without a cable box and once connected to a box voltage dissapeared. Went home and to work and tested every tv I could find and the 2 prong tv's hooked up to just power should voltage shield to ground...3 prong or connected to a box nothing.
 
I had a thread awhile back about the same type of problem. In my case I had 120v on my coax. I found that there was reversed polarity and a boot leg ground. The ground wire was attached to the neutral side of the 3 prong receptacle so there was current on the EGC. When the power strip was plugged in and the Playstation was plugged in it put current where it did not belong.
I don't know if this helps, just a thought.
 
chattahoochee electrical said:
I received a call from a customer stating that he was having intermittent trouble with his internet which is supplied by the local cable service. He called the cable service out to check the problem and they said that he had 47 volts feeding onto their equipment and he had 10 days to get the problem taken care of or they would cut his service. I went to the residence and started checking for the voltage and sure enough there was 47 volts present on the center pin of the cable. The house had 3 tv sets and the pc on the cable equipment. The voltage was only present on the cable supplying the tv in the family room. I first changed out the tv with one from the bedroom and another room and the problem was still there. I then disconnected all tvs and the computer and connected them one at a time ,the voltage did not reappear until I reconnected a tv to the cable in the family room, I also tried a new cable in the family room and I also brought a tv from my house to try and the voltage was still there. Has anyone come across this before?
Click to expand...

47 volts from where to where? Center pin to the cable shield? Or the electrical ground?

The cable sheild should be grounded to the electrical ground at the cable service.

If you are taking readings with a high impedence DVM, the readings probably mean nothing. You are probably trying to measure the voltage between two unconnected pieces of wire.

There is a requirement that all external metal parts on a TV don't have more than "X" volts to ground, but this measurment is taken with a specific resistor and capacitor placed in parallel with the meter to bleed off "ghost" voltages. (I forget the specific voltage limits and component sizes, but it used to be printed in every single television service manual. It was a safety test required after any TV was serviced. The requirement started back in the days when TV's had a "hot" chassis, and putting the wrong screw in the wrong place could create a shock hazzard. Not sure if these tests are still required for LCD TV's, but the ghost voltage effect would stilll apply).

You may want to search this forum for more info on ghost voltages. The cable guy should do the same.
 
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Here is an intro to a monthly engineering study publication w.r.t. cable TV with a voltage of 35 volts that was developed on the cable ground.

I got permission from the authors to use it in this forum.


Note:The PDF is too large to be inserted as an attachment.




Where is all that current coming from?

This is a really interesting case study. Definitely a phenomena you don?t run across everyday.

We initially got the call from a very frustrated cable TV engineer. Apparently, the plastic cable TV splitter boxes were melting off the wall at many homes, however, only in one specific area of town. The outside house cable TV grounds would also generate a ?spark? each time an attempt was made to connect it to the outside house electrical utility ground.

Actual measurements between these 2 grounds (cable & utility) at this site revealed a 35 Volt difference and over 20 Amps of current flow between these 2 grounding systems.
 
We have had a case of that ourselves. The cablevision coax would spark when disconnected, and ended up frying some appliances. Our neutral connection had a high resistance, and since the cablevision connects to our grounds at the house and at the pole, the coax was effectively the neutral for the house.

Call the POCO
 
had similar issue in shich cable lines were melting throught the house. found nuetral and grounds tied together all over the place as well as lights with no nuetral using tapped wire from metal boxes to act as a return. GEC was completly rotted at grounding rod as well. I would check to ensure proper grounding as one thing I forgot to mention is that water lines were also measuring voltage.This cable issue should be taken very seriously as other issues may be present as well.Yes place was a nightmare, needles to say we ended up rewiring entire place. If there is 47 volts on cable line it sounds like grounds and nuetrals are one in the same and voltage return is taking any path it can find.
 
It has been suggested but not really outright stated - make sure the voltage is coming from within the home. If it is coming from outside the home, it still needs to be determined if the source is the cable or the POCO. Also measure with a low impedance meter, as has been stated.
 
Definitely try to measure the voltage with a shunt resistor if using a digital meter. I typically use around 500 ohms. If the voltage disappears, then it is not really there. And you have verified the voltage is AC and not DC?
 
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