# Voltage on house coax cable



## kevinm34232

Hi, I'm a cable co. technician and have a question I can't seem to get a straight answer on, I'll try to describe as best I can.
From time to time I run into customers who have ongoing intermittent issues with their cable service, most of the time their modems. Everything has been checked out with the cable yet problem persists. Then either I feel a little shock outside at the ground block or I will just pull out the multimeter, pull of the coax feeding inside, and I find 50-60v AC on the line, measured with the red lead on the center conductor of the cable, and black lead on the ground wire.

This last time was just the other day, it was 1 cable feeding 2 TV's, on a common circuit. I tried to troubleshoot by unplugging devices and what I came up with was that whenever ANY device is plugged into that circuit and also connected to the coax, the voltage backfed on the cable. I assumed it is something wrong with the electrical wiring.
One of the outlets looked like it had been added aswell, so I suggested she call an electrician. Not sure how often electricians run into this and if it is easy for one to troubleshoot. 
Does anyone have an idea of what's going on. Everytime I've run into this, it's always 45-60v and same symptoms with the cable equipment. 

Oh also the last tech that was there before me said that the cable at one of the connectors outside was melted when he got there.


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## Dennis Alwon

I will allow this thread as it may be helpful to you but this site is for electricians and work done should be done by a trained electrician. Please keep response to what causes it and not how to fix it.


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## kevinm34232

Thank you I'm not interested in fixing it, just what causes it so I can tell customer what to tell the electrician to look for.


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## dronai

B4T just had this problem, and found the source. I don't remember exactly what he did, but I think some wiring splices were done by someone not qualified.


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## guest

Most common causes, in order of number of times I have run into them:



Defective cable box;
Defective TV/VCR;
Miswired receptacle;
Loss of the customer's grounding and/or neutral;
Lightning/surge damage;
Faulty CATV line amp;
Customer making illegal (and poorly done) tap into cable line.
I am sure others on here will come up with more. 

Based on the melted cable outside it is possible that #4 on my list is the culprit, and it requires the attention of an electrician.


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## ecelectric

Loose neutral weather head


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## kevinm34232

Thanks for input so far..

I'm curious how the voltage actually gets onto the center conductor of the coax though just from the cable device being plugged in to an outlet. Is there any significance of the voltage always being around 50-60v?


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## oldtimer

mxslick said:


> Most common causes, in order of number of times I have run into them:
> 
> 
> 
> Defective cable box;
> Defective TV/VCR;
> Miswired receptacle;
> Loss of the customer's grounding and/or neutral;
> Lightning/surge damage;
> Faulty CATV line amp;
> Customer making illegal (and poorly done) tap into cable line.
> I am sure others on here will come up with more.
> 
> Based on the melted cable outside it is possible that #4 on my list is the culprit, and it requires the attention of an electrician.



My first thought was, the added outlet has hot/neutral reversed!

#3 Miswired receptacle

An electrician should be able to determine this very easily!


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## Dennis Alwon

I also think it is a problem with the ground or neutral. If there is too much current on the ground then the coax can pick it up.


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## CraigV

Agreed, dropped service neutral. The grounding system, including phone, cable and any other bonded systems, is carrying the return current imbalance.


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## jza

Most cable modems operate around the 60hz frequency, if the cable service isn't properly grounded at the service entrance and there's anything on the line it's likely to knock out the modem.

Keep in mind that the hard line cable has up to 120vac on it, to power the amplifiers. This voltage isn't supposed to hit any of the drops, but you never know. Also check for defective equipment in the house, old TV's, amplifiers, power inserters.


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## guest

kevinm34232 said:


> Thanks for input so far..
> 
> I'm curious how the voltage actually gets onto the center conductor of the coax though just from the cable device being plugged in to an outlet. _*Is there any significance of the voltage always being around 50-60v?*_



Ask one of your senior techs...but my bet is that the 50-60v number just happens to be in the range of the hard line cable jza referred to to power your system's own amplifier/boosters. :thumbsup:


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## CraigV

mxslick said:


> Ask one of your senior techs...but my bet is that the 50-60v number just happens to be in the range of the hard line cable jza referred to to power your system's own amplifier/boosters. :thumbsup:


 
I assumed he had disconnected the cable-co's connection before troubleshooting. Hmm...not a wise assumption.


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## jza

mxslick said:


> Ask one of your senior techs...but my bet is that the 50-60v number just happens to be in the range of the hard line cable jza referred to to power your system's own amplifier/boosters. :thumbsup:


Bingo, take a reading right off the tap. If there's voltage there, escalate the call to the appropriate department.

This **** could run deep, for all you know it's a back feed from a neighbouring house.


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## Roadhouse

Can this voltage mess with the picture? I ask because I have a brand new _*240hz*_ 1080 HD LCD flat screen and only on a certain few channels is the picture really crystal clear. Cable.


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## Roadhouse

Or is it because the cable company isn't broadcasting in 1080? I have no idea if there is voltage on my coaxial or not.


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## frank

I don't think this is a service problem The modem or TV will usually be a non grounded appliance. The inductive /capacitive component of the internal circcuitry will be fed through all extenal components by virtue of contact or connection.The current levelis insignificant and voltage levels are consistent with this type of problem. The cure is to ensure that coax cables are gounded. You will not find any voltage to measure if you try to take a reading with good grounding in place. This is not a dangerouse problem but a factor consistent with most electronic equipment

Frank


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## CraigV

frank said:


> I don't think this is a service problem The modem or TV will usually be a non grounded appliance. The inductive /capacitive component of the internal circcuitry will be fed through all extenal components by virtue of contact or connection.The current levelis insignificant and voltage levels are consistent with this type of problem. The cure is to ensure that coax cables are gounded. You will not find any voltage to measure if you try to take a reading with good grounding in place. This is not a dangerouse problem but a factor consistent with most electronic equipment
> 
> Frank


Without first determining the source of the voltage it's ill-advised to state a problem isn't dangerous.


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## Roadhouse

Dennis Alwon said:


> I will allow this thread as it may be helpful to you but this site is for electricians and work done should be done by a trained electrician. Please keep response to what causes it and not how to fix it.


 
Weird. Down here it's a one stop shop. Be it coaxial or romex, we do it all.


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## B4T

dronai said:


> B4T just had this problem, and found the source. I don't remember exactly what he did, but I think some wiring splices were done by someone not qualified.


It was a bunch of bad splices with a nail touching ground and hot.. it would not blow the circuit because the ground was not connected in one of the splices..

120v was back feeding through ground into coax through TV.. really wired set of events with the hack changing wire colors in receptacles to get 120V at the recdeptacles..

My handy receptacle tester showed all receptacles wired properly.. :blink::blink:


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## chicken steve

_calling all Kirchoffs commandos....._


A basic assessment question to ask here would be, just how many residents in the nieghborhood are _common_ to a serving poco xformer

and next would be, just how many residents are _common_ to _your_ coax system _(which would be bonded, by rights, back to said xformer)_

~CS~


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## jza

chicken steve said:


> _calling all Kirchoffs commandos....._
> 
> 
> A basic assessment question to ask here would be, just how many residents in the nieghborhood are _common_ to a serving poco xformer
> 
> and next would be, just how many residents are _common_ to _your_ coax system _(which would be bonded, by rights, back to said xformer)_
> 
> ~CS~


I've seen 12 spigot taps, strobes can have well over double that.


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## kevinm34232

Yes it was disco'd from the tap the voltage is coming from the house not the cable system.


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## jza

Good, on other calls you can troubleshoot using what's been posted above.


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## CraigV

kevinm34232 said:


> Yes it was disco'd from the tap the voltage is coming from the house not the cable system.


Excellent, you should then be able to use process of elimination to isolate the voltage source on your cable system.


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## frank

CraigV

You are right of course but rather than waffle on for pages and since this is a site for professionals I take it as 'said' that any hot scources have been eliminated and the fault I described is common and danger free, I will be more precise in my explanations in future.

Frank


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## chicken steve

kevinm34232 said:


> Yes it was disco'd from the tap the voltage is coming from the house not the cable system.


 
10-4 Kev

the V is from the house

but....how many paths is it utilizing to return to _'source'_?

~CS~


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## crosport

Sometimes voltage can be induced into the coax if run really close to branch circuit wiring,coax can be like a capacitor and should be grounded properly to bleed off induced voltage.


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## CraigV

frank said:


> CraigV
> 
> You are right of course but rather than waffle on for pages and since this is a site for professionals I take it as 'said' that any hot scources have been eliminated and the fault I described is common and danger free, I will be more precise in my explanations in future.
> 
> Frank


 I just get nervous whenever an unknown condition is assumed safe. Accidents are often the result of two or more failures; in this case, the voltage is one, and assuming it isn't dangerous is the potential second.


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## CraigV

chicken steve said:


> 10-4 Kev
> 
> the V is from the house
> 
> but....how many paths is it utilizing to return to _'source'_?
> 
> ~CS~


There's the rub, aye?


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## bobelectric

Dennis Alwon said:


> I will allow this thread as it may be helpful to you but this site is for electricians and work done should be done by a trained electrician. Please keep response to what causes it and not how to fix it.


 Bless You,this could help us all.


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## kevinm34232

I guess Im still unclear how voltage gets onto the coax from a cable device like a modem. The modem is plugged into an outlet, the coax is plugged into the modem, but how is the voltage from the outlet getting onto the coax is it just jumping the circuits inside the modem? Because the coax is grounded to the braid at the ground block which is the outside part of the cable or connector, but the voltage is on the center conductor of the coax.??? Just trying to understand the concept...


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## Magnettica

kevinm34232 said:


> I guess Im still unclear how voltage gets onto the coax from a cable device like a modem. The modem is plugged into an outlet, the coax is plugged into the modem, but how is the voltage from the outlet getting onto the coax is it just jumping the circuits inside the modem? Because the coax is grounded to the braid at the ground block which is the outside part of the cable or connector, but the voltage is on the center conductor of the coax.??? Just trying to understand the concept...


The cable block, normally on the exterior of the house/ building, should be connected to grounding electrode system. I'll bet that's where your issue begins. Call an electrician.

Sent from your sisters iPhone!


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## kevinm34232

But the problem starts from the circuit that the modem is plugged into, not at the ground block outside. So we identify the offending circuit, if the modem is plugged in anywhere else it's fine, but in the bad circuit it somehow feeds voltage back through the coax that is also connected to the modem.


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## CraigV

kevinm34232 said:


> But the problem starts from the circuit that the modem is plugged into, not at the ground block outside. So we identify the offending circuit, if the modem is plugged in anywhere else it's fine, but in the bad circuit it somehow feeds voltage back through the coax that is also connected to the modem.


 
If the modem has an internal power supply, not a wall-wart, check the receptacle it's plugged into for a reversed hot and neutral.


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## mikeh32

I have seen this with a shorted out router. The board had a bad terminal, and caused the voltage to jump on to the ethernet cable. 

It messed up ****. 

And this work performed by Low voltage electricians here, not the regular guys


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## CraigV

mikeh32 said:


> I have seen this with a shorted out router. The board had a bad terminal, and caused the voltage to jump on to the ethernet cable.


In this particular case, the trouble is that the problem only appears when the modem is plugged into a particular receptacle. It's still possible there's a fault in the modem. In fact I'm guessing it might be a combination of miswired recep and bad modem.


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## jza

crosport said:


> Sometimes voltage can be induced into the coax if run really close to branch circuit wiring,coax can be like a capacitor and should be grounded properly to bleed off induced voltage.


No it can't.


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